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Heat pump is the name given to heating and cooling equipment that utilizes a reversible refrigerant flow system. The heat pump does not use electricity to directly produce heat nor does it burn fossil fuel. A compressor unit is placed outside the home and refrigerant lines connect it to a tubing coil inside the home. This system functions as a typical air conditioner during warm weather and as a heating system in cool weather.
This type of system draws whatever warmth that is available from the outside during cool weather and deposits it in the coil on the inside of the home. In climates that have very mild winters, this system can work well, but in colder climates the heat pump must be supplemented with another heat source, and this is known as hybrid heat.
HOW IT WORKS
In a standard air conditioning system there is a piece of equipment known as a condensing unit placed outside the home. Inside the home there is an evaporator coil. These two units are connected via copper refrigerant lines, which allow the refrigerant to circulate through the outside unit, through a tube to the evaporator and then through another tube back to the outside condenser. This is a sealed system.
Cooling the Home
The outside unit contains a compressor and a coil. The compressor pumps the refrigerant to a high pressure, which also produces heat. The refrigerant passes through the coil to cool it back down and then goes on to the evaporator coil inside under high pressure. When the refrigerant reaches the evaporator it must pass through a series of tiny tubes known as capillaries or through a tiny orifice valve. The refrigerant then spews into the larger evaporator tubing where it expands quickly, and in doing so removes heat from the air. Air forced through the duct system of the home passes over this cold coil where it is cooled and then is distributed throughout the home. The heat is drawn into the refrigerant, is carried out to the outside unit where it is expelled into the great outdoors.
Heating the Home
A heat pump works like an air conditioner, and uses the same technology described above. However, when the temperature drops below a preset threshold, a heat pump reverses the flow of the system so that it picks up the heat from the outdoors and expels it inside of the home. The air it expels is warmer than the outside air temperature because of the heat generated by pressurizing the refrigerant. This is similar to the geoexchange system illustrated opposite, the difference being that a geoexchange system takes heat from the ground rather than from the outside air.
The heat-pump system is more efficient at producing heat than running electricity through heating strips. But the colder the outside temperature is, the less warmth there is outside to be drawn from, and the less efficient the system becomes, until at about 20 degrees F (-6 C) it is no more efficient than strip heat. When outside temperatures reach this point, in a hybrid system, the strip heat would take over and more efficiently warm the home.
As with any work done around the house there are rules to follow to be safe. Never work on any system that uses electricity until the electric circuit is turned off and verified as being off. Handle wiring as though it were hot just to be sure. Always wear eye protection when working with any hand or power tools. Wear gloves when working with anything that might cut or burn. Read all directions on product packaging and always use only as directed.
The biggest complaint about heat pumps is the temperature of the air that comes out of the vents in the home. Instead of very warm air, it is usually very little warmer than the ambient air already in the home. This can be a disappointment if the homeowner expects to feel warmth coming out of the vents. It also takes longer to warm the home from a cold condition. These two things will usually cause the homeowner to override the heat pump system and go directly to the strip heat. It warms the home in a way that they enjoy, but the heat pump efficiency is lost. For this reason, heat pumps are most commonly and effectively used in warmer climates typically across the south and southwestern United States.