This article was originally published in the March/April 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online March/April 1996
A.C.S. (Skip) Hayden is head of Energy Conservation Technology at the Combustion and Carbonization Research Laboratory (CCRL) of CANMET in Ottawa, Canada.
When something must be done about an old or inefficient heating system, the challenge can be daunting. The decision will affect comfort, energy costs, and the air quality and safety of a home for years to come. Whatever the reasons for considering a new heating system--adding living space or replacing a system that is inefficient or just plain falling apart--this is a long-term investment, and it makes good economic sense to look carefully at energy efficiency.
As this guide points out, there are a number of factors that determine the right choice for a particular home. Costs will include the price of the unit and distribution system, installation charges, and operating expenses. We will focus on the latter, keeping in mind that long after the last payment has been made on the new system, energy bills will keep coming in.
Finding the Perfect Fit
It is counterproductive to invest in a new or improved heating system, only to allow much of its heat to escape. Be sure to air seal and insulate effectively before having a heating system sized, installed, or upgraded (see Home Energy's Guide to Insulation, HE Jan/Feb '92, p. 29 and Air Sealing in Occupied Homes, HE Nov/Dec '95, p. 33).
SizingContractors often put in a furnace with a higher capacity than is really needed for the home. This is partly to avoid callbacks from unhappy customers, or to compensate for leaky ducts or poor house insulation. However, furnaces and boilers are most efficient when they have been running for a while, and since oversized furnaces can meet the demand for heat in a shorter time, they may never achieve their best efficiency.
This is why it is very important to make sure the contractor does proper load and sizing calculations, instead of using a rule of thumb that includes a margin of safety that oversizes for most homes. It doesn't make sense to pay more for a bigger furnace or boiler, yet heat the house less efficiently.
Condensing furnaces (see Heating Equipment Guide) are the exception to this rule. They are actually slightly more efficient when they run for shorter periods. Thus a model that is a little larger than the house heat demand can be installed without suffering an efficiency penalty.
Go to the Source
Check local prices to see if energy will be saved by switching energy sources or making modifications to employ more than one energy source. Table 1 gives the energy content for the various energy sources, per unit in which they are commonly sold. It also provides spaces to enter the local prices of energy sources available in your area. Table 2 lists the typical seasonal efficiency of common heating equipment used with each energy source. Use the information in these tables and follow the Heating Cost Worksheet to determine savings in energy cost from switching heating equipment.
Central Question: Air or Water?
Forced warm air is by far the most common type of central heating system used in North American homes (See Figure 1). Air ducts and registers distribute heat from a central furnace, providing heat very quickly. The system can also be used to filter and humidify the household air, to provide central air conditioning, and to circulate air for ventilation.
Forced warm air systems have some disadvantages. Air coming from the heating registers sometimes feels cool (especially with certain heat pumps), even when it is warmer than the room temperature. There can also be short bursts of very hot air, especially with oversized units. Ductwork may transmit furnace noise and can circulate dust and odors throughout the house. Ducts are also notoriously leaky, typically raising a home's heating costs by 20% to 30%. Homeowners who are having a forced-air system put in should demand a tight duct system, and be willing to pay the contractor to install it properly. With an existing forced air system, have the ducts sealed before upgrading the equipment (see Diagnosing Ducts, HE Sept/Oct '93, p. 26).
Figure 1. In a forced-air system, heat is distributed through the house by air ducts. Supply ducts bring hot air to the living space while return ducts return house air to the furnace.
Instead of circulating warm air throughout a home, a hydronic system pumps hot water from a boiler through pipes and radiators and then returns it to be reheated and recirculated.
Hot-water heating systems once used large wrought-iron pipes and massive cast-iron radiators, but for many years now installers have been using smaller copper piping, slim baseboard heaters, and smaller, more efficient boilers (see Figure 2). Recently, plastic piping has been approved for use in some areas as an alternative to the more expensive copper pipes.
Some advantages of hydronic systems are the ability to regulate the temperature in each room and to use the same boiler for domestic hot water. However, the installed cost of hydronic systems is higher than that of forced-air systems, they can be slow to warm up, and there is no capability for central air conditioning, air filtering, or ventilation.
