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Types of Heating Systems

 

Types of Heating Systems

A variety of technologies are available for heating your house. In addition to heat pumps, which are discussed separately, many homes use the following approaches:

Furnaces and Boilers
By far the most common way to heat a home.

Wood and Pellet-Fuel Heating
Provides a way to heat your home using biomass or waste sources.

Electric Resistance Heating
Among the most expensive ways to heat a home.

Active Solar Heating
Uses the sun to heat either air or liquid and can serve as a supplemental heat source.

Radiant Heating
Can draw on a number of energy sources, including electricity, boilers, solar energy, and wood and pellet-fuel heating.

Small Space Heaters
Less efficient than central heating systems, but can save energy when used appropriately.
While forced-air heating systems rely on the same type of ducts used by heat pumps and air conditioners, water and steam heat systems use radiators that only deliver heat.

Heat Distribution Systems
Heat is distributed through your home through a variety of ways whether forced-air systems, central air conditioning, heat pump systems, radiant heating, steam radiators or hot water radiators.

Furnaces and Boilers

Most U.S. homes are heated with either furnaces or boilers. Furnaces heat air and distribute the heated air through the house using ducts. Boilers heat water, and provide either hot water or steam for heating. Steam is distributed via pipes to steam radiators, and hot water can be distributed via baseboard radiators or radiant floor systems, or can heat air via a coil. Steam boilers operate at a higher temperature than hot water boilers, and are inherently less efficient, but high-efficiency versions of all types of furnaces and boilers are currently available.

UNDERSTANDING THE EFFICIENCY RATING OF FURNACES AND BOILERS

A central furnace or boiler's efficiency is measured by annual fuel utilization efficiency (AFUE). The Federal Trade Commission requires new furnaces or boilers to display their AFUE so consumers can compare heating efficiencies of various models. AFUE is a measure of how efficient the appliance is in converting the energy in its fuel to heat over the course of a typical year.

Specifically, AFUE is the ratio of annual heat output of the furnace or boiler compared to the total annual fossil fuel energy consumed by a furnace or boiler. An AFUE of 90% means that 90% of the energy in the fuel becomes heat for the home and the other 10% escapes up the chimney and elsewhere. AFUE doesn't include the heat losses of the duct system or piping, which can be as much as 35% of the energy for output of the furnace when ducts are located in the attic, garage, or other partially conditioned or unconditioned space.

An all-electric furnace or boiler has no flue loss through a chimney. The AFUE rating for an all-electric furnace or boiler is between 95% and 100%. The lower values are for units installed outdoors because they have greater jacket heat loss. However, despite their high efficiency, the higher cost of electricity in most parts of the country makes all-electric furnaces or boilers an uneconomic choice. If you are interested in electric heating, consider installing a heat pump system.

The minimum allowed AFUE rating for a non-condensing fossil-fueled, warm-air furnace is currently 78% for all types of furnaces except for those designed and manufactured specifically for use in mobile homes, for which the minimum AFUE is 75%. In  the future, minimum AFUE requirements for furnaces will be based on the type of fuel consumed (i.e., gas, oil, or electricity), whether the unit is non-weatherized (i.e., intended for installation indoors) or weatherized (i.e., intended for installation outdoors), and whether the unit is designed specifically for use in a mobile home. 

Beginning on May 1, 2013, for non-weatherized furnaces and January 1, 2015, for weatherized furnaces, the minimum AFUE requirements will be as follows:

PRODUCT CLASS MINIMUM
AFUE RATING
Non-weatherized gas furnaces
(not including mobile home furnaces)
80%
Mobile home gas furnaces 80%
Non-weatherized oil-fired furnaces
(not including mobile home furnaces)
83%
Mobile Home oil-fired furnaces* 75%
Weatherized gas furnaces 81%
Weatherized oil-fired furnaces* 78%
Electric furnaces* 78%

*The minimum AFUE requirements for these product classes are unchanged from the current requirements.

For boilers, the minimum AFUE rating requirements vary based on the type of fuel used and the heating medium. The minimum AFUE rating for a gas-fired hot water boiler is 82%; the minimum AFUE rating for a gas-fired steam boiler is 80%; the minimum AFUE rating for an oil-fired hot water boiler is 84%; and the minimum AFUE rating for an oil-fired steam boiler is 82%. In addition, gas-fired boilers are not permitted to have a constant burning pilot, and hot water boilers are required to have an automatic means for adjusting the water temperature to match the heating load. 

A condensing furnace or boiler condenses the water vapor produced in the combustion process and uses the heat from this condensation. The AFUE rating for a condensing furnace or boiler can be much higher (by more than 10 percentage points) than a non-condensing furnace or boiler. Although condensing models cost more than non-condensing models, a condensing furnace or boiler can save you money in fuel costs over the approximately 15- to 30-year life of the unit and is a particularly wise investment in cold climates.

You can identify and compare a system's efficiency by not only its AFUE but also by its equipment features, listed below.

Old, low-efficiency heating systems:

  • Natural draft that creates a flow of combustion gases
  • Continuous pilot light
  • Heavy heat exchanger
  • 68%–72% AFUE

Mid-efficiency heating systems:

  • Exhaust fan controls the flow of combustion air and combustion gases more precisely
  • Electronic ignition (no pilot light)
  • Compact size and lighter weight to reduce cycling losses
  • Small-diameter flue pipe
  • 80%–83% AFUE

High-efficiency heating systems:

  • Condensing flue gases in a second heat exchanger for extra efficiency
  • Sealed combustion
  • 90%–97% AFUE

Retrofitting Your Furnace or Boiler

Furnaces and boilers can be retrofitted to increase their efficiency. These upgrades improve the safety and efficiency of otherwise sound, older systems. The costs of retrofits should be carefully weighed against the cost of a new boiler or furnace, especially if replacement is likely within a few years or if you wish to switch to a different system for other reasons, such as adding air conditioning. If you choose to replace your heating system, you'll have the opportunity to install equipment that incorporates the most energy-efficient heating technologies available.

Retrofits are fuel-specific, so see the following sections for retrofit information:

  • Gas-Fired Furnaces and Boilers (includes units fired with natural gas and propane)
  • Oil-Fired Furnaces and Boilers

Other retrofitting options that can improve a system's energy efficiency include installing programmable thermostats, upgrading ductwork in forced-air systems, and adding zone control for hot-water systems, an option discussed in the Heat Distribution Systems section.

REPLACING YOUR FURNACE OR BOILER

Although older furnace and boiler systems had efficiencies in the range of 56% to 70%, modern conventional heating systems can achieve efficiencies as high as 98.5%, converting nearly all the fuel to useful heat for your home. Energy efficiency upgrades and a new high-efficiency heating system can often cut your fuel bills and your furnace's pollution output in half. Upgrading your furnace or boiler from 56% to 90% efficiency in an average cold-climate house will save 1.5 tons of carbon dioxide emissions each year if you heat with gas, or 2.5 tons if you heat with oil.

If your furnace or boiler is old, worn out, inefficient, or significantly oversized, the simplest solution is to replace it with a modern high-efficiency model. Old coal burners that were switched over to oil or gas are prime candidates for replacement, as well as gas furnaces with pilot lights rather than electronic ignitions. Newer systems may be more efficient but are still likely to be oversized, and can often be modified to lower their operating capacity.

Before buying a new furnace or boiler or modifying your existing unit, first make every effort to improve the energy efficiency of your home, then have a heating contractor size your furnace. Energy-efficiency improvements will save money on a new furnace or boiler, because you can purchase a smaller unit. A properly sized furnace or boiler will operate most efficiently, and you'll want to choose a dependable unit and compare the warranties of each furnace or boiler you’re considering.

When shopping for high-efficiency furnaces and boilers, look for the ENERGY STAR® label. If you live in a cold climate, it usually makes sense to invest in the highest-efficiency system. In milder climates with lower annual heating costs, the extra investment required to go from 80% to 90% to 95% efficiency may be hard to justify.

You can estimate the annual savings from heating system replacements by using the table on this page, which assumes that both heating systems have the same heat output. However, most older systems are oversized, and will be particularly oversized if you significantly improve the energy efficiency of your home. Because of this additional benefit, your actual savings in upgrading to a new system could be much higher than indicated in the table.

