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Olive, How Is a Heat Pump Getting Heat Outside in Winter?

Graduation Dog

I love it when I get to put on my “Professor Olive” cap and discuss HVAC science. Now don’t worry—I’m not going to test you on this. And the science I’m going to talk about isn’t too tough. It’s just a bit tricky to approach at first because people aren’t used to hearing about “getting heat out of the cold.”

Okay, let’s jump into the question:

“How can a heat pump move heat into a home during a cold winter day?”

A heat pump works similarly to an air conditioner. During a hot day, the refrigerant circulating through the heat pump absorbs heat from inside the house (using evaporation) and exhausts it outside (using condensation). This seems basic: the inside of your house is hot, so the heat pump shouldn’t have any problem moving excess heat from it.

A heat pump can reverse the direction it works so it absorbs heat from outside the house (evaporation) and releases it inside (condensation). But this sounds backward! You’ll run your heat pump in heating mode when it’s cold outside. Where is the heat pump getting the heat it brings indoors?

Heat is always around—it just may take more work getting at it

Here’s the secret to how a heat pump works during winter: there’s always some heat available in the air, no matter how cold it is. Heat comes from the movement of molecules; as long as there’s movement, there is heat. (No molecular movement is called absolute zero, but it’s impossible to reach this point.)

The reason it’s easy for us to think there’s no heat during cold weather is that our bodies are 98.6°F. This makes cold temperatures feel really cold. (Okay, I’m a dog and I have a body temperature of 102.5°F. The same thing applies.) It’s hard to imagine we could get heat from this air.

But as long as the refrigerant moving through the outdoor coil of a heat pump is colder than the air around it, it can absorb heat from that air. For example, let’s say it’s 30°F outside. If the refrigerant in the heat pump’s outdoor coil is 20°F, heat from the outside will move to the refrigerant, and the refrigerant will move it indoors.

Yes, heat pumps can lower refrigerant temperature this far—and farther. This is thanks to a part called the expansion valve, which reduces the pressure of the refrigerant before it reaches the outdoor coil and causes its temperature to plummet.

It can take more energy for a heat pump to draw heat from the outdoor air when it gets extremely cold, and sometimes a heat pump can lose energy efficiency in those ice-cold conditions. Heat pumps aren’t ideal for all homes, but they can work in most.

If you’re interested in installing a heat pump in Knoxville, TN in time to start using it for the warmer weather, our team is glad to help you! Heat pumps don’t work for every home, but our experts can find the ideal unit to meet your needs with a careful heat load calculation.

Stay cool (or warm),