It is usually more economical to stay with an existing distribution system than to switch to another, unless major modifications or repairs are needed. However, some homeowners with electric baseboard heating and high electricity rates will find that a switch to a central forced air or hydronic system is worth the investment. Also, if central air conditioning is already installed or if it is planned for the house, a forced air heating system is a sensible choice for heating, since the heating and cooling systems can share the ductwork.
Figure 2. A hydronic system uses hot water to distribute heat through the house. The boiler can be gas, oil, or electric.
Interpreting Energy Efficiency Ratings
Electric space heating equipment using resistance heating is typically defined as being 100% efficient, because all of the electrical energy used is converted into heat and there are no combustion losses through a chimney. The efficiency rating of the equipment does not take into consideration the losses that took place in the production of the electricity and its transportation to the house, which account for its relatively high unit cost.
Heat pumps, another type of electric heating system, can have efficiencies higher than 100%, since they transfer and upgrade heat from the outside air or ground, thereby increasing their heat output without losses. A heat pump's efficiency is given as HSPF (Heating Seasonal Performance Factor). Dividing the HSPF by 3.413 yields a seasonal Coefficient of Performance, which is similar to the AFUE for a furnace.
AFUE and HSPF measure seasonal efficiency, which takes into consideration not only the normal operating losses, but also the fact that most furnaces rarely run long enough to reach their steady-state efficiency, particularly during the milder weather at the beginning and end of the heating season. Steady-state efficiency measures the maximum efficiency a furnace achieves after it has been running long enough to reach its peak level operating temperature. This is an important standardized testing procedure that is also used by a service person to adjust the furnace.
Be sure to look for the seasonal efficiency--the AFUE or HSPF rating is most useful to a homeowner because it provides a good indication of how much annual heating costs will be reduced either by improving existing equipment or by replacing it with a higher efficiency unit.
The Heating Cost Worksheet and Heating Equipment Guide can help narrow the choices for either upgrading or replacing a heating system. Since there are no furnace stores where the different makes and models can be examined and compared, it will be necessary to obtain manufacturer's literature from distributors or heating firms. The local utility can provide information on the cost of purchasing or installing heating systems, and the estimated seasonal heating costs of different equipment.
It is important to hire a contractor who will size and install the equipment properly, so that it will operate efficiently. Before deciding to buy, obtain firm, written bids from several companies on both the cost of upgrading existing equipment and the cost of buying and installing a complete new unit along with any other fittings and adjustments required. These should include changes to ductwork or piping and a final balancing of the heat supply to the house.
To get an estimate of how many years it will take for the investment to pay off, divide the costs of buying new equipment by the energy savings per year calculated in the worksheet. Once it does, all the additional energy savings are money in the bank. That's one kind of payback. The other is that from now on winters will be more comfortable, and annual heating bills will set new records--for low costs!
Understanding Energy Source Measurements
The amount of heat a piece of equipment can deliver is called its capacity, while the amount of energy actually used is called consumption. Heating units are manufactured and sold by their capacity; the monthly bills customers receive are for consumption.
Natural Gas. The heating capacity of gas heating appliances is measured in British thermal units per hour (Btu/h). (One Btu is equal to the amount of energy it takes to raise the temperature of one pound of water by 1 degree Fahrenheit.) Most heating appliances for home use have heating capacities of between 40,000 and 150,000 Btu/h. In the past, gas furnaces were often rated only on heat input; today the heat output is given.
Consumption of natural gas is measured in cubic feet (ft3). This is the amount that the gas meter registers and the amount that the gas utility records when a reading is taken. One cubic foot of natural gas contains about 1,007 Btu of energy. Utilities often bill customers for therms of gas used: one therm equals 100,000 Btu.
Propane. Propane, or liquefied petroleum gas (LPG), can be used in many of the same types of equipment as natural gas. It is stored as a liquid in a tank at the house, so it can be used anywhere, even in areas where natural gas hookups are not available. Consumption of propane is usually measured in gallons; propane has an energy content of about 92,700 Btu per gallon.