Specify a sealed combustion furnace or boiler, which will bring outside air directly into the burner and exhaust flue gases (combustion products) directly to the outside, without the need for a draft hood or damper. Furnaces and boilers that are not sealed-combustion units draw heated air into the unit for combustion and then send that air up the chimney, wasting the energy that was used to heat the air. Sealed-combustion units avoid that problem and also pose no risk of introducing dangerous combustion gases into your house. In furnaces that are not sealed-combustion units, backdrafting of combustion gases can be a big problem.

High-efficiency sealed-combustion units generally produce an acidic exhaust gas that is not suitable for old, unlined chimneys, so the exhaust gas should either be vented through a new duct or the chimney should be lined to accommodate the acidic gas (see the section on maintaining proper ventilation below).

Annual Estimated Savings for Every $100 of Fuel Costs by Increasing Your Heating Equipment Efficiency*

Current System AFUE
New/Upgraded System AFUE
  55% 60% 65% 70% 75% 80% 85% 90% 95%
50% $9.09 $16.76 $23.07 $28.57 $33.33 $37.50 $41.24 $44.24 $47.36
55% ---- $8.33 $15.38 $21.42 $26.66 $31.20 $35.29 $38.88 $42.10
60% ---- ---- $7.69 $14.28 $20.00 $25.00 $29.41 $33.33 $37.80
65% ---- ---- ---- $7.14 $13.33 $18.75 $23.52 $27.77 $31.57
70% ---- ---- ---- ---- $6.66 $12.50 $17.64 $22.22 $26.32
75% ---- ---- ---- ---- ---- $6.50 $11.76 $16.66 $21.10
80% ---- ---- ---- ---- ---- ---- $5.88 $11.11 $15.80
85% ---- ---- ---- ---- ---- ---- ---- $5.55 $10.50
90% ---- ---- ---- ---- ---- ---- ---- ---- $5.30

*Assuming the same heat output 

MAINTAINING FURNACES AND BOILERS 

The following maintenance should be provided by a heating system professional. 

All systems: 

  • Check the condition of your vent connection pipe and chimney. Parts of the venting system may have deteriorated over time. Chimney problems can be expensive to repair, and may help justify installing new heating equipment that won't use the existing chimney.
  • Check the physical integrity of the heat exchanger. Leaky boiler heat exchangers leak water and are easy to spot. Furnace heat exchangers mix combustion gases with house air when they leak—an important safety reason to have them inspected.
  • Adjust the controls on the boiler or furnace to provide optimum water and air temperature settings for both efficiency and comfort.
  • If you're considering replacing or retrofitting your existing heating system, have the technician perform a combustion-efficiency test. 

Forced Air Systems: 

  • Check the combustion chamber for cracks
  • Test for carbon monoxide (CO) and remedy if found
  • Adjust blower control and supply-air temperature
  • Clean and oil the blower
  • Remove dirt, soot, or corrosion from the furnace or boiler
  • Check fuel input and flame characteristics, and adjust if necessary
  • Seal connections between the furnace and main ducts. 

Hot Water Systems: 

  • Test pressure-relief valve
  • Test high-limit control
  • Inspect pressure tank, which should be filled with air, to verify that it's not filled with water
  • Clean the heat exchanger. 

Steam Systems: 

  • Drain some water from the boiler to remove sediments and improve the heat exchange efficiency
  • Test low-water cutoff safety control and high-limit safety control
  • Drain the float chamber to remove sediments, which will prevent the low-water cutoff control from sediment clogs
  • Analyze boiler water and add chemicals as needed to control deposits and corrosion
  • Clean the heat exchanger 

MAINTAINING PROPER VENTILATION FOR COMBUSTION SYSTEMS 

Anytime you maintain, retrofit, or replace a gas or oil heating system you need to be concerned with air quality. Combustion air is needed by all oil and gas heating systems to support the combustion process. This air is provided in some homes by unintentional air leaks, or by air ducts that connect to the outdoors. The combustion process creates several byproducts that are potentially hazardous to human health and can cause deterioration in your home. You can protect yourself from these hazards, as well as maintain energy efficiency, by ensuring that your chimney system functions properly and that your heating system is properly ventilated. In some cases, installing a sealed-combustion furnace or boiler can also help. 

CHIMNEYS 

Properly functioning chimney systems will carry combustion byproducts out of the home. Therefore, chimney problems put you at risk of having these byproducts, such as carbon monoxide, spill into your home. 

Most older furnaces and boilers have naturally drafting chimneys. The combustion gases exit the home through the chimney using only their buoyancy combined with the chimney's height. Naturally drafting chimneys often have problems exhausting the combustion gases because of chimney blockage, wind or pressures inside the home that overcome the buoyancy of the gases. 

Atmospheric, open-combustion furnaces and boilers, as well as fan-assisted furnaces and boilers, should be vented into masonry chimneys, metal double-wall chimneys, or another type of manufactured chimney. Masonry chimneys should have a fireclay, masonry liner or a retrofitted metal flue liner. 

Many older chimneys have deteriorated liners or no liners at all and must be relined during furnace or boiler replacement. A chimney should be relined when any of the following changes are made to the combustion heating system: 

  • When you replace an older furnace or boiler with a newer one that has an AFUE of 80% or more. These mid-efficiency appliances have a greater risk of depositing acidic condensation droplets in chimneys, and the chimneys must be prepared to handle this corrosive threat. The new chimney liner should be sized to accommodate both the new heating appliance and the combustion water heater (if present) by the installer.
  • When you replace an older furnace or boiler with a new 90+ AFUE appliance or a heat pump. In this case, the heating appliance will no longer vent into the old chimney, and if the combustion water heater is present it will now vent through an oversized chimney. This oversized chimney can lead to condensation and inadequate draft. The new chimney liner should be sized for the water heater alone, or the water heater in some cases can be vented directly through the wall. 

OTHER VENTILATION CONCERNS 

Some fan-assisted, non-condensing furnaces and boilers, installed between 1987 and 1993, may be vented horizontally through high-temperature plastic vent pipe (not PVC pipe, which is safely used in condensing furnaces). This type of venting has been recalled and should be replaced by stainless steel vent pipe. If horizontal venting was used, an additional draft-inducing fan may be needed near the vent outlet to create adequate draft. Floor furnaces may have special venting problems because their vent connector exits the furnace close to the floor and may travel 10 to 30 feet before reaching a chimney. Check to see if this type of venting or the floor furnace itself needs replacement. If you smell gases, you have a venting problem that could affect your health. Contact your local utility or heating contractor to have this venting problem repaired immediately.

Wood and Pellet Heating

Before the 20th century, 90% of Americans burned wood to heat their homes. As fossil fuel use rose, the percentage of Americans using wood for fuel dropped, falling as low as one percent by 1970. Then during the energy crises of the 1970s, interest in wood heating resurfaced as a renewable energy alternative.

Newer on the scene are pellet fuel appliances, which burn small pellets that look like rabbit feed and measure 3/8 to 1 inch in length. Pellets are made from compacted sawdust, wood chips, bark, agricultural crop waste, waste paper, and other organic materials. Some pellet fuel appliances can burn a wide variety of biomass fuels, including nutshells, corn kernels, small wood chips, barley, beet pulp, sunflowers, dried cherry pits, and soybeans.

  • Choosing and Installing Wood- and Pellet-Burning Appliances
  • Types of Wood- and Pellet-Burning Appliances
  • Chimney Placement and Sizing
  • Maintenance
  • Wood and Pellet Fuels

Choosing and Installing Wood- and Pellet-Burning Appliances

Today you can choose from a new generation of wood- and pellet-burning appliances that are cleaner burning, more efficient, and powerful enough to heat many average-sized, modern homes. It's also important to use a properly sized appliance for the space to be heated. When an appliance is too big, residents tend to burn fires at a low smolder to avoid overheating, which wastes fuel and is one of the biggest causes of air pollution. A reputable dealer should talk with you about size requirements, but a good rule-of-thumb is that a stove rated at 60,000 British Thermal Units (Btu) can heat a 2,000 square foot home, while a stove rated at 42,000 Btu can heat a 1,300 square foot space.

