A heat pump is a refrigerant system, much like
an air conditioner or refrigerator, that can both heat and cool
your home. Most heat pumps are split systems, with an outdoor
coil, an indoor coil and a compressor. During the winter, the
system extracts low-grade heat from outdoors and transfers it
indoors. During the summer, the heat pump is reversed, absorbing
unwanted heat from indoors and sending it outdoors. A thermostat,
ducts and blowers control and distribute the warm and cool air.
Although it's hard to imagine squeezing any useful heat energy
from 30°F (-1°C) wintertime air, a refrigerant at -20°F (-2 9°C)
will be warmed when circulated through the outdoor coil in air that's 50°F (2 7°C) warmer. During this cycle, the compressor and fans consume energy to deliver the heat. The ratio of the
amount of heat delivered to the amount of energy consumed is
called the coefficient of performance (COP). Depending on the
outdoor temperature, the COP of a good heat pump in the heating
season ranges from 1.0 to 3.0—meaning 1 to 3 BTUs of heat delivered
per BTU of electricity consumed. Most heat pumps are slightly
more efficient during the cooling season.
Other rating systems used for heat pumps are the heating season
performance factor (HSPF) and the season energy efficiency ratio
(SEER). These ratings use the watt as a unit of measure and are
simply 3.4 times the size of the COP because there are 3.4 BTUs
As outdoor temperatures fall, a heat pump’s heating capacity
declines, although high-quality units can still extract heat
with outdoor temperatures as low as 9°F (-13°C). To compensate,
supplementary electric heating elements switch on at the crossover
point, which is the point at which an air-source heat pump does
not have enough heating capacity to satisfy a home’s needs.
Have a heat pump serviced annually by a qualified contractor.
In addition, routinely vacuum the indoor coil, change the blower
filter and wash the outdoor coil with a hose.
|Diagram of Heating Cycle
In heating cycle, refrigerant passes through outdoor
coil as gas, drawing heat from air. Gas moves to compression
where high pressure raises its temperature. Compressed gas moves
to indoor coil, where it releases heat and condenses into liquid.
Expansion valve allows liquid to move from high pressure to low,
lowers temperature and vaporizing again. For cooling, a valve
reverses the system.