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Why do Heat Pumps Work During the Winter?

Heat pumps are amazing. They use the same technology that your air conditioner does to help regulate heat in your home during the winter. Heat pumps, just like air conditioners, work by moving heat from one place to another. Unlike your air conditioner, a heat pump works to pull warmth from the outdoors and fill your home with it. But if it’s just an air conditioner, how does it manage to heat your home during the winter when everything outside is cold and there’s no heat to move?

How a Compressor Works

You might remember the General Gas Equation from high school. It’s a very simply equation that allows us to predict how a gas will react under certain conditions. It states:

Pressure x Volume / Temperature = Pressure2 x Volume2 / Temperature2

Anything to the left of the equal sign is what we start with, while values one the right are final states. How is this useful? Well it tells us what will happen as we change individual values. Take for example the compressor in your air conditioner or heat pump.

As coolant flows through the pipes into the compressor one things is kept constant: Volume cannot change. The size of the pipe itself will not change so the coolant cannot occupy more space than is currently available. The compressor then does exactly as its name implies, it adds pressure to the system by compressing the coolant.

As coolant travels through the compressor, volume remains constant, pressure increases, which means that, to keep the other side of the formula equal, temperature has to increase.

  • Pressure +
  • Volume =
  • Temperature +

In an air conditioner, coolant warms in your home, is heated even further by the compressor, then sent outside to ‘cool’ off before returning. Upon returning, the now-cooled coolant decompresses. Shedding heat and cooling down without changing volume. This is then returned to the home where it absorbs heat and repeats the cycle.

How this Works During the Winter


Heat pump mounted on brick wall.

In simple terms: We reverse the order.

An air conditioner works by pulling heat out of colder air (your home) and sending it to a hotter environment (outside). Heat pumps do the same thing but in reverse. Coolant that has been cooled to a point lower than the outside air, ‘warms up’ (relatively) and is then compressed to increase the overall temperature. Doing this doesn’t magically create heat, it simply uses the principles behind how gasses and fluids work to transfer heat from outside (it’s there even if it doesn’t feel like it) back into your home. All it requires is for coolant to be cooled to a point that’s colder than the outdoors.

In extremely cold environments, this can be a bit of a problem. Heat pumps have to work especially hard to chill the coolant to colder temperatures. To make up for the decreased heat during a particularly cold freeze, many heat pumps include a ‘furnace mode’ or ‘emergency heating mode’ that uses an electrical heating element or emergency gas furnace to give extra heating capability. That way they work regardless of how cold it is outside.