Wood-burning appliances and fireplaces may emit large quantities of air pollutants. Wood smoke contains hundreds of chemical compounds including nitrogen oxides, carbon monoxide, organic gases, and particulate matter, many of which have adverse health effects. In many urban and rural areas, smoke from wood burning is a major contributor to air pollution. Because of this, some municipalities restrict wood heating appliance use when the local air quality reaches unacceptable levels. Others restrict or ban the installation of wood-burning appliances in new construction. Before installing a wood-burning system, you should contact your local building codes department, state energy office, or state environmental agency about wood-burning regulations that may apply in your area.

If you have an older wood-burning appliance, consider upgrading to one of the newer appliances certified by the U.S. Environmental Protection Agency (EPA). They include a catalytic combustor that allows combustion gases to burn at lower temperatures, thereby cleaning the exhaust gas while generating more heat. All wood stoves sold today should bear an EPA certification sticker. High-efficiency appliances not only have lower emissions but they are also often safer, since complete combustion helps to prevent a buildup of flammable chimney deposits called creosote.

If you want to retrofit an existing non-catalytic wood-burning appliance with a catalytic combustor, you can buy a catalytic damper. These are available as kits and are usually installed in the flue collar. To monitor the stove temperature after adding a catalytic combustor, you should also install at least one heat sensor on the stove body or stove pipe. Several manufacturers sell retrofit kits, and they may be available from wood stove retailers. They are not appropriate for all types of stoves. Again, be sure to follow the manufacturer's installation and operating instructions.

The location of the appliance (and chimney) will influence how well heat is distributed and conserved in your home. Most wood- and pellet-burning appliances are essentially space heaters, and should be put in the room where you spend most of your time. Ideally, there should be a way for heat to circulate to the rest of the house.

For safety, and to maximize efficiency, you should consider having a professional install your wood- or pellet-burning appliance. A professional will carefully evaluate everything from your chimney to your floor protection. A certified professional can also help you choose the best appliance to heat your home.

Types of Wood- and Pellet-Burning Appliances

The following is a brief overview of the different types of appliances available.

High-efficiency fireplaces and fireplace inserts

Designed more for show, traditional open masonry fireplaces should not be considered heating devices. Traditional fireplaces draw in as much as 300 cubic feet per minute of heated room air for combustion, then send it straight up the chimney. Fireplaces also produce significant air pollution. Although some fireplace designs seek to address these issues with dedicated air supplies, glass doors, and heat recovery systems, fireplaces are still energy losers. When burning a fire, you should turn your heat down or off and open a window near the fireplace.

Only high-efficiency fireplace inserts have proven effective in increasing the heating efficiency of older fireplaces. Essentially, the inserts function like wood stoves, fitting into the masonry fireplace or on its hearth, and use the existing chimney. You must install a flue collar that continues from the insert to the top of the chimney. A well-fitted fireplace insert can function nearly as efficiently as a wood stove.

Studies have shown that proper installation of fireplace inserts is very important. Have a professional installer examine the fireplace and chimney to determine if they are suitable for an insert. Inserts should be as airtight as possible. The more airtight it is, the easier it is to control the fire and the heat output. The installer should use only approved fireplace insulating materials to fill any gaps between the fireplace mouth and insert shield.

Moving an insert to clean the chimney or liner can be difficult, and is a job best left to a professional chimney sweep. In some situations, a clean-out door can be installed above the insert connection so the insert does not have to be moved as often. Some models have wheels to simplify installation, cleaning, repairs, and other adjustments.

Some modern fireplaces heat at efficiencies near those of wood stoves and are certified as low emission appliances. Although designed to include the fire-viewing benefits of a traditional fireplace, this generation of fireplaces can effectively provide heat as well. Through vents under the firebox, room air is drawn in, heated through a heat exchanger, and sent back into the house either through vents at the top of the fireplace or through ducts leading to nearby rooms. Some of these fireplaces are approved to route heated air to a basement auxiliary fan. The air then travels through ducts to other rooms in the house. The fireplace should have a dedicated supply of outside air for combustion.

Flues are ideal for leaking heat and warm air out of your home. If you have a fireplace that you don't use, plug and seal the flue. If you use the fireplace, be sure to close the flue when the fireplace is not in use. You could also use an inflatable stopper, available commercially, to temporarily seal the chimney and avoid air leakage through the flue.

Catalytic Wood Stoves, Advanced Combustion Wood Stoves, and Centralized Wood-Burning Boilers

Wood stoves are the most common appliance for burning wood. New catalytic stoves and inserts have advertised efficiencies of 70%–80%.

Advanced combustion wood stoves provide a lot of heat but only work efficiently when the fire burns at full throttle. Also known as secondary burn stoves, they can reach temperatures of 1100°F—hot enough to burn combustible gases.

These stoves have several components that help them burn combustible gases, as well as particulates, before they can exit the chimney. Components include a metal channel that heats secondary air and feeds it into the stove above the fire. This heated oxygen helps burn the volatile gases above the flames without slowing down combustion. While many older stoves only have an air source below the wood, the secondary air source in advanced combustion stoves offers oxygen to the volatile gases escaping above the fire. With enough oxygen, the heated gases burn as well. In addition, the firebox is insulated, which reflects heat back to it, ensuring that the turbulent gases stay hot enough to burn. New advanced combustion stoves have advertised efficiencies of 60%–72%.

Another benefit is that the secondary channels funnel hot air toward the glass doors, keeping them clean for viewing the fire. They can also be slightly less expensive than conventional wood stoves fitted with catalytic combustors. Like wood stoves, centralized wood-burning boilers have been improved over the last several years. Modern, centralized wood heaters use wood gasification technology that burns both the wood fuel and the associated combustible gases, rendering them efficient up to 80%. In addition, systems are available that can switch to oil or gas if the fire goes out.

Masonry heaters

Masonry heaters are also known as "Russian," "Siberian," and "Finnish" fireplaces. They produce more heat and less pollution than any other wood- or pellet-burning appliance. Masonry heaters include a firebox, a large masonry mass (such as bricks), and long twisting smoke channels that run through the masonry mass. Their fireboxes are lined with firebrick, refractory concrete, or similar materials that can handle temperatures of over 2,000°F (1,093°C).

A small hot fire built once or twice a day releases heated gases into the long masonry heat tunnels. The masonry absorbs the heat and then slowly releases it into the house over a period of 12–20 hours. Masonry heaters commonly reach a combustion efficiency of 90%.

Most are intended for burning wood, but they were historically designed to burn almost any type of solid fuel. The relatively small, but intense fire also results in very little air pollution and very little creosote buildup in the chimney. Because most of the heat from the fuel is transferred to the masonry and slowly released into the room over the day, this type of heater does not need to be loaded with fuel as often as other types of wood heating appliances. In addition, if the masonry heater is built where sunlight can directly shine on it in the winter, the heater will absorb the sun's heat and release it slowly into the room.

A wide variety of masonry heater designs and styles are available. Larger models resemble conventional fireplaces and may cover an entire wall. Smaller models take up about as much space as a wood or pellet stove. They can be custom-built or purchased as prefabricated units. Some large designs may cost $5,000 or more. Plans and kits are available, but they are not easy do-it-yourself projects and require experience in working with masonry.

In addition to their expense, masonry heaters have one significant disadvantage when compared to conventional wood stoves and fireplaces: They cannot provide heat quickly from a cold start.

Pellet Fuel Appliances

Pellet fuel appliances burn small, 3/8–1 inch (100–254 millimeter [mm])-long pellets that look like rabbit feed. Pellets are made from compacted sawdust, wood chips, bark, agricultural crop waste, waste paper, and other organic materials. Some models can also burn nutshells, corn kernels, and small wood chips. They are more convenient to operate and have much higher combustion and heating efficiencies than ordinary wood stoves or fireplaces. As a consequence of this, they produce very little air pollution. In fact, pellet stoves are the cleanest of solid fuel-burning residential heating appliances. With combustion efficiencies of 78%–85%, they are also exempt from United States Environmental Protection Agency (EPA) smoke-emission testing requirements. Pellet stoves have heating capacities that range between 8,000 and 90,000 Btu per hour. They are suitable for homes as well as apartments or condominiums.

Most pellet stoves cost between $1,700 and $3,000. However, a pellet stove is often cheaper to install than a cordwood-burning heater. Many can be direct-vented and do not need an expensive chimney or flue. As a result, the installed cost of the entire system may be less than that of a conventional wood stove.

