An air-source Heat Pump uses the unique heat-exchange capabilities of refrigerant to move heat energy from one place to another. For efficient home heating, refrigerant flowing through a heat pump system extracts heat from the cold outside air and transfers it inside, where the resulting heated air is distributed via ducts, just as with a combustion furnace system.
For cooling, a heat pump works very similarly to a central air conditioner, extracting heat energy from the inside air and expelling it outside, again using refrigerant as the medium of heat exchange. A reversing valve controls the direction of refrigerant flow, for either heating or cooling.
While a well-maintained heat pump vastly exceeds the efficiency of a gas furnace, it comes at a price. Most heat pumps begin struggling to extract heat energy from the outside air when the temperature falls below freezing. At this point in most heat pump systems, an auxiliary electric heating strip will kick on for supplemental heating. This can get expensive, however, since the backup electric heating costs about the same to operate as an electric space heater or baseboard.
A HYBRID SYSTEM
Fortunately, there’s a third option, a hybrid or dual-fuel system. In a hybrid system, the air-source heat pump uses a gas furnace for auxiliary or backup heating rather than the electric heating element. When outside temperatures do plummet, you’re saved by a relatively inexpensive natural gas furnace, which can quickly heat up a home and keep it warm even in the coldest weather.
In most hybrid systems, the homeowner has the ability to manually switch the fuel source from electric heat pump to gas furnace and back again. If you find that heating with natural gas is cheaper than the air-source heat pump, even with the efficiency advantage of the heat pump, you can elect to use the furnace.