Fuel Oil. Several grades of fuel oil are produced by the petroleum industry, but only #2 fuel oil is commonly used for home heating. The heating (bonnet) capacity of oil heating appliances is the steady-state heat output of the furnace, measured in Btu/h. Typical oil-fired central heating appliances sold for home use today have heating capacities of between 56,000 and 150,000 Btu/h.
Oil use is generally billed by the gallon. One gallon of #2 fuel oil contains about 140,000 Btu of potential heat energy.
Electricity. The watt (W) is the basic unit of measurement of electric power. The heating capacity of electric systems is usually expressed in kilowatts (kW); 1 kW equals 1,000 W. A kilowatt-hour (kWh) is the amount of electrical energy supplied by 1 kW of power over a 1-hour period. Electric systems come in a wide range of capacities, generally from 10 kW to 50 kW.
When converted to heat in an electric resistance heating element, one kWh produces 3,413 Btu of heat.
This article is part of a series om energy-effiecient remodeling, which is being funded by the Environmental Protection Agency and the Department of Energy.
HEATING EQUIPMENT GUIDE
Natural Gas and Propane Equipment
Conventional Warm-Air FurnacesA basic, conventional natural gas-fired warm-air furnace is shown in Figure 3. The furnace has a naturally aspirating burner, which means that air for combustion is drawn in from the surrounding area by the natural forces of hot air rising. The gas and air burn, forming combustion gases, which give up heat across a heat exchanger and are exhausted to the outside via a flue pipe and vent. A dilution device, known as a draft hood, isolates the burner from outside pressure fluctuations by pulling varying quantities of heated house air into the exhaust. A circulation fan passes house air from the return ducts over the furnace heat exchanger. The warmed air then flows into the ductwork for distribution around the house. These older natural gas systems usually have seasonal efficiencies of about 60%.
A minor improvement in efficiency comes with adding a vent damper in the flue exhaust. By closing off the vent during the off cycle, the damper prevents some of the warm household air from being drawn up the flue and lost to the outdoors. These furnaces usually have an electric or electronic ignition. Fuel savings are generally in the 3%-9% range, relative to a conventional furnace. However, some of this savings potential can be lost if a conventional gas-fired water heater is connected to the same chimney. The water heater is still vented and now has a higher draft imposed upon it, increasing the heat loss through the water heater.
Figure 3. Conventional gas-fired warm air furnace with vent damper.
Neither of the previous furnace types meets current U.S. or Canadian minimum standards for energy efficiency. So those planning to replace an existing system with a new gas furnace will choose one of the mid-efficiency or high-efficiency units discussed below. The seasonal efficiency (AFUE) of the furnace will be listed on an energy guide label.
The combustion of natural gas produces heat and some by-products, including water vapor and carbon dioxide. In a conventional gas furnace, such by-products are vented through a chimney, where a considerable amount of heat (both in the combustion products and in heated room air) also escapes. The newer designs have been modified to increase energy efficiency by reducing the amount of heated air that escapes, and by extracting more of the heat contained in the combustion by-products before they are vented.
Mid-Efficiency Gas Furnaces with Induced-Draft FanMid-efficiency gas furnaces usually have a naturally aspirating burner like conventional units. They do not have a continuous pilot, however, and instead of a draft hood, they are equipped with a powered exhaust--usually a built-in induced draft fan. They save 15%-25% of the energy used by conventional gas furnaces.
One word of caution: do not buy a mid-efficiency furnace that is more than 82% efficient. These systems often have condensation problems in the furnace or venting system. There is also some concern about the longevity of high temperature plastic pipe used to vent many of these mid-efficiency units. For higher efficiency, get a condensing furnace.
High-Efficiency Condensing Gas FurnacesCondensing gas furnaces (see Figure 4) are the most energy efficient furnaces available, with seasonal efficiencies between 90% and 96%. They are called condensing furnaces because the combustion gases are cooled to the point where the water vapor condenses, releasing additional heat into the home. The resulting liquids (condensate) are piped to a floor drain. Because the flue gas temperature is low, plastic piping can be used for venting out the side wall of the house.