Pellet fuel appliances are available as freestanding stoves or fireplace inserts. Freestanding units resemble conventional cordwood heaters in that they generally heat a single room well, but not adjacent rooms unless you use a fan to force the warm air into those other spaces. There are also fireplace inserts that fit into existing fireplaces. Several companies now make pellet-fired furnaces and boilers for replacement of, or a supplement to, gas or oil fired furnaces and boilers in residential space heating systems.

All pellet fuel appliances have a fuel hopper to store the pellets until they are needed for burning. Most hoppers hold 35 and 130 pounds (16 and 60 kilograms [kg]) of fuel, which will last a day or more under normal operating conditions. A feeder device, like a large screw, drops a few pellets at a time into the combustion chamber for burning. How quickly pellets are fed to the burner determines the heat output. The exhaust gases are vented by way of a small flue pipe that can be directed out a side wall or upwards through the roof. More advanced models have a small computer and thermostat to govern the pellet feed rate.

Pellet appliances usually require refueling only once a day, and since the fuel is compressed and bagged, the operator does not have to lift heavy, dirty logs. Most pellet appliance exteriors (except glass doors) stay relatively cool while operating, reducing the risk of accidental burns. Since pellet stoves burn fuel so completely, very little creosote builds up in the flue, posing less of a fire hazard.

Unfortunately, pellet appliances are also more complex and have expensive components that can break down. They also require electricity to run fans, controls, and pellet feeders. Under normal usage, they consume about 100 kilowatt-hours (kWh) or about $9 worth of electricity per month. Unless the stove has a back-up power supply, the loss of electric power results in no heat and possibly some smoke in the house.

Chimney Placement and Sizing

Chimneys harness the heat of the fire to create what's called a stack effect. As the warm air from the fire rises, cooler house air rushes into the wood-burning appliance through vents, providing the oxygen the fire needs to burn. Starting a fire with a good hot burn will encourage this healthy draft to flow. Also, between the higher and lower pressure zones of the home lies a neutral pressure zone. The neutral pressure zone tends to move toward the largest air leak. When the top of the chimney is located above the home ceiling (as it should be), the chimney's neutral pressure zone is above the neutral pressure zone of the house. Such proper chimney placement creates a gentle flow of air into the appliance and out the chimney even when no fire burns.

If you are designing or building a new home, consider placing the chimney inside your home. A more traditional chimney, constructed along the outside of a home, will lose valuable heat to the cold, outside air. If the chimney air temperature falls below that of the inside air, the cold, smelly chimney air will be pulled into the house by the low pressure of the stack effect. In such a scenario, the house has become a better chimney than the chimney. So when a fire is lit, smoke fills the room.

Chimneys must match the size of the appliance, meaning the flue size should match the stove outlet. If the chimney is bigger than the stove or fireplace outlet, exiting exhaust slows, increasing creosote buildup and decreasing efficiency. High-performance chimneys are also insulated. Older masonry chimneys can be relined to safely and efficiently connect them to newer high-efficiency, wood-burning appliances. Again, the chimney liner should be continuous from the appliance outlet to the chimney top. It is not uncommon to pay as much for the chimney as for your appliance.

Free-standing woodstoves exhaust into a connecting pipe, which then connects into the chimney. If the connecting pipe is longer than 8 feet (as in a vaulted ceiling), you should consider investing in double-layer pipe with 1-inch airspace between pipe layers. Efficient modern stoves produce large amounts of heat. Much of this heat can radiate from a longer length of single-layer pipe, slowing down the draft, which can impact the overall efficiency of your wood-burning system.

Maintenance

To keep your wood- or pellet-burning system operating efficiently and safely, you'll need to maintain it on a regular basis.

Every year, preferably before each heating season, have a chimney sweep certified by the Chimney Safety Institute of America inspect your wood-burning system. In addition to cleaning the chimney, a certified chimney sweep should have the knowledge to help make sure your appliance, hearth, connecting pipe, air inlets, chimney, and all other components are functioning efficiently and safely.

Catalytic combustors need to be inspected at least three times every heating season and replaced according to the manufacturer's recommendations. Most catalytic stoves or inserts have a view window or thermometer to help you check the combustor. The catalytic cell is removable and replaceable and costs between $75 and $160.

Cleaning out the inside of the appliance with a wire brush periodically will also help your wood-burning appliance heat your home efficiently. Even a one-tenth inch of soot can drop the heat transfer efficiency of the metal by 50%.

For pellet-fuel appliances, it is very important to follow the manufacturer's instructions for operation and maintenance. Inspect fans and motors regularly, and maintain them properly. Manufacturers advise removing unused pellets from the stove hopper and feed system at the end of the heating season. This reduces the chance of rusting, which can cause expensive damage to the appliance. It also minimizes difficulties in lighting the appliance at the start of the next heating season. Clean the flue vent on a regular basis to prevent soot building up.

Wood and Pellet Fuels

Selecting and Storing Wood

Because a lot of energy can be wasted burning wet wood, you should use wood that has been properly seasoned. Properly seasoned wood is harvested in the spring and allowed to dry throughout the summer. Look for wood that is of even color, without any green. It should have a moisture content of just over 20%–25% by weight. Some well-seasoned wood can in fact be too dry for today's airtight modern stoves. If you place wood that is too dry on a bed of coals, it will instantly give up its gases as smoke, wasting unburned smoke and producing creosote buildup.

All species of wood have a similar heat (Btu) content on a per pound basis when completely dry. Therefore, denser woods will generally cost more and burn longer. Woods like oak, hickory, and pine will burn overnight. Aspen builds a hot fire, which helps clean the chimney.

When selecting wood, you might also want to find out whether the supplier uses sustainable harvesting practices. Unsustainable practices can negatively impact the environment, causing soil erosion and loss of biopersity. At least ascertain that the wood was not the result of clear-cutting. Clear-cutting is when all, or nearly all, of the trees are cut down on a piece of land.

Store your wood away from the house in case termites discover the woodpile. The top of the pile should be covered, but leave the sides open so air can circulate. If possible, store the wood a foot off the ground (on concrete blocks, for example) to keep it dry.

Pellet Fuel

Pellet fuel is normally sold in 40 pound (18 kg) bags at about $3–$4 each, or about $120–$200 a ton. You can estimate how much fuel you will need for a heating season by noting that one ton of pellets is equivalent to approximately 1.5 cords of firewood. Most homeowners who use a pellet appliance as a main source of heat use two to three tons of pellet fuel per year. Pellet fuel appliances are often less expensive to operate than electric resistance heating and propane-fueled appliances.

Most pellet fuels have a 5%–10% moisture content. Well-seasoned firewood is usually around 20%. Some pellets contain either petroleum or non-petroleum lignin used as a lubricant in the pellet production process, though most contain no additives. Pellets made from agricultural waste contain more ash, but they may produce more heat than pellets made from wood.

The Pellet Fuels Institute (PFI) maintains National Residential Pellet Fuel Standards, although fuel quality certification is the responsibility of the pellet manufacturer. Under the standards, there are two pellet fuel grades: premium and standard. The only difference between grades is in the inorganic ash content: premium should be less than 1%, and standard less than 3%. Premium is usually made of core wood (not bark). There are five fuel characteristics prescribed for both grades:

  • Bulk density per cubic foot (0.028 cubic meters) shall not be less than 40 pounds (18 kg)
  • The diameter shall be 1/4 to 5/16 inch (635-794 mm)
  • Maximum length shall be 1 and 1/2 inches (254-127 mm)
  • Fines (dust) of not more than 0.5% by weight shall pass through a 1/8 inch (317.5 mm) screen
  • Sodium content shall be less than 300 parts per million (ppm).

You can check pellet fuel quality by inspecting the bag for excessive dirt and dust. (Dirt can form clinkers in the stove.) There should be less than one half of a cup of dust at the bottom of a 40 pound (18 kg) bag. Pellet stoves designed for low-ash (typically top-fed stoves) tend to operate poorly when used with pellets of a higher ash content. Many pellet appliance manufacturers are redesigning their products to burn pellets with varying ash contents.

