Efficiency Losses in a Heating System
If you heat with a conventional heating fuel (such as oil, gas, propane or wood), not all the energy in the fuel you buy contributes to heating your house. Your heating system’s efficiency is the amount of useful heat you get as a percentage of purchased energy. Anything less than 100 percent represents waste, or lost efficiency. There are three kinds of heating system losses: combustion loss, cycling loss, and distribution loss. Combustion loss is simply the heat that goes up the chimney; it varies from about 30 percent to as little as 3 percent. That leaves a combustion efficiency between 70 percent and 97 percent; that is, the percentage of the fuel you buy that is converted to heat in the equipment. Combustion efficiency is mostly affected by equipment design; unless the burners are badly out of tune, it can’t be improved much unless you buy a new unit.
Cycling losses occur when the heating equipment starts and stops. Each time the unit shuts off, some heat remains inside; some of that heat goes up the chimney. Cycling losses are affected by the equipment design, location, and controls; some efficiency improvements can be made if the controls are set poorly to begin with (especially with some types of boilers). Distribution losses occur when heat generated in the equipment gets out of the house before it reaches its destination. Distribution losses include duct air leaks and conductive losses through uninsulated ducts or pipes. Distribution losses can be significant: 5 percent to 30 percent, especially if there are ducts in an attic or garage, so in some homes a large improvement can be made.
Comfort and energy use
Air leaks in a duct system can have an enormous effect on your house. Duct leaks rob a furnace, heat pump, or central air system of energy— sometimes close to half. Leaks can cause comfort problems, drive moisture flows, and sometimes even lead to dangerous back drafting of combustion equipment. Leaky basement ducts don’t usually waste much energy; sealing them is pretty easy, but there’s not much value to it. But ducts located in attics, garages, or vented crawlspaces can be a big energy problem. Sealing them can be the biggest opportunity for energy savings in many homes. Leaky ducts do more than let heat out. They also increase the air exchange in a house, from a small amount to as much as triple, whenever the air handler is running. That is why furnaces have the reputation for drying out houses. Return leaks in a mechanical room or basement can backdraft a nearby water heater or even the furnace itself. Return leaks can also draw in radon gas, water vapor, subsoil pesticides, or airborne mold spores, distributing them throughout the house.
Supply air leaking into an attic or enclosed cathedral ceiling cavity can deliver moisture at an accelerated rate to cold roof sheathing, causing condensation. If the ducts also carry central air-conditioning, return leaks can bring in outdoor humidity in the summer, reducing cooling efficiency, and leaks can draw outside air into building cavities, where moisture can lead to mold inside the house. Duct leaks are even more important than shell air leaks: Each hole and crack is under pressure from the furnace fan and leaks the hot air from your furnace (or cold air from your air conditioner).
Return ducts can be a bigger challenge. Return systems are frequently patched together using a combination of plywood, sheet metal, joist or wall cavities, and a lot of wishful thinking. Many HVAC installers don’t pay much attention to returns, focusing mostly on supplies that get the air to where it’s supposed to go. But well-sealed returns are important for furnace efficiency, occupant health and safety, and building durability. Fortunately, it’s not typically necessary to seal ducts that are buried inside interior wall and floor cavities. However, you do want to ensure that those cavities don’t leak to outside spaces; if they do, seal those leaks at the thermal boundary to keep the ducts and the indoor air inside.
Ductwork that runs through unconditioned space should be insulated, if it is not already. Typically, supply ducts are insulated but often have weak spots at register boots and other major connections. Often, return ductwork is left uninsulated.
Ducts should be insulated with vinyl-faced fiberglass wrap; R-6 is the minimum allowed by current residential codes; R-8 is required for supply ducts in attics. If possible, attic ducts should have higher R-values. Think about it: In winter, attic temperatures are nearly as cold as outdoors; in summer (for air-conditioning ducts), they are often much hotter than outdoors. If the ducts are close to the attic floor, cover them completely with loose-fill insulation. If that is not possible, try to find R-10 or higher duct wrap. Of course, seal all the duct connections tightly before insulating.
