Skip navigation

Estimates are free!

24 hour Emergency Service


Northwest Vermont & Northeast New York


How Can a Heat Pump Get Heat From Outside in Winter?


This is a common question people have when they first hear about how a heat pump works. The conversation usually goes something like this …

HVAC Tech: A heat pump is basically like an air conditioner. But it can work in reverse.

Homeowner: What happens when an AC runs in “reverse”?

HVAC Tech: Well, normally an AC draws heat from out of your house. That’s why the air feels cooler indoors.

Homeowner: I thought an AC generated cooling.

HVAC Tech: Cooling is the lack of heat. The AC takes the heat away, making the air cooler. But the AC has to put that heat somewhere, so it releases it outdoors. Now imagine doing it the other way. A heat pump when in heating mode draws heat from the outside and releases it indoors. Presto! Home heating.

Homeowner: Oh, I see. But … wait a minute, I’m only going to run the heat pump in heating mode when it’s cold. So that means the heat pump is bringing in heat from the cold air outside?

HVAC Tech: Yes.

Homeowner: How does that work? It’s … cold outdoors!

The explanation isn’t as confusing as it might seem

We understand the confusion. Hot objects lose their heat to cooler objects, which is why your 98.6°F body feels plenty cold when you walk outside in 20°F weather. You lose heat fast. It’s hard to imagine how that 20°F air can have any heat in it.

But it does have heat. There’s always some heat in the air! (Absolute zero is the only point where there’s no heat, but nothing can be lowered to absolute zero. It’s only a hypothetical.) As long as an object is colder than the outdoor temperature, it can absorb heat.

Heat pumps are designed to operate in colder temperatures and effectively draw thermal energy from the outdoor air. The refrigerant moving through the heat pump can drop to extremely low temperatures before it passes through the outside refrigerant coil. This is thanks to a component called the expansion valve. As the cool refrigerant passes through the expansion valve, the drop in pressure causes a huge drop in temperature. This is one of the properties of gases: when allowed to expand, their temperature drops.

The drop for refrigerant in a heat pump is big—enough for it to be colder than the outside air and absorb heat.

Heat pumps aren’t always efficient in deep chills

There’s a caveat to this. Although newer heat pumps are built to work in some very cold temperatures, some may suffer from a drop in efficiency in extreme cold. In homes where this may be the case, the alternative is to install a dual fuel heat pump. This is a heat pump that has a backup furnace (usually powered with propane) to turn on and make up the difference if the heat pump begins to lose its efficiency.

If you have a heat pump that’s currently having trouble proving warmth, schedule heater repair in Colchester, VT with our technicians. In some cases, efficiency declines may be due to a malfunction with the heat pump.

Turn to our experts for steady heating! Red Rock Mechanical, LLC serves Northwest Vermont and Northeast New York.

Comments are closed.