Although pellet fuel availability is increasing, you should be sure there is a reliable pellet fuel supplier in your area before purchasing a pellet stove. It is also important to know the type of pellet fuel available before you shop for an appliance. Most pellet fuel appliance dealers either maintain a supply of pellets or recommend a supplier. You may also check the local telephone listings under "Fuel" or "Pellet Fuel," or inquire at a local tree nursery, or at home and garden supply stores.

Electric Resistance Heating

Electric resistance heating converts nearly 100% of the energy in the electricity to heat. However, most electricity is produced from coal, gas, or oil generators that convert only about 30% of the fuel's energy into electricity. Because of electricity generation and transmission losses, electric heat is often more expensive than heat produced in the home or business using combustion appliances, such as natural gas, propane, and oil furnaces.

If electricity is the only choice, heat pumps are preferable in most climates, as they easily cut electricity use by 50% when compared with electric resistance heating. The exception is in dry climates with either hot or mixed (hot and cold) temperatures (these climates are found in the non-coastal, non-mountainous part of California; the southern tip of Nevada; the southwest corner of Utah; southern and western Arizona; southern and eastern New Mexico; the southeast corner of Colorado; and western Texas). For these dry climates, there are so few heating days that the high cost of heating is not economically significant.

Electric resistance heating may also make sense for a home addition if it is not practical to extend the existing heating system to supply heat to the new addition.

TYPES OF ELECTRIC RESISTANCE HEATERS

Electric resistance heat can be supplied by centralized forced-air electric furnaces or by heaters in each room. Room heaters can consist of electric baseboard heaters, electric wall heaters, electric radiant heat, or electric space heaters. To learn about electric radiant heat and electric space heaters, see radiant heating and small space heaters. It is also possible to use electric thermal storage systems to avoid heating during times of peak power demand.

ELECTRIC FURNACES

Electric furnaces are more expensive to operate than other electric resistance systems because of their duct heat losses and the extra energy required to distribute the heated air throughout your home (which is common for any heating system that uses ducts for distribution). Heated air is delivered throughout the home through supply ducts and returned to the furnace through return ducts. If these ducts run through unheated areas, they lose some of their heat through air leakage as well as heat radiation and convection from the duct's surface.

Blowers (large fans) in electric furnaces move air over a group of three to seven electric resistance coils, called elements, each of which are typically rated at five kilowatts. The furnace's heating elements activate in stages to avoid overloading the home's electrical system. A built-in thermostat called a limit controller prevents overheating. This limit controller may shut the furnace off if the blower fails or if a dirty filter is blocking the airflow.

As with any furnace, it's important to clean or replace the furnace filters as recommended by the manufacturer, in order to keep the system operating at top efficiency.

ELECTRIC BASEBOARD HEATERS

Electric baseboard heaters are zonal heaters controlled by thermostats located within each room. Baseboard heaters contain electric heating elements encased in metal pipes. The pipes, surrounded by aluminum fins to aid heat transfer, run the length of the baseboard heater's housing, or cabinet. As air within the heater is warmed, it rises into the room, and cooler air is drawn into the bottom of the heater. Some heat is also radiated from the pipe, fins, and housing.

Baseboard heaters are usually installed underneath windows. There, the heater's rising warm air counteracts falling cool air from the cold window glass. Baseboard heaters are seldom located on interior walls because standard heating practice is to supply heat at the home's perimeter, where the greatest heat loss occurs.

Baseboard heaters should sit at least three-quarters of an inch (1.9 centimeters) above the floor or carpet. This is to allow the cooler air on the floor to flow under and through the radiator fins so it can be heated. The heater should also fit tightly to the wall to prevent the warm air from convecting behind it and streaking the wall with dust particles.

The quality of baseboard heaters varies considerably. Cheaper models can be noisy and often give poor temperature control. Look for labels from Underwriter's Laboratories (UL) and the National Electrical Manufacturer's Association (NEMA). Compare warranties of the different models you are considering.

ELECTRIC WALL HEATERS

Electric wall heaters consist of an electric element with a reflector behind it to reflect heat into the room and usually a fan to move air through the heater. They are usually installed on interior walls because installing them in an exterior wall makes that wall difficult to insulate.

ELECTRIC THERMAL STORAGE

Some electric utilities structure their rates in a way similar to telephone companies and charge more for electricity during the day and less at night. They do this in an attempt to reduce their "peak" demand.

If you are a customer of such a utility, you may be able to benefit from a heating system that stores electric heat during nighttime hours when rates are lower. This is called an electric thermal storage heater, and while it does not save energy, it can save you money because you can take advantage of these lower rates.

The most common type of electric thermal storage heater is a resistance heater with elements encased in heat-storing ceramic. Central furnaces incorporating ceramic block are also available, although they are not as common as room heaters. Storing electrically heated hot water in an insulated storage tank is another thermal storage option.

Some storage systems attempt to use the ground underneath homes for thermal storage of heat from electric resistance cables. However, this requires painstaking installation of insulation underneath concrete slabs and all around the heating elements to minimize major heat losses to the earth. Ground storage also makes it difficult for thermostats to control indoor temperatures.

Any type of energy storage systems suffers some energy loss. If you intend to pursue an electric thermal storage system, it would be best for the system to be located within the conditioned space of your home, so that any heat lost from the system actually heats your home, rather than escaping to the outdoors. It would also be best to know how quickly heat will escape from the system. A system that leaks too much heat could cause control problems, such as the accidental overheating of your home.

CONTROL SYSTEMS

All types of electric resistance heating are controlled through some type of thermostat. Baseboard heaters often use a line-voltage thermostat (the thermostat directly controls the power supplied to the heating device), while other devices use low-voltage thermostats (the thermostat uses a relay to turn the device on and off). Line-voltage thermostats can be built into the baseboard heater, but then they often don't sense the room temperature accurately. It's best to instead use a remote line-voltage or low-voltage thermostat installed on an interior wall. Both line-voltage and low-voltage thermostats are available as programmable thermostats for automatically setting back the temperature at night or while you're away.

Baseboard heaters supply heat to each room individually, so they are ideally suited to zone heating, which involves heating the occupied rooms in your home while allowing unoccupied area (such as empty guest rooms or seldom-used rooms) to remain cooler. Zone heating can produce energy savings of more than 20% compared to heating both occupied and unoccupied areas of your house.

Zone heating is most effective when the cooler portions of your home are insulated from the heated portions, allowing the different zones to truly operate independently. Note that the cooler parts of your home still need to be heated to well above freezing to avoid freezing pipes.

If electricity is the only choice, heat pumps are preferable in most climates, as they easily cut electricity use by 50% when compared with electric resistance heating. The exception is in dry climates with either hot or mixed (hot and cold) temperatures (these climates are found in the non-coastal part of California; the southern tip of Nevada; the southwest corner of Utah; southern and western Arizona; southern and eastern New Mexico; the southeast corner of Colorado; and western Texas). For these dry climates, there are so few heating days that the high cost of heating is not economically significant.

Electric resistance heating may also make sense for a home addition if it is not practical to extend the existing heating system to supply heat to the new addition.

  • Types of Electric Resistance Heaters
  • Control Systems

Types of Electric Resistance Heaters

Electric resistance heat can be supplied by centralized forced-air electric furnaces or by heaters in each room. Room heaters can consist of electric baseboard heaters, electric wall heaters, electric radiant heat, or electric space heaters. To learn about electric radiant heat and electric space heaters, see the radiant heating and small space heaters sections. It is also possible to use electric thermal storage systems to avoid heating during times of peak power demand.

Electric Furnaces

Electric furnaces are more expensive to operate than other electric resistance systems because of their duct heat losses and the extra energy required to distribute the heated air throughout your home. Heated air is delivered throughout the home through supply ducts and returned to the furnace through return ducts. If these ducts run through unheated areas, they lose some of their heat through air leakage as well as heat radiation and convection from the duct's surface.

Blowers (large fans) in electric furnaces move air over a group of three to seven electric resistance coils, called elements, each of which are typically rated at five kilowatts. The furnace's heating elements activate in stages to avoid overloading the home's electrical system. A built-in thermostat called a limit controller prevents overheating. This limit controller may shut the furnace off if the blower fails or if a dirty filter is blocking the airflow.

As with any furnace, it's important to clean or replace the furnace filters as recommended by the manufacturer, in order to keep the system operating at its top efficiency.

