The heat pump serves as an air conditioner by absorbing heat from indoor air and pumping it outdoors. The heat pump contains an indoor coil which, in turn, contains a very cold liquid refrigerant. As indoor air passes over the indoor coil, the refrigerant-cooled coil absorbs heat from the air and so quickly cools that air. The cooled air cannot hold as much moisture as it did at a higher temperature. The excess moisture condenses on the outside of the coil, resulting in the dehumidfication of the air. The cooled, dehumidified air is then forced (by a fan) into the duct system which, in turn, circulates it throughout the building.
At the same time, the absorption of heat by the refrigerant turns the refrigerant from a liquid into a vapor. A compressor pumps the heat laden vapor through a vapor line to an outdoor coil which discharges the heat extracted from the indoor air. As the heat is discharged, the vapor is cooled and changes back into a liquid refrigerant. The refrigerant is then pumped back through a liquid line to the indoor coil and the cycle is repeated.
In addition to serving as an air conditioner, the heat pump contains a reversal valve which reverses the flow of refrigerant and thus allows the heat pump to serve as a heater during cold weather.
The Heat Pump as a Heater
The heat pump serves as a heater by absorbing heat from outdoor air and pumping it indoors. All air, even cold winter air, contains a certain amount of heat. As the outdoor air passes over the outdoor coil, heat from that air is absorbed by the refrigerant contained inside the coil. This absorption of heat changes the refrigerant from a low-temperature liquid to a low-temperature, low-pressure vapor. The vapor then passes through a compressor where it is compressed into a high pressure, high-temperature vapor. The hot vapor then circulates into the indoor coil. As indoor air passes over the indoor coil, it absorbs heat from the coil. The warmed air is then redistributed through the duct system.
The Need for a Supplemental Heater
As explained above, as outdoor air passes over the outdoor coil, its heat is absorbed by the refrigerant contained inside that coil. The temperature of the outdoor air passing over the outdoor coil is reduced by about 10º F (or 5.56º). This means that even if the outdoor temperature is above freezing (say, 35º to 40º F), the air closest to the outdoor coil will be reduced to below freezing (32º F or 0º). This reduction in temperature will cause the moisture contained in that air to freeze and to form frost on the surface of the outdoor coil. When the coil is iced over, it must be defrosted. Methods for defrosting the coil vary among manufacturers. The most popular method uses a reversal of the refrigerant flow. As described above, warm air is absorbed by the refrigerant as it passes through the indoor coil. This raises the temperature of the refrigerant and turns it from a liquid into a vapor. As the hot vapor passes, in turn, through the outdoor coil, it defrosts it. Because the heat pump is operating in a cooling mode (drawing heat from indoors and pumping it outdoors) in order to defrost the outdoor coil, it must be supplemented by a secondary heater. Once all the frost on the outside coil has melted, the defrost controls cause the reversal valve to switch over and the unit returns to its heating mode.
Efficiency Ratings
Heat pumps are assigned two efficiency ratings, a SEER rating based on a unit's cooling efficiency and a HSPF rating based on a unit's heating efficiency.
SEER Rating
The SEER (Seasonal Energy Efficiency Ratio) rating is used to identify the cooling efficiency of both traditional air conditioners and heat pumps. The SEER rating indicates how efficiently the unit utilizes electricity: the higher the rating, the less electricity the unit requires to cool a given area.
HSPF Rating
The HSPF (Heating Seasonal Performance Factor) rating is used to identify the heating efficiency of heat pumps: the higher the rating, the less electricity the heat pump uses to heat a given area.
Initial Cost versus Long Term Expense
Generally speaking, heat pumps with the highest SEER and HSPF ratings are more expensive to purchase than their lower rated counterparts. However, because they utilize less electricity, they can actually save you money in the long run. If you are planning to sell your residence in the near future, you may not wish to invest in a unit with a high rating. However, if you plan to be in your home for a while, it may be more cost effective to purchase a more high efficiency unit.
Comfort Features
Some heat pumps come with additional features that provide greater comfort. Two-speed units can run on low-speed (using about 50% of the energy) 80% of the time. Consequently, they use fewer on/off cycles and produce fewer drafts. Likewise, they produce much smaller temperature swings: only two or three degrees rather than the four degree temperature swing commonly experienced with single-speed units. Finally, the improved air circulation provided by a two-speed unit helps to prevent air stratification (warm air rising to the ceiling and cold air settling near the floor).