The furnace is by far the most common type of heating system in the country. In fact, it’s so common that many people use the word furnace to describe any type of heating equipment. Furnaces create heat by burning fuel, usually natural gas. Oil and liquefied petroleum (LP, or propane) gas are also used in areas where piped gas is unavailable. By contrast, heat pumps and electric furnaces use electricity to heat the air; they also use a duct system to deliver heat, but they are not furnaces and do not have a fuel supply, chimney, or vent pipe. Boilers burn gas or oil but create hot water or steam instead of warm air. Furnace efficiency can be improved by performing regular maintenance, improving airflow, adjusting control settings, and sealing leaky ductwork.
Regular furnace maintenance is important to keep things running well, just as with any machine, but tune-ups and filter replacements don’t really save energy. Be sure to change the filter once or twice a year; more frequently is not much benefit because filters actually work better once some dust has built up on them. You may need to change it more often if you have pets or lots of dust. If the filter never gets dirty, however, then you may have a duct leak that is causing a loss of efficiency. Keeping registers open and free of obstructions may help your comfort, but this also won’t affect your energy bills noticeably.
If you have a gas furnace, you should have it cleaned and tuned every two to four years. An oil burner should be tuned every year. Don’t wait until cold weather hits—that’s when the service companies are busy. You may get a better price if you get the tune-up in the spring.
Operating efficiency and resistance heating
Heat pumps are rated using Heating System Performance Factor, or HSPF. An HSPF of 6.8 corresponds to an efficiency of 200 percent. Most new heat pumps vary from 8.2 to about 12 HSPF; the larger the number, the more efficient the unit. An HSPF of 10 is approximately equal to 290-percent efficient—for every kilowatt-hour (kWh) of electricity you buy, you get almost 3 kWh of heat delivered to your house. How can a heat pump generate more energy than it consumes? The difference is made up by heat energy absorbed from the outdoor air. That energy goes into the system, but because it’s free, it’s not counted in the measurement of efficiency. Older heat pumps, by themselves, operate well at reasonable efficiencies until outside air temperatures drop below about 35°F. As the outdoor air gets colder, less heat is available, and the heat pump output drops off, even as the house needs more and more heat. So cold-weather performance has conventionally been supplemented with electric-resistance backup heaters built into the ductwork. This “auxiliary” heat costs about three times more per unit of heat than the heat pump would, and isn’t well-suited to the Northeast, where electricity is expensive and winters are cold. One fairly inexpensive way to reduce the use of electric-resistance heating in an existing heat pump is to install an outdoor cutout thermostat. For about $200 to $300 installed, this device locks out the supplemental electric heat when the outdoor air temperature is above 30°F or 35°F.
Note that new heat pumps are a different story: in New England, cold-climate models are now available that have very high heating capacities, and efficiencies, down to 0°F or even lower, so they can heat a house with little or no need for a backup heating supply, and save significantly on heating costs compared with oil, propane, or electric resistance heating. Many utilities and state agencies have incentives for installing listed, high-efficiency cold climate units.
Heat pump service
Like furnaces, heat pumps should be serviced regularly. Basic service can be done by anyone. Air filters should be replaced once or twice a year, or more as needed, and the outdoor coil should be kept free of snow and debris. Regular service calls— typically every two to three years—should include testing the controls, cleaning the blower and both the indoor and the outdoor coils as needed, and checking the insulation on the refrigerant lines. To service your heat pump, try to find a contractor whose technicians are certified by North American Technician Excellence (NATE, see www.hvacradvice.com), or who demonstrates a commitment to attending manufacturers’ programs frequently. An initial service appointment should include testing and fixing airflow problems, then carefully measuring and correcting refrigerant charge. Once that has been done, service technicians generally should not attach refrigerant gauges to the system unless the system performance drops off or there is other evidence that something is wrong. Refrigerant does not escape unless there is a leak.
Most electric-resistance heating systems are simple electric baseboard heaters. There are also various types of radiant-heat systems (usually panels mounted on ceiling or walls, built into valances, or built into ceiling or wall-finish systems). Cheap to install but expensive to operate, electric-resistance heat operates on the same principle as that of a toaster or hot plate. An electric current passes through a wire element, heating it. Air passes by and is heated by the hot surfaces. Electric-resistance heat is always 100-percent efficient; every kilowatt-hour of electricity that you pay for is converted to heat. The reason electric baseboards are expensive to operate is that electricity costs two to five times as much as natural gas, and 1.5 to three times as much as oil. One advantage of electric heat is the ease with which rooms can be zoned. But with the high fuel cost and hidden emissions, it’s difficult to justify except in special situations, such as isolated rooms. Because these systems are so simple, there is very little that can be done to improve their efficiency. Supplementing or completely replacing them with an air source heat pump can be a very cost-effective option. The best approach (as always) is to insulate and thoroughly seal the building to reduce heating loads, regardless of what else you do.