Figure 4. High-efficiency condensing gas furnace.
In general, the same technologies and comments apply to propane as to natural gas, with slight differences in the efficiencies. Propane has a lower hydrogen level than natural gas. About 3% less energy is tied up in the form of latent heat with propane systems than with natural gas. This means that conventional and mid-efficiency propane furnaces can be expected to be slightly more efficient than comparable natural gas units. On the other hand, propane's lower hydrogen content makes it more difficult to condense the combustion products, so that propane-fired condensing furnaces will be 2%-3% less efficient than the same unit fired with natural gas.
BoilersGas-fired boilers use either a power burner or the same type of burner as furnaces. A circulating pump pushes heated water through the pipes and the radiator system. Conventional boilers have seasonal efficiencies of about 60%.
Condensing gas-fired boilers in hydronic heating systems can have difficulty condensing in practice, because the return water temperature is above the dew point of the flue gases. By installing a water-to-water heat exchanger and storage tank upstream of the boiler, the return water temperature can be brought below the dew point, flue gases will condense, and efficiency will improve significantly.
Sealed-Combustion (Direct Vent) SystemsHeating costs may be lowered slightly by reducing the amount of combustion air drawn from inside the house. One way to do this is to use outside air, brought in through piping directly to the burner. This is known as sealed combustion or direct vent and can prevent backdrafting--hazardous flue gas spillage--caused by exhaust fans or conventional fireplaces in airtight homes. It can also prevent depressurization of the house caused by the furnace itself.
Figure 5. Mid-efficiency oil furnace.
Mid-Efficiency Oil FurnacesMany noncondensing mid-efficiency oil furnaces use an even more efficient high-static retention burner (see Figure 5). This type of furnace also features an improved low-mass combustion chamber (usually ceramic fiber) and passes the hot combustion gases through a superior heat exchanger that enables the circulating house air to extract more heat. The barometric damper, with its large requirement for house air to dilute the combustion gases, has been eliminated in the most efficient of these designs.
Benefits of a good mid-efficiency furnace are much lower combustion and dilution air requirements as well as more power to exhaust the combustion products (both advantages in new, tighter housing); a safety shutoff in case of draft problems; and a more effective venting system.
Mid-efficiency oil furnaces can have seasonal efficiencies of 85%-89% and use 25%-30% less fuel than a conventional oil furnace producing the same amount of heat.
Condensing Oil FurnacesWhile a natural-gas condensing furnace has a significant efficiency advantage over a mid-efficiency gas furnace, a condensing oil furnace is only marginally more efficient than a well-designed mid-efficiency oil furnace. Oil produces only half the water vapor of gas, and so has much less energy tied up in the form of latent heat; the furnace must work harder to condense less. In addition, the condensate is much more corrosive than with natural gas, so the condensing oil heat exchanger must be made of special materials. For these reasons, a mid-efficiency oil furnace is a better bet than a condensing oil furnace.
Sealed CombustionSome newer oil furnaces have optional sealed combustion to save a bit more energy and prevent backdrafts and spillage. However, on a very cold winter's day, if the air is not warmed before reaching the burner, it could cool the fuel oil in some units, causing start-up problems.
Oil Boilers for Hydronic SystemsAn oil-fired boiler uses the same types of burners as an oil-fired forced-air furnace, although the boiler itself is often somewhat smaller and heavier (see Figure 6). There is no circulating fan and filter housing as there is in a forced-air furnace. Instead, most boilers require a circulating pump to push hot water around the house through the pipes and the radiator system. The seasonal efficiency of old conventional boilers is similar to that of conventional furnaces (about 60%).
Figure 6. Oil-fired boiler.
Oil System UpgradeThere are many ways to improve the efficiency of an old oil boiler or furnace. If the system is oversized (a properly sized oil burner should run 45 to 50 minutes per hour when the temperature outside is the lowest expected for the area), simply replacing the existing oil burner nozzle with a smaller one can downsize it. Reduct the nozzle only one size on a conventional cast-iron head burner, so as not to reduce the firing rate too much. Do not reduce the size below the minimum firing rate given on the manufacturer's rating plate.
