Boilers are special-purpose water heaters. While furnaces carry heat in warm air, boiler systems distribute the heat in hot water or steam, which gives up heat as it passes through radiators or radiant floor heaters in rooms throughout the house. The cooled water then returns to the boiler to be reheated. In a hot-water system, also called a “hydronic” system, the water is typically heated to about 180°F (or less in a high efficiency system). In steam boilers, which are much less common in modern homes, the water is boiled and steam carries heat through the house, condensing to water in the radiators as it cools. Instead of a fan and duct system, a hot-water boiler uses a pump to circulate hot water through pipes to radiators.
Oil or natural gas is commonly used to heat water in boilers; as with gas- and oil-fired furnaces, boilers can be designed to condense water vapor in the exhaust pipe to reclaim some escaping heat.
Radiant floor heating generally refers to systems that warm the floor, either with electric elements or, more commonly, by circulating warm water in tubing in or under the floor. This warms the room gently, without the noise of blowers and air rushing through ducts. These systems are fairly easy to configure in separate zones, with controls for heating individual rooms. Hydronic (liquid-based) systems are the most popular and cost-effective radiant heating systems for cold climates. For added efficiency, the circulating water can be heated by solar hot water collectors, with the boiler providing a temperature boost, if needed.
Electric radiant floors typically consist of electric cables built into the floor. Mats of electrically conductive plastic are also available, and are attached to the subfloor, below a floor covering (usually ceramic tile). Because of the relatively high cost of electricity, electric radiant floors are usually only cost effective in small areas like bathrooms, or if they include thermal mass, such as a thick concrete floor, and your electric utility offers time-of-use rates, which allow you to “charge” the concrete floor with heat during less expensive, off-peak hours. If the floor’s thermal mass is large enough, and your home is well insulated, the heat stored in the thermal slab will keep the house comfortable for several hours without any further electrical input.
Heat pumps are just two-way air conditioners. During summer, an air conditioner works by moving heat from the relatively cool indoors to the relatively warm outside. In winter, a heat pump reverses this trick, scavenging heat from the outdoors and discharging that heat inside the house. Almost all heat pumps use forced warm-air delivery systems to move heated air throughout the house.
Air-source heat pumps use the outside air as the heat source in winter and heat sink in summer and are installed much like a central air conditioner. Heat pumps are far more energy efficient than electric furnaces, and they can be used for both heating and air conditioning. But before deciding to replace your present system with a heat pump, you should carefully look into whether it makes sense in your climate. Because air-source heat pumps rely on the outside air as the heat source in the wintertime, the colder that air, the worse the energy performance. Air-source heat pumps make more sense in warmer climates, where summer cooling loads are considerable. Cold-climate air-source heat pumps, which are specially designed for optimal winter use, are currently offered by some manufacturers, and are in field trials by several utilities.
Because underground temperatures are nearly constant year-round—warmer than the outside air during the winter and cooler than the outside air during the summer—a ground-source heat pump (also called geothermal, GeoExchange, or GX) can be much more efficient than an air-source unit and appropriate for both warm and cold climates. These heat pumps require that a pipe loop (typically, polyethylene) be buried in the ground, usually in long, shallow (3- to 6-foot-deep) trenches or in one or more vertical boreholes, from 100 to 400 feet deep. Alternatively, some systems draw in groundwater and pass it through the heat exchanger instead of using a refrigerant. The groundwater is then returned to the aquifer.