Beginning in the 1970s, air-source heat pumps came into
common use. They have the advantage of no combustion,
and thus no possibility of indoor pollutants like carbon
monoxide. Heat pumps provide central air conditioning as
well as heating as a matter of course. And they are
installation-cost competitive with a central combustion
furnace/central air conditioner combination.
Heat pumps operate by moving or transferring heat,
rather than creating it. During the summer, a heat pump
captures heat from inside a home or business and
transfers it to the outdoor air through a condensing
unit. During the winter, the process is reversed. Heat
is captured from outdoor air, compressed, and released
inside.
Much less electricity is used to move heat rather than
create it, making heat pumps more economical than
resistance heating. However, in all but the most
moderate climates, the heating ability of the heat pump
is limited by freezing outdoor temperatures. So electric
resistance heat is used to supplement outdoor-air-source
heat pump during the coldest weather, preventing "cold
blow."
Depending on climate, air-source heat pumps (including
their supplementary resistance heat) are about 1.5 to 3
times more efficient than resistance heating alone.
Operating efficiency has improved since the 70s, making
their operating cost generally competitive with
combustion-based systems, depending on local fuel
prices. With their outdoor unit subject to weathering,
some maintenance should be expected.
