The majority of North American families depend on a central furnace to provide heat. A furnace works by blowing hot air through ducts that carry hot air through air registers or grills to rooms throughout the house. This type of heating system is called a ducted warm-air or forced warm-air distribution system. It can be powered by electricity, natural gas, or fuel oil.
Inside the furnace with gas- or oil, fuel is mixed into the air and ignited. The flames heat a metal heat exchanger where the heat is transferred to the air. Air is pushed through the heat exchanger to the furnace fan of the “air handler” and then forced through the ductwork downstream of the heat exchanger. In the furnace, combustion products are pumped out of the building through a flue pipe. Older “atmospheric” furnaces went directly into the atmosphere and wasted about 30% of fuel energy so that the exhaust would remain hot enough to safely lift through the chimney. Current minimum-efficiency furnaces significantly reduce this waste by using a “chimney” fan to pull exhaust gases through the heat exchanger and create a draft in the chimney. “Condensing” furnaces are designed to reclaim this residual heat by cooling exhaust gases well below 140 ° F, where exhaust vapor becomes water vapor. This is a primary feature of a high-efficiency furnace (or boiler). These usually vent through a sidewall with a plastic pipe.
New furnace standards are currently being developed by the US Department of Energy and are being finalized in the spring of 2016. The current furnace standards have not been updated since 1987.
The heating system controls when various components of the heating system are turned on and off. The most important control from your point of view is the thermostat, which turns the system – or at least the delivery system – on and off to keep you comfortable. A typical forced air system will have a single thermostat. But, a heating system has other internal controls, such as “high limit” switches that are part of an invisible but important set of safety controls.
The efficiency of a fossil-fuel furnace or boiler is a measure of the amount of useful heat per unit of input energy (fuel). The simplest measure is combustion efficiency; It is just the efficiency of the system while it is running. Combustion efficiency is like miles per gallon. Your car hovers at 55 mph on the highway.
The efficiency of the furnace is controlled by the minimum AFUE (annual fuel use efficiency). AFUE estimates seasonal efficiency, average peak, and part-load conditions. AFUE includes estimates of the power used by the air handlers, indenter fans, and controls for start-ups, cool-downs, and other operating losses that occur in actual operating conditions. AFUE is like the mileage of your car, which fills both highway driving and stop-and-go traffic.
Boilers are special-purpose water heaters. While furnaces carry heat to hot air, boiler systems distribute heat to hot water, which gives heat as it passes through radiators or other devices in rooms throughout the home. The cooler water returns to the boiler again. Hot water systems are often called hedonic system. Residential boilers typically use natural gas or heating oil for fuel.
In steam boilers, which are rarely used in homes today, the water is boiled and the steam transmits heat to the houses, which condenses the water in the radiators when cooled. Oil and natural gas are commonly used.
Instead of a fan and duct system, a boiler uses a pump to circulate hot water from the pipe to the radiator. Some hot water systems circulate water through plastic tubing in the floor, a system known as radiant floor heating (see “State of the Art Heating”). Important boiler controls include thermostats, aqua states, and valves that control circulation and water temperature. Although the cost is not trivial, it is generally much easier to install “zone” thermostats and control for individual rooms with hydroid systems with forced air. The new boilers have some control standard features, while others can be added to save energy (see the “Modification by heating system technicians” section on the heating maintenance page).
With furnaces, condensing gas-fired boilers is relatively common and significantly more efficient than non-condensing boilers (unless very sophisticated controls are employed). Oil-fired condensing boilers are associated with conventional fuel oil.
Heat pumps are only two-way air conditioners (see detailed description in the section of the cooling system). During summer, an air conditioner works to heat the heat relatively out of the relatively cool home. In winter, the heat pump reverses this trick, removing heat from the cold road with the help of an electrical system, and discharging that heat indoors. Almost all heat pumps use forced warm-air delivery systems to move hot air throughout the home.
There are two common types of heat pumps. Air-source heat pumps use the outside air as a heat source in winter and a heat sink in summer. Ground-source (also called geothermal, geo-exchange) heat pumps get their heat from underground, where the temperature is high throughout the year. Air-source heat pumps are far more common than ground-source heat pumps because they are inexpensive and easy to install. Ground-source heat pumps, however, are much more efficient and are often chosen by consumers who plan to stay in the same house for a longer period of time or have a strong desire to live more sustainably. To determine whether a heat pump makes sense in your climate, it is discussed further under “Fuel Options”.
While an air-source heat pump is installed much like a central air conditioner, ground-source heat pumps require that a “loop” be buried in the ground, usually long, shallow (3-6 ” Deep) in trenches or in a more vertical borehole. The particular method used will depend on the installer’s experience, the size of your lot, the subsoil, and the landscape. Alternatively, some systems draw in groundwater and pass it through a heat exchanger instead of using refrigerants. The groundwater is then converted into aquifers.
Because electricity is used in a heat pump to generate heat rather than transfer it, a heat pump can provide more energy than it consumes. The ratio of the distributed heat energy to the consumed energy is called the coefficient of performance, or COP, with typical values ranging from 1.5 to 3.5. It is a “steady-state” measure and is not directly equivalent to the heating season performance factor (HSPF), a seasonal measure for rating the heating efficiency of air-source heat pumps. Converting between measures is not straightforward, but ground-source units are generally more efficient than air-source heat pumps.
