How a Heat Pump Works
The mystery of how it extracts warmth from the chilly air outside
Could you give me an explanation of how heat pumps work?
—Anthony, Pearl River, NY
A heat pump is an air conditioner in reverse. With an air conditioner, heat is extracted from the building as it is absorbed into refrigerant inside the fan unit in your basement or closet. The refrigerant then carries the heat to an outdoor unit where it is transferred outside. This leaves the air in the building cooler. This same basic cycle happens millions of times an hour in American refrigerators and freezers.
In a heat pump, heat is extracted from the air outside and brought into the house. Surprisingly, there is a decent amount of heat to be extracted until the outdoor air gets to approximately 30 to 35 degrees F. Then you need a backup heater.
A heat pump is an excellent solution for climates that don't often get below freezing. In colder climates, a geo-exchange (geothermal) heat pump might be a better heat pump choice. A loop of pipe in the ground filled with antifreeze replaces the outdoor air conditioning unit and always sees the earth's 52 degrees F. The initial cost is dramatically higher because of drilling or loop installation cost, but it works in cold weather.
— This Old House plumbing and heating contractor Richard Trethewey