Flame retention head burners do a much better job of mixing the air and fuel than old cast-iron head burners. They are now almost standard on new furnaces and can also be added to most older furnaces. Replacing the burner can increase the seasonal efficiency of an old oil-fired furnace by about 15%. Use of a new high-static burner can give even greater savings and better performance. The nozzle should be reduced at least one size or even more, and a ceramic fiber combustion chamber liner should be used. (An experienced contractor should check the system's chimney or flue whenever any change is made to the system.)
Central Electric FurnaceIn a central electric furnace, house air is blown over electric heating coils and then distributed through ductwork around the house. This type of furnace is much simpler than a fuel-fired one, because no combustion air or exhaust is needed. These units therefore have an efficiency rating of 100%, meaning that all of the heat created goes into heating the house air. However, this figure can be misleading. A lot of energy is lost producing and transporting electricity to the house, and it shows up on the energy bill. As with all forced-air systems, leaky ducts in a poor distribution system can lead to hefty additional heat losses. Electricity rates vary, but in most places a central electric furnace is the most expensive type of heating system to run.
Heat PumpsMany forced-air systems use a heat pump instead of an electric furnace because of its high efficiency and capability to air-condition. A heat pump is an electrical device that extracts heat from one place and transfers it to another (see Figure 7). It transfers the heat by circulating a refrigerant through a cycle of alternating evaporation and condensation. A compressor pumps the refrigerant between two heat exchanger coils. In one coil, the refrigerant is evaporated at low pressure and absorbs heat from its surroundings. The refrigerant is then compressed en route to the other coil, where it condenses at high pressure. At this point, it releases the heat it absorbed earlier in the cycle.
Figure 7. An air-source heat pump during the heating cycle. Heat pumps use a refrigerant to transfer heat from outside into the house.
A heat pump can be used for both heating and cooling. In the summer, it acts as an air conditioner, removing heat from the air inside the house and transferring it outside. In the winter, the heat pump operates in reverse, removing heat from the outside air or ground, and transferring it inside the house. Residential heat pumps are divided into two major groups: air source (air-to-air) systems, which draw heat from the air, and ground source (earth energy) systems, which draw heat from the ground or underground water.
Air Source. Air source heat pumps can be either add-on, all-electric, or bivalent. Add-on heat pumps are designed to be used with another source of supplementary heat, such as a fuel-fired furnace. All-electric air source heat pumps come equipped with their own supplementary heating system in the form of electric-resistance heaters. Bivalent heat pumps are a special type, developed in Canada, that use a gas- or propane-fired burner to increase the temperature of the air entering the outdoor coil. This allows these units to operate efficiently at somewhat lower outdoor temperatures. A problem with most air source heat pumps is that the heat output (and efficiency) drops with colder outside temperatures, exactly the opposite of what the house requires.
Ground Source. A ground source heat pump uses the relatively constant temperature of the earth or groundwater or both as a source of heat in the winter (see Figure 8). This allows it to maintain its output in cold weather and makes it more efficient than an air source heat pump, which must work harder as the air temperature drops. In ground source pumps, heat is removed from the earth through a liquid, such as groundwater or an antifreeze solution, upgraded by the heat pump, and transferred to indoor air. During the summer months, the process is reversed: heat is extracted from indoor air and transferred to the earth through the groundwater or antifreeze solution.
Heat Pump Efficiency. Heat pump efficiency is measured separately for the cooling and heating cycles. For cooling, the Seasonal Energy Efficiency Ratio (SEER) of an air source heat pump ranges from a minimum of 9 to a maximum of about 16. The Heating Seasonal Performance Factor (HSPF) for the same units ranges from a minimum of 5.9 to a maximum of 8.8. The SEER of a ground source heat pump ranges from 11 to 17, and the HSPF ranges from 8.3 to 11.6.
At the lower end of the product range, both air and ground source heat pumps have single-speed reciprocating compressors. Heat pumps with the highest SEERs and HSPFs invariably use variable or two-speed scroll compressors.