Electric Baseboard Heaters

Electric baseboard heaters are zonal heaters controlled by thermostats located within each room. Baseboard heaters contain electric heating elements encased in metal pipes. The pipes, surrounded by aluminum fins to aid heat transfer, run the length of the baseboard heater's housing, or cabinet. As air within the heater is warmed, it rises into the room, and cooler air is drawn into the bottom of the heater. Some heat is also radiated from the pipe, fins, and housing.

Baseboard heaters are usually installed underneath windows. There, the heater's rising warm air counteracts falling cool air from the cold window glass. Baseboard heaters are seldom located on interior walls because standard heating practice is to supply heat at the home's perimeter, where the greatest heat loss occurs.

Baseboard heaters should sit at least three-quarters of an inch (1.9 centimeters) above the floor or carpet. This is to allow the cooler air on the floor to flow under and through the radiator fins so it can be heated. The heater should also fit tightly to the wall to prevent the warm air from convecting behind it and streaking the wall with dust particles.

The quality of baseboard heaters varies considerably. Cheaper models can be noisy and often give poor temperature control. Look for labels from Underwriter's Laboratories (UL) and the National Electrical Manufacturer's Association (NEMA). Compare warranties of the different models you are considering.

Electric Wall Heaters

Electric wall heaters consist of an electric element with a reflector behind it to reflect heat into the room and usually a fan to move air through the heater. They are usually installed on interior walls because installing them in an exterior wall makes that wall difficult to insulate.

Electric Thermal Storage

Some electric utilities structure their rates in a way similar to telephone companies and charge more for electricity during the day and less at night. They do this in an attempt to reduce their "peak" demand.

If you are a customer of such a utility, you may be able to benefit from a heating system that stores electric heat during nighttime hours when rates are lower. This is called an electric thermal storage heater, and while it does not save energy, it can save you money because you can take advantage of these lower rates.

The most common type of electric thermal storage heater is a resistance heater with elements encased in heat-storing ceramic. Central furnaces incorporating ceramic block are also available, although they are not as common as room heaters. Storing electrically heated hot water in an insulated storage tank is another thermal storage option.

Some storage systems attempt to use the ground underneath homes for thermal storage of heat from electric resistance cables. However, this requires painstaking installation of insulation underneath concrete slabs and all around the heating elements to minimize major heat losses to the earth. Ground storage also makes it difficult for thermostats to control indoor temperatures.

Any type of energy storage systems suffers some energy loss. If you intend to pursue an electric thermal storage system, it would be best for the system to be located within the conditioned space of your home, so that any heat lost from the system actually heats your home, rather than escaping to the outdoors. It would also be best to know how quickly heat will escape from the system. A system that leaks too much heat could cause control problems, such as the accidental overheating of your home.

Control Systems

All types of electric resistance heating are controlled through some type of thermostat: baseboard heaters often use a line-voltage thermostat (the thermostat directly controls the power supplied to the heating device), while other devices use low-voltage thermostats (the thermostat uses a relay to turn the device on and off). Line-voltage thermostats can be built into the baseboard heater, but then they often don't sense the room temperature accurately. It's best to instead use a remote line-voltage or low-voltage thermostat installed on an interior wall. Both line-voltage and low-voltage thermostats are available as programmable thermostats for automatically setting back the temperature at night or while you're away.

Since baseboard heaters supply heat to each room individually, they are ideally suited to zone heating, which involves heating the occupied rooms in your home while allowing unoccupied sections (such as empty guest rooms or seldom-used rooms) to remain cooler. Zone heating can produce energy savings of more than 20% compared to heating both occupied and unoccupied areas of your house.

Zone heating is most effective when the cooler portions of your home are insulated from the heated portions, allowing the different zones to truly operate independently. Note that the cooler parts of your home still need to be heated to well above freezing to avoid freezing pipes.

Active Solar Heating

Active solar heating systems use solar energy to heat a fluid -- either liquid or air -- and then transfer the solar heat directly to the interior space or to a storage system for later use. If the solar system cannot provide adequate space heating, an auxiliary or back-up system provides the additional heat. Liquid systems are more often used when storage is included, and are well suited for radiant heating systems, boilers with hot water radiators, and even absorption heat pumps and coolers. Both liquid and air systems can supplement forced air systems.

LIQUID-BASED ACTIVE SOLAR HEATING

Solar liquid collectors are most appropriate for central heating. They are the same as those used in solar domestic water heating systems. Flat-plate collectors are the most common, but evacuated tube and concentrating collectors are also available. In the collector, a heat transfer or "working" fluid such as water, antifreeze (usually non-toxic propylene glycol), or other type of liquid absorbs the solar heat. At the appropriate time, a controller operates a circulating pump to move the fluid through the collector.

The liquid flows rapidly, so its temperature only increases 10° to 20°F (5.6° to 11°C ) as it moves through the collector. Heating a smaller volume of liquid to a higher temperature increases heat loss from the collector and decreases the efficiency of the system. The liquid flows to either a storage tank or a heat exchanger for immediate use. Other system components include piping, pumps, valves, an expansion tank, a heat exchanger, a storage tank, and controls.

The flow rate depends on the heat transfer fluid. To learn more about types of liquid solar collectors, their sizing, maintenance, and other issues.

STORING HEAT IN LIQUID SYSTEMS

Liquid systems store solar heat in tanks of water or in the masonry mass of a radiant slab system. In tank type storage systems, heat from the working fluid transfers to a distribution fluid in a heat exchanger exterior to or within the tank.

Tanks are pressurized or unpressurized, depending on overall system design. Before choosing a storage tank, consider cost, size, durability, where to place it (in the basement or outside), and how to install it. You may need to construct a tank on-site if a tank of the necessary size will not fit through existing doorways. Tanks also have limits for temperature and pressure, and must meet local building, plumbing, and mechanical codes. You should also note how much insulation is necessary to prevent excessive heat loss, and what kind of protective coating or sealing is necessary to avoid corrosion or leaks.

Specialty or custom tanks may be necessary in systems with very large storage requirements. They are usually stainless steel, fiberglass, or high temperature plastic. Concrete and wood (hot tub) tanks are also options. Each type of tank has its advantages and disadvantages, and all types require careful placement because of their size and weight. It may be more practical to use several smaller tanks rather than one large one. The simplest storage system option is to use standard domestic water heaters. They meet building codes for pressure vessel requirements, are lined to inhibit corrosion, and are easy to install.

DISTRIBUTING HEAT FOR LIQUID SYSTEMS

You can use a radiant floor, hot water baseboards or radiators, or a central forced-air system to distribute the solar heat. In a radiant floor system, solar-heated liquid circulates through pipes embedded in a thin concrete slab floor, which then radiates heat to the room. Radiant floor heating is ideal for liquid solar systems because it performs well at relatively low temperatures. A carefully designed system may not need a separate heat storage tank, although most systems include them for temperature control. A conventional boiler or even a standard domestic water heater can supply back-up heat. The slab is typically finished with tile. Radiant slab systems take longer to heat the home from a "cold start" than other types of heat distribution systems. Once they are operating, however, they provide a consistent level of heat. Carpeting and rugs will reduce the system's effectiveness.

Hot-water baseboards and radiators require water between 160° and 180°F (71° and 82°C) to effectively heat a room. Generally, flat-plate liquid collectors heat the transfer and distribution fluids to between 90° and 120°F (32° and 49°C). Therefore, using baseboards or radiators with a solar heating system requires that the surface area of the baseboard or radiators be larger, temperature of the solar-heated liquid be increased by the backup system, or a medium-temperature solar collector (such as an evacuated tube collector) be substituted for a flat-plate collector.

There are several options for incorporating a liquid system into a forced-air heating system. The basic design is to place a liquid-to-air heat exchanger, or heating coil, in the main room-air return duct before it reaches the furnace. Air returning from the living space is heated as it passes over the solar heated liquid in the heat exchanger. Additional heat is supplied as necessary by the furnace. The coil must be large enough to transfer sufficient heat to the air at the lowest operating temperature of the collector.

VENTILATION PREHEATING

Solar air heating systems use air as the working fluid for absorbing and transferring solar energy. Solar air collectors can directly heat individual rooms or can potentially pre-heat the air passing into a heat recovery ventilator or through the air coil of an air-source heat pump.

