A heat pump is a device that transfers heat from one place to another. The name is a little misleading, because a heat pump does not only heat your home: it also cools and dehumidifies it through the warm months. In winter it moves heat from the outside air into the house; in summer it moves the heat inside the house back out.
The cycle itself. The heat pump captures heat from the outside air and carries it indoors using a refrigerant and a pair of coils. In summer it runs the same cycle in reverse, taking excess heat out of the house and releasing it outdoors. The refrigeration cycle is the same one at work in a modern refrigerator, which is a useful way to picture it: a fridge makes its inside cold by moving heat to its outside, and a heat pump does that to a whole house, in whichever direction you need.
What happens when it gets very cold? There is less heat available in the outside air as the temperature falls. In deep cold the heat pump automatically brings in an electric resistance element, usually a coil located in the furnace or the air handling unit, to hold the temperature. How far the unit gets before it needs that help depends on the model, which is why low-temperature capacity is worth checking before you buy.
Paired with a furnace. When a heat pump is installed alongside an oil or gas furnace, the furnace supplies the last few degrees instead. The two alternate and work in sequence, each running where it is most efficient, which is the principle behind a bi-energy setup and is covered in our comparison of heat pumps and air conditioners.
What it saves. A heat pump reduces heating bills substantially, and the reason is the one difference above. An electric furnace spends one unit of electricity to produce one unit of heat, because that is all resistance heating can do. A heat pump delivers up to three units of heat for that same unit of electricity, because it is moving heat rather than creating it. Geothermal pushes the same principle further by drawing on the ground instead of the air.
If you are weighing a heat pump for your own home, speak with one of our specialists. Sizing it correctly is what turns the theory above into a lower bill.
