How cold outdoor air can help warm a home

Omar looks after a small apartment building. Tonight the courtyard is cold, and the flat upstairs needs warming. The heat pump’s outdoor unit sits right there in that cold courtyard. So where does the warmth come from? Cold air isn’t empty of heat. A heat

pump uses electricity to move some of that heat indoors. Meet our heat courier. It isn’t a real part. It stands for energy, so we can follow it. Follow the sealed loop. Outside, very cold refrigerant flows through a coil. It’s even colder than the

courtyard air, so it absorbs heat and turns into a gas. Next comes the compressor. It runs on electricity, and it squeezes the gas, raising its pressure and its temperature. Now the refrigerant is hotter than the room. In the indoor coil, it gives up

its heat and turns back into a liquid. Then an expansion valve drops its pressure, and its temperature falls, so it can collect heat outside again. Round and round. The refrigerant stays inside the loop. No outdoor air is being piped into the room. Here’s

a made-up example, not a measurement. Three units of heat collected outside, plus one unit of electricity, deliver four units of heat inside. That’s not free energy. Three of the four came from the courtyard. The electricity is the effort that moves heat uphill, from

cold to warm. Real results vary. The colder it is outside, the bigger the lift, and the harder the compressor has to work. The system, its installation and the building matter too. Many heat pumps can also run in reverse, to cool a room in

summer. Not all of them do. So if you’re weighing one up, ask three questions. Where will it collect heat? How big a lift must it make? And is it designed for this building? A heat pump doesn’t make most of its heat. It moves

it, and it needs electricity to do that.