A homeowner who has an electric furnace and wants to stay with electricity as an energy source may be able to reduce heating costs by up to 50% by converting to an air source heat pump, and by 65% by converting to a ground source heat pump. Actual dollar savings will vary depending on factors such as local climate, the efficiency of the current heating system, the cost of electricity, and the size and HSPF of the heat pump installed.
Figure 8. Piping for a ground source heat pump is usually run vertically in a deep trench (shown here) or horizontally in a shallow trench if there is enough space on the property.
Integrated Systems for Space and Water Heating
An integrated condensing gas-fired space- and water-heating system can have an efficiency of over 90% for both applications. Space heating can be hydronic or forced-air. The overall purchase and installation cost may even be lower than for individual appliances, and the efficiency is high.
Mid-efficiency gas-fired combined systems also exist, but their overall efficiency potential is lower than that of the condensing systems. Some integrated systems have even used a conventional gas-fired water heater as the basic energy generator, connecting it to a fan coil for warm air heating; however the result is a system with quite low overall efficiencies.
Mid-efficiency oil-fired integrated systems, based on a low-mass boiler, high static burner, and external water storage tanks, offer high seasonal efficiencies--83%-84% for both space and tap water heating.
Some space heaters can also be very effective radiant heat sources, warming solid bodies (like people) in their line-of-sight without necessarily having to heat up all the air. Good examples are the new direct-vent gas fireplaces, advanced combustion wood fireplaces and portable electric infrared radiant heaters. (Never use an unvented combustion appliance in a tight house.) If properly located in a major living space, a radiant space heater can lower the overall heat demands of the house, while making the occupants feel more comfortable.
Direct-Vent Wall FurnacesDirect-vent gas wall furnaces are self-contained heating appliances that draw in combustion air and discharge by-products through a vent to the outside. They are permanently attached to the structure of a building and are not connected to ductwork. These units circulate heated air by gravity or with the help of a circulating fan. Furnaces with circulating fans are usually more efficient.
Wall furnaces are compact and less expensive than central furnaces, but they are generally less efficient. They come in a variety of heating capacities and with efficiencies that range from 70% for a standard efficiency unit with a pilot light to 82% for a mid-efficiency unit with electric ignition and induced draft.
Freestanding Room HeatersRoom heaters are self-contained and have heat outputs much lower than those of central furnaces. Often they resemble new, freestanding wood stoves and are fired by wood, wood pellets, oil, or gas. They are not connected to ductwork. A vent pipe allows combustion by-products to escape to the outdoors. Heat is circulated by radiation, natural convection, or with the aid of a circulating fan. Standard and mid-efficiency units are available with AFUE ratings between 60% and 82%.
If electricity rates are low, an electric space heater can minimize use of an inefficient central system. Portable electric room heaters are available as convection, radiant, and fan-assisted units, ranging from 500W to 1500W capacity. These can be very expensive to run at typical electricity rates.
Advanced Efficient FireplacesWhile conventional fireplaces are extremely inefficient and can cause serious indoor air quality problems, new advanced wood- and gas-fired designs offer safe, efficient, attractive alternatives (see Fireplaces: Studies in Contrasts, HE Sept/Oct '94, p. 27). Gas fireplaces have simulated fire logs and flames, which are visible through glass doors. They are similar to room heaters, except that they include a number of decorative touches. They have the potential to provide good, efficient heating. However, many models don't live up to the sales literature. Look for direct-vent gas fireplaces with radiation-transparent pyro-ceramic glass, intermittent ignition, good heat transfer to the house, an insulated outer casing, and an effective venting system to ensure safe removal of the combustion products.
Ductless Minisplit Heat PumpsA new type of heat pump, called a ductless minisplit, is ideal for retrofit to homes with hydronic or electric-resistance baseboard heating in areas where electricity costs are reasonable. Inside, wall-mounted units can be installed in individual rooms, all served by one outdoor section. This is less expensive than installing a central heat pump with ducts, or putting individual wall or window heat pumps in each room.
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