The higher the AFUE, the more efficient the furnace or boiler. Boilers are special-purpose water heaters. While furnaces carry heat in warm air, boiler systems distribute the heat in hot water, which gives up heat as it passes through radiators or other devices in rooms throughout the house. The cooler water then returns to the boiler to be reheated. Hot water systems are often called hydronic systems. Residential boilers generally use natural gas or heating oil for fuel.
In steam boilers, which are much less common in homes today, the water is boiled and steam carries heat through the house, condensing to water in the radiators as it cools. Oil and natural gas are commonly used. Instead of a fan and duct system, a boiler uses a pump to circulate hot water through pipes to radiators. Important boiler controls include thermostats, aquastats, and valves that regulate circulation and water temperature.
As with furnaces, condensing gas-fired boilers are relatively common, and significantly more efficient than non-condensing boilers unless very sophisticated controls are employed.
Oil-fired condensing boilers are uncommon in the U. Heat pumps are just two-way air conditioners see detailed description in the cooling systems section.
During the summer, an air conditioner works by moving heat from the relatively cool indoors to the relatively warm outside. In winter, the heat pump reverses this trick, scavenging heat from the cold outdoors with the help of an electrical system, and discharging that heat inside the house.
Almost all heat pumps use forced warm-air delivery systems to move heated air throughout the house. There are two relatively common types of heat pumps. Air-source heat pumps use the outside air as the heat source in winter and heat sink in summer. Ground-source also called geothermal, GeoExchange, or GX heat pumps get their heat from underground, where temperatures are more constant year-round. Air-source heat pumps are far more common than ground-source heat pumps because they are cheaper and easier to install.
Ground-source heat pumps, however, are much more efficient, and are frequently chosen by consumers who plan to remain in the same house for a long time, or have a strong desire to live more sustainably. The particular method used will depend on the experience of the installer, the size of your lot, the subsoil, and the landscape.
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. Because electricity in a heat pump is used to move heat rather than to generate it, the heat pump can deliver more energy than it consumes. The ratio of delivered heating energy to consumed energy is called the coefficient of performance, or COP, with typical values ranging from 1.
Converting between the measures is not straightforward, but ground-source units are generally more efficient than air-source heat pumps. In some areas, gas-fired direct heating equipment is popular. This includes wall-mounted, free-standing, and floor furnaces, all characterized by their lack of ductwork and relatively small heat output.
Because they lack ducts, they are most useful for warming a single room. If heating several rooms is required, either the doors between rooms must be left open or another heating method is necessary. These units can provide acceptable performance, particularly for cabins and other buildings where large temperature differences between bedrooms and main rooms are acceptable.
The models can be fired with natural gas or propane, and some burn kerosene. Gas or kerosene space heaters that do not have an exhaust vent have been sold for decades, but we strongly discourage their use for health and safety reasons. However, this claim is only valid if you keep a nearby window open for adequate fresh air— which defeats the purpose of supplemental heat. For homeowners seeking the most efficient heating system available, heat pumps are the best candidate.
They work on the principle of capturing heat and moving it from place to place, removing it from your home to provide cooling in the summer and bringing in heat from the outdoors for warming in the winter. Heat pumps can be astonishingly efficient, sometimes producing four times as much energy as the electricity that powers them. In general, heat pumps are a good choice for a moderate climate like that of the Raleigh area. Heat pumps are also very easy on the environment.
Their high efficiency means they use less electricity yet provide effective heating. They do not burn fossil fuels to generate heat, which means they do not generate exhaust gases that could affect the environment. They are very safe to operate, producing no potentially harmful gases such as carbon monoxide.
Air-source heat pumps capture heat from and release heat into the air surrounding the equipment. When heating, the refrigerant captures heat from outdoor air and brings it into your home.
Heat pumps can extract heat even from cooler outdoor air, although their efficiency is dramatically reduced when temperatures fall below about 32 degrees. During the heating process, a liquid refrigerant is circulated between the indoor and outdoor units of the heat pump. As the refrigerant circulates, it changes state from liquid to gas and back again. When the refrigerant evaporates into a gas, it also absorbs heat from the air around the outdoor unit.
The gaseous refrigerant travels into your home, after being compressed to its liquid form. Inside, it releases the heat it contains. The heat is then used to warm your indoor living spaces. Geothermal systems work similarly, except they use the soil outside your home or a nearby body of water as the source of heat capture and release.
A series of pipes, called the loop, is buried several feet below the surface of the ground, drilled very deep in a vertical loop system or submerged in a water source. Even a few feet underground, temperatures remain between about 45 to 60 degrees all year long. Either water or a refrigerant solution circulates through the loop, picking up heat or releasing it as necessary.
Geothermal systems provide the most efficient type of heating. They can cut heating bills by up to 70 percent. Like other types of heat pumps, they are also very safe and environmentally friendly to operate. The initial investment in a geothermal system can be higher than other types of heating equipment, and it will be necessary to dig holes and trenches in your yard for placement of the ground loop pipes.
However, a geothermal heat pump can usually pay for itself with the monthly savings within about five years. Federal and state tax credits may be available for these systems—check with your heating contractor for more information!
The second most efficient heating system is a residential furnace. Furnaces are probably the most common type of heating system in use today, with industry estimates placing gas furnaces in about 60 percent of American homes.
Older furnaces are not very efficient, but newer models contain new technologies and features that can boost efficiency to extremely high levels. It indicates how much of the energy in the fuel is turned into usable heat and how much is likely to be wasted. The higher the number, the more efficient your new unit will be. They also are larger in size than traditional air-conditioning units, giving them more space through which to transfer heat to the outside of a building. Boilers heat water to keep an entire home warm during cool weather.
By regulatory requirement, boilers must have an efficiency of at least 80 percent, but that percentage can go as high as This efficiency rating is known as the annual fuel-utilization efficiency AFUE , but as the rating increases, so does the cost. There are three types of high-efficiency boilers: gas-fired, oil-fired, and electric. Boilers with extreme efficiency are more likely to be worth the expense in an area of the country with long periods of extremely cold temperatures each year.
A great deal of energy can be leaked through the ductwork of a home. Ductless mini-split systems eliminate this loss by operating completely without ductwork.
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