Air collectors produce heat earlier and later in the day than liquid systems, so they may produce more usable energy over a heating season than a liquid system of the same size. Also, unlike liquid systems, air systems do not freeze, and minor leaks in the collector or distribution ducts will not cause significant problems, although they will degrade performance. However, air is a less efficient heat transfer medium than liquid, so solar air collectors operate at lower efficiencies than solar liquid collectors.

Although some early systems passed solar-heated air through a bed of rocks as energy storage, this approach is not recommended because of the inefficiencies involved, the potential problems with condensation and mold in the rock bed, and the effects of that moisture and mold on indoor air quality.

Solar air collectors are often integrated into walls or roofs to hide their appearance. For instance, a tile roof could have air flow paths built into it to make use of the heat absorbed by the tiles.

Most solar air heating systems are room air heaters, but relatively new devices called transpired air collectors have limited applications in homes.

ROOM AIR HEATERS

Air collectors can be installed on a roof or an exterior (south-facing) wall for heating one or more rooms. Although factory-built collectors for on-site installation are available, do-it-yourselfers may choose to build and install their own air collector. A simple window air heater collector can be made for a few hundred dollars.

The collector has an airtight and insulated metal frame and a black metal plate for absorbing heat with glazing in front of it. Solar radiation heats the plate that, in turn, heats the air in the collector. An electric fan or blower pulls air from the room through the collector, and blows it back into the room. Roof-mounted collectors require ducts to carry air between the room and the collector. Wall-mounted collectors are placed directly on a south-facing wall, and holes are cut through the wall for the collector air inlet and outlets.

Simple "window box collectors" fit in an existing window opening. They can be active (using a fan) or passive. In passive types, air enters the bottom of the collector, rises as it is heated, and enters the room. A baffle or damper keeps the room air from flowing back into the panel (reverse thermosiphoning) when the sun is not shining. These systems only provide a small amount of heat, because the collector area is relatively small.

TRANSPIRED AIR COLLECTORS

Transpired air collectors use a simple technology to capture the sun's heat to warm buildings. The collectors consist of dark, perforated metal plates installed over a building's south-facing wall. An air space is created between the old wall and the new facade. The dark outer facade absorbs solar energy and rapidly heats up on sunny days—even when the outside air is cold.

A fan or blower draws ventilation air into the building through tiny holes in the collectors and up through the air space between the collectors and the south wall. The solar energy absorbed by the collectors warms the air flowing through them by as much as 40°F. Unlike other space heating technologies, transpired air collectors require no expensive glazing.

Transpired air collectors are most suitable for large buildings with high ventilation loads, a fact which makes them generally unsuitable for today's tightly sealed homes. However, small transpired air collectors could be used to pre-heat the air passing into a heat recovery ventilator or could warm the air coil on an air source heat pump, improving its efficiency and comfort level on cold days. No information is currently available on the cost effectiveness of using a transpired air collector in this way, however.

ECONOMICS AND OTHER BENEFITS OF ACTIVE SOLAR HEATING SYSTEMS

Active solar heating systems are most cost-effective in cold climates with good solar resources when they are displacing the more expensive heating fuels, such as electricity, propane, and oil. Some states offer sales tax exemptions, income tax credits or deductions, and property tax exemptions or deductions for solar energy systems.

The cost of an active solar heating system will vary. Commercially available collectors come with warranties of 10 years or more, and should easily last decades longer. The economics of an active space heating system improve if it also heats domestic water, because an otherwise idle collector can heat water in the summer.

Heating your home with an active solar energy system can significantly reduce your fuel bills in the winter. A solar heating system will also reduce the amount of air pollution and greenhouse gases that result from your use of fossil fuels for heating or generating the electricity.

SELECTING AND SIZING A SOLAR HEATING SYSTEM

Selecting the appropriate solar energy system depends on factors such as the site, design, and heating needs of your house. Local covenants may restrict your options; for example homeowner associations may not allow you to install solar collectors on certain parts of your house (although many homeowners have been successful in challenging such covenants).

The local climate, the type and efficiency of the collector(s), and the collector area determine how much heat a solar heating system can provide. It is usually most economical to design an active system to provide 40% to 80% of the home's heating needs. Systems providing less than 40% of a home’s heat are rarely cost-effective except when using solar air heater collectors that heat one or two rooms and require no heat storage. A well-designed and insulated home that incorporates passive solar heating techniques will require a smaller and less costly heating system of any type, and may need very little supplemental heat other than solar.

Besides the fact that designing an active system to supply enough heat 100% of the time is generally not practical or cost-effective, most building codes and mortgage lenders require a back-up heating system. Supplementary or back-up systems supply heat when the solar system cannot meet heating requirements. Backups can range from a wood stove to a conventional central heating system.

CONTROLS FOR SOLAR HEATING SYSTEMS

Controls for solar heating systems are usually more complex than those of a conventional heating system, because they have to analyze more signals and control more devices (including the conventional back-up heating system). Solar controls use sensors, switches, and/or motors to operate the system. The system uses other controls to prevent freezing or extremely high temperatures in the collectors.

The heart of the control system is a differential thermostat, which measures the difference in temperature between the collectors and storage unit. When the collectors are 10° to 20°F (5.6° to 11°C) warmer than the storage unit, the thermostat turns on a pump or fan to circulate water or air through the collector to heat the storage medium or the house.

The operation, performance, and cost of these controls vary. Some control systems monitor the temperature in different parts of the system to help determine how it is operating. The most sophisticated systems use microprocessors to control and optimize heat transfer and delivery to storage and zones of the house.

It is possible to use a solar panel to power low voltage, direct current (DC) blowers (for air collectors) or pumps (for liquid collectors). The output of the solar panels matches available solar heat gain to the solar collector. With careful sizing, the blower or pump speed is optimized for efficient solar gain to the working fluid. During low sun conditions the blower or pump speed is slow, and during high solar gain, it runs faster.

When used with a room air collector, separate controls may not be necessary. This also ensures that the system will operate in the event of utility power outage. A solar power system with battery storage can also provide power to operate a central heating system, though this is expensive for large systems.

BUILDING CODES, COVENANTS, AND REGULATIONS FOR SOLAR HEATING SYSTEMS

Before installing a solar energy system, you should investigate local building codes, zoning ordinances, and subdivision covenants, as well as any special regulations pertaining to the site. You will probably need a building permit to install a solar energy system on an existing building.

Not every community or municipality initially welcomes residential renewable energy installations. Although this is often due to ignorance or the comparative novelty of renewable energy systems, you must comply with existing building and permit procedures to install your system.

The matter of building code and zoning compliance for a solar system installation is typically a local issue. Even if a statewide building code is in effect, your city, county, or parish usually enforces it. Common problems homeowners have encountered with building codes include the following:

  • Exceeding roof load
  • Unacceptable heat exchangers
  • Improper wiring
  • Unlawful tampering with potable water supplies.

Potential zoning issues include these:

  • Obstructing side yards
  • Erecting unlawful protrusions on roofs
  • Siting the system too close to streets or lot boundaries.

Special area regulations—such as local community, subdivision, or homeowner's association covenants—also require compliance. These covenants, historic district regulations, and flood-plain provisions can easily be overlooked. To find out what's needed for local compliance, contact your local jurisdiction's zoning and building enforcement divisions and any appropriate homeowner, subdivision, neighborhood, and/or community association(s).

INSTALLING AND MAINTAINING YOUR SOLAR HEATING SYSTEM

How well an active solar energy system performs depends on effective siting, system design, and installation as well as the quality and durability of the components. Today’s collectors and controls are high quality, but it can still be a challenge finding an experienced contractor who can properly design and install the system.

Once a system is in place, it has to be properly maintained to optimize its performance and avoid breakdowns. Different systems require different types of maintenance, and you should set up a calendar listing the maintenance tasks that the component manufacturers and installer recommends for your installation.

Most solar water heaters are automatically covered under your homeowner's insurance policy. However, damage from freezing is generally not. Contact your insurance provider to find out what its policy is. Even if your provider will cover your system, it is best to inform them in writing that you own a new system.

 

Radiant Heating

Radiant heating systems supply heat directly to the floor or to panels in the wall or ceiling of a house. The systems depend largely on radiant heat transfer -- the delivery of heat directly from the hot surface to the people and objects in the room via infrared radiation. Radiant heating is the effect you feel when you can feel the warmth of a hot stovetop element from across the room. When radiant heating is located in the floor, it is often called radiant floor heating or simply floor heating.

