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The air in your home is circulated by the blower. It pulls the air through the returns and across the evaporator coils which cool or heat the air and then send it through the supply ducts back into the house.
The evaporator coils are heated or cooled by circulating freon (a chemical refrigerant) through refridgeration lines. The freon travels to an outdoor unit know as a condenser, containing a compressor and condenser coils, where heat exchange takes place. Freon then travels back to the evaporator coils to cool the air, and the cycle continues. The thermostat setting determines wheather the coils are heated or cooled. In a furnace system, the coils are typically used for cooling only. When the heat is on, the heat exchanger heats the air and the colis are left unchanged. A heat pump system may also have electric heat strips to suppliment the heat pump.
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Click here to see a diagram and learn more. There are two main types of systems; split or package.
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Homeowners typically have a choice between two system configurations: a split system (depicted above) or a package system (depicted to the left).
The diagram above depicts a split system which includes an outdoor unit, known as the condenser, containing the compressor and condenser coils, and an indoor unit, commonly refered to as an air handler, which contains the blower and evaporator coils. The condenser is located outside, next to the home, and the air handler is generally located in a crawlspace, basement, attic or closet. In split systems, the outdoor and indoor units must be properly matched to achieve the rated efficiency.
To the left is a single-package system. In a package unit all of the mechanical parts from both the condenser and the air handler are housed in one single cabinet, located outside the house, with both return and supply duct work connecting directly to it. These systems are widely used on commercial properties but many homes have them as well.
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A furnace heats your home by pulling cool indoor air into the furnace, heating it and then redistributing
the warmed air back into the rooms through ductwork. These units can be used in conjunction with heat
pumps or central air conditioners.
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Natural Gas Furnace
With a natural gas furnice, gas is mixed with air and burns to heatup a heat exchanger. A blower pulls cool air in from the rooms through
air ducts in the furnace. The air is heated by passing over a heat exchanger. Warm air then flows back into rooms through ductwork.
Exhaust gases from the burners are vented outside through ventiilation pipe.
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Electric Furnace
An electric furnace uses a blower unit to blow air over electric heating elements.
The warm air is then distributed through the home through ducts.
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The heat pump works based on heat transfer. To cool, it transfers heat from the inside air to the outside,
thereby cooling the home. For heat, it does the opposite.
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Cooling
To cool your home, the heat pump transfers heat from the air inside the home to the outside air. To perform this function, a heat pump works as a central air conditioning system, converting freon from a gas to a liquid and back again. The indoor unit, which contains the evaporator coil, removes undesirable indoor warmth and humidity from the air. The outdoor unit, which contains the condenser and compressor, expells the heat that was captured indoors.
The cycle of cooling begins when the refrigerant arrives at the compressor as a cool, low-pressure gas. The compressor squeezes the refrigerant and packs the molecule of the refrigerant closer together causing them to increase in temperature. The refrigerant leaves the compressor as a hot, high-pressure gas and flows into the condenser. In the condensing coil, the refrigerant releases its heat. A fan draws outdoor air in through louvers surrounding the outdoor cabinet and blows air across the hot coil. As the air blows across the coil, it transfers the heat to the outside air and cools the refrigerant inside the coil.
When the refrigerant leaves the condenser coil, its temperature is much cooler and it has changed from a gas to a liquid. The refrigerant then flows indoors through tubing and passes through a small opening in the expansion valve. In this valve, the refrigerant expands and the liquid's pressure drops and becomes a low-temperature, low-pressure liquid that flows into the evaporator coil where it evaporates absorbing heat from the return air blown over the indoor coil by a fan. At the same time, moisture in the air is removed as it condenses on the indoor coil. Humidity removed from the air, as it makes contact with the indoor coil, is collected in a pan at the bottom of the coil and flows to a drain.
Refrigerant leaving the evaporator then returns to the compressor and the cycle continues. When the room reaches the temperature set on the thermostat, the air conditioner turns off. As the room warms up, the thermostat turns the air conditioner back on.
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Heating
When an air-source heat pump is heating your home, the cooling cycle is reversed. In the outdoor unit, the heat pump evaporates a low temperature refrigerant. As the liquid evaporates, it absorbs heat from the outside air. A heat pump can do this because heat exists in all air down to absolute zero (-460 F or -273 c). Even cold winter air contains heat.
After the gas is compressed in the outdoor unit’s compressor, it passes into the indoor coil and condenses, releasing heat to the inside of the house. The pressure changes caused by the compressor and the expansion valve allow the gas to evaporate at a low temperature outside and condense at a higher temperature indoors.
Types of Air-source Heat Pumps
Most central heat pumps are split-systems, which means they have an indoor unit and an outdoor unit. Some heat pumps are packaged systems, which means they usually have the compressor, condenser, evaporator coil and the fan located outdoors in a single cabinet.
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