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Professor Olive on How Heat Pumps Work


The team members at MK Russell & Abbott are kind enough to leave books about heating and air conditioning technology scattered around my favorite couch. This gives me an opportunity to catch up reading about the fine details of how the equipment that keeps your home warm in winter and cool in summer actually works. Our technicians often receive questions about how HVAC installations operate, and they can provide answers in the field. But it makes me proud to know that if any of them asked me, they’d get the same helpful, knowledgable answer. (Provided they understand barking.)

Okay, Olive. Impress me. How’s a heat pump work?

Hah! I thought you were going to ask a hard question. Heat pumps are popular around these parts, so I know plenty about how these great installations can keep a home both warm and cool… all with only a change on the thermostat.

First, you need to know how a standard air conditioner works, because a heat pump operates in a similar fashion. An air conditioner uses a compressor (housed in the outside cabinet) to place chemical refrigerant under high pressure and raise its temperature. The refrigerant travels first to an outdoor set of coils, where it releases this heat through condensation. Cooled down, the refrigerant now goes to the indoor unit and moves through coils where it absorbs heat—lowering the temperature of the air—through evaporation. The warm refrigerant now returns to the compressor to start over.

What a heat pump does is the same… except it can reverse which direction the refrigerant flows using a device called a reversing valve. As the hot refrigerant gas leaves the compressor, this valve can send the refrigerant to the indoor coils first, in which case it releases heat to the indoors. Then it moves to the outdoor cabinet where it absorbs heat. Basically, this makes the two sets of coils swap jobs.

But wait, how does a heat pump draw heat from the outdoors when it’s cold?

Ah, that’s the big question, isn’t it? You switch the heat pump over to heating mode only during cold days. So how does the heat pump absorb heat when it’s cold outside?

Your bare human skin may find it hard to believe that there’s actually heat in the air even when it’s deeply cold outdoors, but it’s the truth. There is still thermal energy in the air, and the modern heat pump is effective at drawing it out. Today, most heat pumps can operate in heating mode during below freezing weather without losing energy efficiency.

Okay, next time ask me a hard one. Like, “What’s up with cats?”  

MK Russell & Abbott is here to help if you have more questions about heat pumps or wish to schedule service in Friendsville and Maryville, TN.