Radiant heating has a number of advantages. It is more efficient than baseboard heating and usually more efficient than forced-air heating because it eliminates duct losses. People with allergies often prefer radiant heat because it doesn’t distribute allergens like forced air systems can. Hydronic (liquid-based) systems use little electricity, a benefit for homes off the power grid or in areas with high electricity prices. Hydronic systems can use a wide variety of energy sources to heat the liquid, including standard gas- or oil-fired boilers, wood-fired boilers, solar water heaters, or a combination of these sources.

Despite its name, radiant floor heating depends heavily on convection, the natural circulation of heat within a room as air warmed by the floor rises. Radiant floor heating systems are significantly different from the radiant panels used in walls and ceilings. For this reason, the following sections discuss radiant floor heat and radiant panels separately.

RADIANT FLOOR HEAT

There are three types of radiant floor heat -- radiant air floors (air is the heat-carrying medium), electric radiant floors, and hot water (hydronic) radiant floors. You can further categorize these types by installation. Those that make use of the large thermal mass of a concrete slab floor or lightweight concrete over a wooden subfloor are called "wet installations,” and those in which the installer "sandwiches" the radiant floor tubing between two layers of plywood or attaches the tubing under the finished floor or subfloor are called "dry installations."

TYPES OF RADIANT FLOOR HEAT

AIR-HEATED RADIANT FLOORS

Air cannot hold large amounts of heat, so radiant air floors are not cost-effective in residential applications, and are seldom installed. Although they can be combined with solar air heating systems, those systems suffer from the obvious drawback of only producing heat in the daytime, when heating loads are generally lower. The inefficiency of trying to heat a home with a conventional furnace by pumping air through the floors at night outweighs the benefits of using solar heat during the day. Although some early solar air heating systems used rocks as a heat-storage medium, this approach is not recommended.

ELECTRIC RADIANT FLOORS

Electric radiant floors typically consist of electric cables built into the floor. Systems that feature mats of electrically conductive plastic mounted on the subfloor below a floor covering such as tile are also available.

Because of the relatively high cost of electricity, electric radiant floors are usually only cost-effective if they include a significant thermal mass such as a thick concrete floor and your electric utility company offers time-of-use rates. Time-of-use rates allow you to "charge" the concrete floor with heat during off-peak hours (approximately 9 p.m. to 6 a.m.). If the floor's thermal mass is large enough, the heat stored in it will keep the house comfortable for eight to ten hours without any further electrical input, particularly when daytime temperatures are significantly warmer than nighttime temperatures. This saves a considerable number of energy dollars compared to heating at peak electric rates during the day.

Electric radiant floors may also make sense for home additions if it would be impractical to extend the heating system into the new space. However, homeowners should examine other options, such as mini-split heat pumps, which operate more efficiently and have the added advantage of providing cooling.

HYDRONIC RADIANT FLOORS

Hydronic (liquid) systems are the most popular and cost-effective radiant heating systems for heating-dominated climates. Hydronic radiant floor systems pump heated water from a boiler through tubing laid in a pattern under the floor. In some systems, controlling the flow of hot water through each tubing loop by using zoning valves or pumps and thermostats regulates room temperatures. The cost of installing a hydronic radiant floor varies by location and depends on the size of the home, the type of installation, the floor covering, remoteness of the site, and the cost of labor.

TYPES OF FLOOR INSTALLATIONS

Whether you use cables or tubing, the methods of installing electric and hydronic radiant systems in floors are similar.

So-called "wet" installations embed the cables or tubing in a solid floor and are the oldest form of modern radiant floor systems. The tubing or cable can be embedded in a thick concrete foundation slab (commonly used in "slab" ranch houses that don't have basements) or in a thin layer of concrete, gypsum, or other material installed on top of a subfloor. If concrete is used and the new floor is not on solid earth, additional floor support may be necessary because of the added weight. You should consult a professional engineer to determine the floor's carrying capacity.

Thick concrete slabs are ideal for storing heat from solar energy systems, which have a fluctuating heat output. The downside of thick slabs is their slow thermal response time, which makes strategies such as night or daytime setbacks difficult if not impossible. Most experts recommend maintaining a constant temperature in homes with these heating systems.

Due to recent innovations in floor technology, so-called "dry" floors, in which the cables or tubing run in an air space beneath the floor, have been gaining in popularity, mainly because a dry floor is faster and less expensive to build. But because dry floors involve heating an air space, the radiant heating system needs to operate at a higher temperature.

Some dry installations involve suspending the tubing or cables under the subfloor between the joists. This method usually requires drilling through the floor joists to install the tubing. Reflective insulation must also be installed under the tubes to direct the heat upward. Tubing or cables may also be installed from above the floor, between two layers of subfloor. In these instances, liquid tubing is often fitted into aluminum diffusers that spread the water's heat across the floor in order to heat the floor more evenly. The tubing and heat diffusers are secured between furring strips (sleepers), which carry the weight of the new subfloor and finished floor surface.

At least one company has improved on this idea by making a plywood subfloor material manufactured with tubing grooves and aluminum heat diffuser plates built into them. The manufacturer claims that this product makes a radiant floor system (for new construction) considerably less expensive to install and faster to react to room temperature changes. Such products also allow for the use of half as much tubing or cabling, because the heat transfer of the floor is greatly improved compared with more traditional dry or wet floors.

FLOOR COVERINGS

Ceramic tile is the most common and effective floor covering for radiant floor heating, because it conducts heat well and adds thermal storage. Common floor coverings like vinyl and linoleum sheet goods, carpeting, or wood can also be used, but any covering that insulates the floor from the room will decrease the efficiency of the system.

If you want carpeting, use a thin carpet with dense padding and install as little carpeting as possible. If some rooms, but not all, will have a floor covering, then those rooms should have a separate tubing loop to make the system heat these spaces more efficiently. This is because the water flowing under the covered floor will need to be hotter to compensate for the floor covering. Wood flooring should be laminated wood flooring instead of solid wood to reduce the possibility of the wood shrinking and cracking from the drying effects of the heat.

Radiant Panels

Wall- and ceiling-mounted radiant panels are usually made of aluminum and can be heated with either electricity or with tubing that carries hot water, although the latter creates concerns about leakage in wall- or ceiling-mounted systems. Most commercially available radiant panels for homes are electrically heated.

Like any type of electric heat, radiant panels can be expensive to operate, but they can provide supplemental heating in some rooms or can provide heat to a home addition when extending the conventional heating system is impractical.

Radiant panels have the quickest response time of any heating technology and -- because the panels can be individually controlled for each room—the quick response feature can result in cost and energy savings compared with other systems when rooms are infrequently occupied. When entering a room, the occupant can increase the temperature setting and be comfortable within minutes. As with any heating system, set the thermostat to a minimum temperature that will prevent pipes from freezing.

Radiant heating panels operate on a line-of-sight basis -- you'll be most comfortable if you're close to the panel. Some people find ceiling-mounted systems uncomfortable because the panels heat the top of their heads and shoulders more effectively than the rest of their bodies.

Portable Heaters

Small space heaters are typically used when the main heating system is inadequate or when central heating is too costly to install or operate. In some cases, small space heaters can be less expensive to use if you only want to heat one room or supplement inadequate heating in one room. They can also boost the temperature of rooms used by individuals who are sensitive to cold, especially elderly persons, without overheating your entire home.

Space heater capacities generally range between 10,000 Btu to 40,000 Btu per hour. Common fuels used for this purpose are: electricity, propane, natural gas, and kerosene (see the wood and pellet section for information on wood and pellet stoves).

Although most space heaters rely on convection (the circulation of air in a room) to heat a room, some rely on radiant heating; that is, they emit infrared radiation that directly heats up objects and people that are within their line of sight. Radiant heaters are a more efficient choice when you will be in a room for only a few hours, if you can remain within the line of sight of the heater. They can be more efficient when using a room for a short period because they avoid the energy needed to heat the entire room by instead directly heating the occupant of the room and the occupant's immediate surroundings.

Safety is a top consideration when using space heaters. The U.S. Consumer Product Safety Commission estimates that more than 25,000 residential fires every year are associated


Source: U.S. Department of Energy

 

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