Supplementing Electric Heat
Hot-Water and Steam Boilers
Boilers are common in the Northeast. Instead of heating air, a boiler heats water. The classic boiler heats water that is pumped through pipes to hot-water radiators or baseboard fin-tube convectors to heat the house. Some older systems use a boiler to make steam, which expands to fill steam radiators to heat the house.
Boiler controls and upgrades
A typical hot-water boiler operates throughout the winter with a water temperature determined by the low-limit setting, a thermostatic switch that senses the water temperature inside the boiler and shuts off the burner when the water is hot enough. Standard boiler controls maintain full boiler temperature throughout the year, which is inefficient but necessary if a tankless coil water heater heats the domestic hot water. In the absence of a tankless coil, efficient boiler controls have a “cold start” feature that heats the boiler water only when it’s needed, and a “purge” control that runs the circulating pump for a few minutes after the boiler shuts off, to extract any heat that remains inside. These controls, often standard or optional on new, high-efficiency models, can also be retrofitted to existing boilers—but you have to be careful. Cold-start boilers must be protected from condensing on start-up, which may take some extra piping and a thermic valve (made by Danfoss™ or Termovar™) to maintain adequate return-water temperature.
Steam boilers are very different from hot water boilers. Steam systems were popular in the early 20th century, and many of them are still in service in older homes. Instead of relying on pumps to circulate hot water, steam boilers boil water to create steam. The steam naturally expands to fill all the pipes and radiators in the system, distributing heat throughout the house. Although steam-heat distribution is simple and reliable, steam boilers have a number of unique maintenance and efficiency issues. Most residential steam systems are one-pipe systems. When the thermostat calls for heat, the boiler fires and steam begins expanding to fill the system. As the steam expands, it displaces air contained in the pipes and radiators, and that air must be allowed to escape through air vents typically located on the side of each radiator. Once the steam reaches the radiator, the air vent senses the heat and closes, keeping the steam inside.
Air-venting systems that aren’t working properly can lead to uneven heating, large temperature fluctuations, and discomfort. In the case of a steam boiler, a tune-up can increase energy efficiency and comfort dramatically. If the air vents are too small, if there are not enough of them, or if they are clogged with mineral deposits, air gets trapped in the system, and the steam cannot reach its destination. Having a professional replace malfunctioning air vents, add extra vents to radiators, or add special high-volume air vents to large central supply pipes can improve steam-system efficiency by quickly getting the heat where it is needed. Thermostatically controlled vents can reduce the rate of steam delivery to some rooms, reducing overheating and improving overall comfort.
A common problem with steam systems is too much pressure. The boiler pressure controls should be set just high enough so that the boiler shuts down just as each radiator fills with steam. Too much steam pressure increases the boiler’s cycling losses and leads to overheating. That can particularly be a problem if the building envelope’s efficiency has been improved and/or some radiators have been removed during remodeling. Timed-cycle controllers vary the boiler cycle length to match the outdoor temperature, further improving the system’s efficiency.
Upgrading or Replacing Heating Equipment
All new boilers and furnaces have standardized efficiency ratings, just like the gas-mileage ratings on automobiles. When selecting equipment, look for the highest efficiency ratings. Furnaces and boilers use Annual Fuel Utilization Efficiency (AFUE), which accounts for burner efficiency, pilot fuel, and off-cycle losses. New gas furnaces and boilers have AFUEs of 80 per-cent to 97 percent. AFUE ratings are available from manufacturers or at www.ahridirectory.org. Air-source heat pumps are rated using HSPF; GSHPs are rated using COP. The U.S. Environmental Protection Agency and Department of Energy (DOE) maintain a listing of high-efficiency equipment on their Energy Star website (www.energystar.gov/products); always look for the Energy Star label, which indicates that the equipment meets certain above-minimum efficiency standards set by the government. Remember that there are many products that exceed the Energy Star minimum standards; it’s still worth shopping for the highest-efficiency unit you can find. For example, state or local incentive programsmay set their own standards for equipment to qualify for rebates.
Material adapted from Build Like a Pro: Insulate & Weatherize, published by The Taunton Press, 2012; used by permission.