Gas fired space heater
In some areas, gas-fired direct heating devices are popular. This includes wall-mounted, free-standing, and floor furnaces, characterized by a lack of ductwork and relatively small heat production. Because they lack ducts, they are most useful for heating a room. If several rooms require heating, either the doors between the rooms must be left open or another heating method is required. The improved models use “sealed combustion air” systems, with pipes installed through the wall for both, providing combustion air and closing combustion products. These units can provide acceptable performance, particularly for cabins and other buildings where large temperature differences between bedrooms and main rooms are acceptable. The model can be fired with natural gas or propane, and some can burn kerosene.
Gasless Heaters: A Bad Idea
Gas or kerosene space heaters that do not have exhaust vents have been sold for decades, but we strongly discourage their use for health and safety reasons. Known by manufacturers as “vent-free” gas heating devices, they include wall-mounted and free-standing heaters as well as open flame gas fireplaces with ceramic logs that are not actually connected to the chimney. The manufacturers claim that because the products have very high combustion capacity, they are safe for the occupants of the building. However, this claim is valid only if you keep a nearby window open for enough fresh air – which defeats the purpose of supplemental heat. Hazards include the performance of combustion by-products, as discussed in ventilation and oxygen depletion (these heaters must be equipped with oxygen reduction sensors). Because of these threats, at least five states (California, Minnesota, Massachusetts, Montana, and Alaska) have restricted their use in homes, and many cities in the United States and Canada have also banned them.
Electric space heater
Portable (plug-in) electric heaters are inexpensive to buy, but expensive to use. These resistive heaters include “oil-filled” and “quartz-infrared” heaters. They convert the electric current directly from the wall socket to heat like a toaster or cloth iron. As further explained under “Selecting a new system”, it takes too much electricity to deliver the same useful heat that natural gas or oil can provide onsite. A 1,500-watt plug-in heater will use the full capacity of approximately 15-amp branch circuits; thus, adding too much additional load will trip the circuit breaker or blow the fuse. Calculating the cost of operating a 1,500-watt unit for one hour is simple: it is 1.5 times your electricity cost per kilowatt-hour. The national average rates for electricity — 12h kWh — that the heater would cost 18 12 per hour to run – and would cost quicker than its purchase price. On the other hand, for intermittent use, it is the “least bad” solution when alternatives will require major investment to improve ductwork for a specific area, for example. Just remember, lightning resistance heat is usually the most expensive form of heat, and it is, therefore, rarely recommended.
“Electric baseboard heat” is yet another type of resistive heat, similar to a plug-in space heater, except that it is hard-wired. It has two key properties: installation costs are low, and individual room thermostats are easy to install so you can turn off the heat in rooms that are not being used. Operating costs for all resistive systems are typically very high unless the home is “super-insulated”.
Wood-burning and pellet stoves
If you enjoy woodpiles and stoves or furnaces, wood heating in rural areas can make great sense. Wood prices are generally lower than gas, oil, or electricity. If you cut your own wood, the savings can be huge. Pollutants from wood burning have become a problem in some parts of the country, prompting the US Environmental Protection Agency (EPA) to implement regulations governing pollution emissions from wood stoves. As a result, the new models are quite clean-burning. Pellet stoves offer many advantages over wood stoves. They are less polluting than wood stoves and provide users with more convenience, temperature control, and indoor air quality.
Gas (and most wood) fireplaces are basically part of a room’s decoration, providing a warm glow (and a way of disposing of secret documents), but usually not an effective heat source. With customary installations, which rely on air drawn from the chimney to the room for combustion and dilution, the chimney typically loses more heat than it provides, because too much hot air is drawn through the unit and it should be replaced by the cold air outside. On the other hand, if the chimney is provided with a tight-sealing glass door, external air source, and a good chimney sponge, it can provide useful heat.
State of the art heat
Radiant floor heat generally refers to systems that circulate hot water in tubes under the floor. This heats the floor, which in turn heats people using the room. It is highly controllable, considered efficient by its advocates, and expensive to install. This requires a very experienced system designer and installer, and limits carpet options and other floor finishes: you don’t want to “blanket” your heat source.
Ductless multi-split. Residential ductwork is relatively rare outside North America. “Ductless” heat pumps, which distribute energy through refrigerant lines instead of water or air, are widely used. Large field trials in the Pacific Northwest show that they can have good cold-weather performance, and can be very cost-effective where electrical resistance can replace heating. Like ground-source systems, the relative immaturity of the market helps assure that a multi-split system throughout the home carries premium prices.
Combined heat and electricity or cogeneration for homes are being seriously studied in some countries. The basic premise is to use a small generator to meet some of the home’s electricity demand and to recover wastewater (typically more than 70% of the heating value of the fuel) to heat the home (hydroid or Water-to-air system) and domestic hot water. These systems are not yet widely available. They are likely to have the best economics in homes with high heat bills because the house cannot be solidly insulated, such as concrete stone or brick houses.