A heat pump doesn't generate heat the way a furnace does by burning fuel. Instead, it moves heat from one place to another, and in winter, that means pulling heat out of the outdoor air, even air that feels plenty cold to a person, and delivering it inside. That idea trips people up at first, but the physics behind it is the same refrigeration cycle your AC and refrigerator already use, just running in reverse.
The basic mechanism: moving heat, not making it
Even at outdoor temperatures that feel cold to us, there's still usable heat energy present in the air, there just isn't any at absolute zero, far colder than anything we experience. A heat pump's refrigerant absorbs that heat outside at a low pressure and temperature, then a compressor raises its pressure and temperature, and that now-hot refrigerant releases its heat inside the house through the indoor coil. In summer, the same system runs the opposite direction, pulling heat out of the house and releasing it outside, which is exactly why a heat pump is really an air conditioner that can also run in reverse.
Why colder air still has heat to give up
This is the part that feels wrong intuitively but holds up physically: refrigerant can be engineered to boil and absorb heat at temperatures well below freezing, so as long as the outdoor air is warmer than the refrigerant circulating through the outdoor coil, heat will transfer from the air into the refrigerant. That's true even when the outdoor air itself is uncomfortably cold to stand in.
Where efficiency actually starts to drop
The mechanism keeps working at low temperatures, but it gets less efficient as the gap between outdoor air and the refrigerant's operating temperature shrinks, since there's less of a temperature difference driving the heat transfer. Every heat pump has what's called a balance point, the outdoor temperature below which it can no longer heat the house entirely on its own, even running continuously. Below that point, a backup heat source has to make up the difference.
Where auxiliary heat comes in
Most heat pump systems pair with a backup heat source, commonly electric resistance heat strips, that kicks in automatically once the system can't keep up on its own, usually visible as an "Aux Heat" indicator on the thermostat. Seeing it light up during a genuinely cold stretch is the system working as designed, not a problem. We've covered the difference between auxiliary and emergency heat in more detail in a separate post.
Why heat pumps still make sense in North Texas winters
This area's winters rarely stay below a typical heat pump's balance point for extended periods, which is exactly the climate where heat pumps perform well: efficient heating most of the winter, with backup heat covering the relatively rare, sharper cold snaps. That's a different calculation than it would be in a climate with long stretches of deep cold, which is part of why heat pumps are such a common, sensible choice for homes in this region.
Have a question about your own system?