Heat pumps are excellent inventions that provide indoor comfort without breaking the bank. Many homes here in the South have them in lieu of furnaces, which can be expensive to install and operate. But they’re not very common in the North, and there is a very good reason for that.

Heat pumps are known to not work well in colder climates during the winter months. The way they’re built makes it so they’re more efficient in climates where the temperatures don’t drop below freezing.

So how do heat pumps work, and at what temperature do they stop being effective?

How Heat Pumps Work

Heat pumps warm the home using heat from the outside. Air source heat pumps take air from outside and bring it into the system, whereas geothermal heat pumps take heat from the ground.

In a nutshell, the way heat pumps work is that they take thermal energy from an outside source and push it into an enclosed space. Similar to an air conditioner, heat pumps use refrigerant which is pumped by a compressor between two heat exchanger coils in a constant process of condensation and evaporation. However, unlike an air conditioner, heat pumps have the ability to act as a home cooling device and a home heating device. This is because of a reversing valve that changes the direction of the heat transfer.

Heat pumps do require energy to operate. However, the less of a difference between the ambient air temperature and the desired home temperature, the less energy is used.

What Temperature Do Heat Pumps Stop Being Effective?

Heat pumps are highly efficient because of how thermal energy works. Thermal energy exists in air, even when the air temperature feels very cold. To a human, the difference between 0 degrees Fahrenheit and 70 degrees Fahrenheit is huge: however, air at 0 degrees contains 85% of the thermal energy as air at 70 degrees. Heat pumps don’t pump in air directly: they take the heat from the air and use it to heat the home. 

However, that marginal difference in thermal energy does make a big difference for efficiency. In order to make up the shortfall, the heat pump has to work harder to maintain the same indoor temperature. For this reason, heat pumps start to lose efficiency at around 40 degrees F and become less efficient than furnaces at around 25 degrees F.

Heat pumps continue to be effective at cooling the indoors, even at high temperatures.

In the South, it rarely gets below 25 degrees. For this reason, air source heat pumps are very common in homes. However, if you live in a climate where it regularly gets that low in the winter (Minneapolis, for example) then it’s unlikely that a heat pump is your best option for winter heating.