Heat Pump Operation
Heat pumps work like air conditioners during the summer and reverse to become air heaters during the winter.
During the summer it operates in the cooling mode. Refrigerant is piped through the indoor coils, absorbs heat from the room air, and vaporizes. The cooled room air is then re-circulated in the house by a blower.
The "vaporized" refrigerant flows into the compressor, which pumps the refrigerant to the outdoor coil, where it condenses back into a liquid by releasing its heat to the outdoor air. Air is circulated through the outside unit by a fan. The cooled refrigerant then flows back to the indoor coil, where the heat transfer cycle is repeated.
In the heating mode, the refrigerant flow is reversed, bringing heat inside from outdoors, essentially working like a "conventional" Air Conditioner in reverse. Cold refrigerant is piped through the Outdoor Coils, absorbing heat from the outside air. The refrigerant vaporizes and flows into the compressor, which pumps it to the Indoor Coil, where it condenses back into a liquid by releasing its heat to the indoor air. The Refrigerant then flows back to the outdoor coils, where the heat transfer cycle starts again.
Like refrigerators, most heat pumps have defrost cycles that minimize "frost" buildup on the evaporator during the winter heating cycle. Defrost occurs automatically at "pre-set" time intervals. Defrosting works against the "efficiency" of the Unit when it switches into the defrost mode unnecessarily, wasting heating and cooling capacity. Microprocessor controls in some Units prevent this from happening.
Some controls even determine whether the heat pump or "back-up" heat is more economical at a particular outdoor air temperature and switch to that heating system. An example of this might be a heat pump with natural gas back-up heat. When it gets very cold, and the heat pump's efficiency drops off, the control would turn the electric heat pump off and the natural gas system on. This is because even thought the gas system may only be 70% or 80% efficient, it still may be more economical to operate than the electric heat pump because of the difference in cost between gas and electricity
Heat Pump Efficiency
There are four different measurements used to describe heat pump efficiency. Since you might see any of these measurements used, we'll explain them all here. Here is a simple rule of thumb: No matter which measurement is used: the higher the number, the higher the efficiency of the system.
Heat pump efficiency is determined by comparing the amount of energy delivered by the heat pump to the amount of energy it consumes. Heat pump efficiency is dependent on "outdoor" temperature. In the heating mode, performance drops as the air's "outside" temperature drops. In cold climates, heat pumps require a supplemental heating system for when they cannot produce enough heat to maintain the desired indoor temperature.
Electric resistance "backup" is most common; but gas, oil and propane back-up heating systems can also be used. The efficiency of the heat pump when the back-up heat is running is really just the efficiency of the back-up system. If it is resistance electric, the efficiency of the heat pump falls to about 100%, which is the efficiency of electric resistance heat. If a gas furnace backs-up the heat pump, when it operates, the heat pump's efficiency is that of the gas furnace.
Coefficient of Performance (COP) is the most common measurement used to rate heat pump efficiency. COP is the ratio of the heat pump's BTU heat output to the BTU electrical input. Conventional electric resistance heaters have a COP of 1.0. meaning it takes one watt of electricity to deliver the heat equivalent of one watt. Air-source heat pumps generally have COPs of 2-to-4, meaning they deliver 2-to-4 times more energy than they consume. Water and ground-source heat pumps normally have even higher COPs of 3-to-5.
Energy Efficiency Ratio (EER) is used for evaluating a heat pump's efficiency in the cooling cycle. It is the same rating system used for Air Conditioners, making it easy to compare different Units. EER is the number of BTUs of cooling provided per watt of electricity consumed.
- EER ratings of greater than 10 are the most desirable.
COP and EER measurements are based on laboratory tests and do not necessarily measure how the heat pump performs in actual use. A heat pump's Performance will vary depending on the weather and how much supplementary heat is required. Therefore, a more realistic measurement, especially for air-to-air heat pumps, is calculated on a "seasonal" basis. These measurements are referred to as the Heating Season Performance Factor (HSPF) for the heating cycle and the Seasonal Energy Efficiency Ratio (SEER) for the cooling cycle.
HSPF is the estimated seasonal heating output in BTUs divided by the seasonal power consumption in watts. It can be thought of as the "average COP" for the entire heating system. An HSPF of 6.8 corresponds roughly with an average COP of 2.
- HSPFs of 7 or 8 are considered good..