How Does a Heat Pump Work in the Winter? (SURPRISING Answer!)

When looking for energy-efficient heating solutions, few options compare to heat pumps. 

There are 2 popular types of heat pumps: ground-based and air-based. Ground-based systems use the heat from under the ground. This stays quite stable through the year so the winter doesn’t affect its operations. Air-based systems have to work harder during winter as the air contains less heat. The technology is still evolving but you can get heat from air-based systems during the winter as well.

So, how does a heat pump work in the winter? Keep reading to find out!


What Is a Heat Pump and how do they look like?

Heat pumps use the same principles as refrigerators and air conditioners. They transport a fluid to change the temperature.

It’s a system of indoor and outdoor coils that moves (pumps) heat energy by using a refrigerant. It can either heat or cool the air inside, depending on the season. There are 2 main types: air and ground-based systems. Ground-based systems use geothermal energy. This is collected by drilling a hole in the ground. The core of the earth is warm and we use this heat. Air-based systems use the warmth of the air.

Heat pumps consist of:

  • Outdoor Unit: Looks like an air-conditioning outdoor unit in air-source heat pumps and is buried underground in ground-source (geothermal) heat pumps. It contains coils, a compressor, an expansion valve, and a reversing valve.
  • Indoor Unit: Can either be a series of coils buried in the walls (like those in HVAC systems) or a separate unit with fans, which looks like an air-conditioning unit in mini-split systems.
  • Refrigerant: The substance that flows through the outdoor and indoor coils. It’s responsible for the heat moving or pumping action through serial vaporization and liquefaction cycles.

How Do Heat Pumps Work in the Winter?

Heat pumps rely on the idea of manipulating the boiling point of the refrigerant. 

Under high pressure or low temperature, the refrigerant is in a liquid state. When depressurized or heated, it turns into gas. 

You start with a cool, low-pressure vapor that’s heated by the outside air (or by geothermal energy in Ground Heat Pumps). As it enters the compressor, it’s pressurized and heated up even more.

The laws of physics dictate that heat moves from the hotter object to the cooler object until they’re both at the same temperature. So, since the emerging vapor is hot and at a higher pressure, the cold air inside the home draws the heat and becomes warmer.

Once the refrigerant has given up enough heat and is liquified as a result, the liquid moves out to pass through an expansion valve or device. 

The expansion valve releases the pressure the refrigerant liquid is under. It then vaporizes, loses even more heat, and passes through the outdoor coil to reach the compressor once again.

This is a simplified version of the refrigeration cycle, but it’s enough to know that the heat pump is a carrier system, not a generator.

Air-based systems produce less heat when it is really cold but can still provide some comfort. Ground-based systems are less affected by the cold and keep working.

Some things that we have to take into account during winter:

  1. Due to the difference in temperature between the refrigerant inside the outdoor coil and the ambient air, water vapor condenses and freezes on the coils. This requires “defrostingcycles that run the heat pump in reverse, which pauses indoor heating for a while.
  2. Heat pumps can work in freezing temperatures that reach -13℉, but lower than that and the mechanism can’t work as well and might even stop completely. This requires a supplemental heating system, such as a furnace or multiple space heaters, to be used when the temperature drops too low.

Can Heat Pumps Work in the Summer?

Unlike air conditioners, heat pumps are reversible systems. This means they can cool and dehumidify the inside air in the summer just as well as they heat it in the winter thanks to the reversing valve.

The reversing valve simply reverses the direction of the cycle so the low pressure, cool liquid refrigerant is pumped into the indoor coil. 

The hot air indoors heats and vaporizes the liquid, and the ambient moisture condenses on the surface of the coils, dehumidifying the air. The resulting refrigerant vapor in the outdoor coils is pressurized and liquified by the compressor. 

Lastly, the hot, high-pressure liquid passes through the expansion valve, which depressurizes and cools it, and the cycle starts again.

How Do Heat Pumps Save Energy?

To examine this, we can take a look at two measurements: COP and HSPF. These will allow us to understand how efficient our heat pump is.

Unlike space heaters that turn electric energy into heat by using resistors, a heat pump can use the same amount of electric energy to move more heat energy from the outside indoors. This results in a better Coefficient of Performance (COP).

Coefficient of Performance is a metric used to tell us how much heat you can get out of one watt of energy. The COP of space heaters is 1 since all the electricity that reaches them is turned into heat. Heat pumps, on the other hand, can reach COPs of 4 or more.

This is simply due to the fact that the compressor that pressurizes the refrigerant uses a fraction of the energy needed by a space heater. The resulting high-pressure vapor then carries the outdoor heat inside to be diffused into the air. 

In the USA the HSPF (Heating Seasonal Performance Factor) rating is used by the Department of Energy. It measures the efficiency of the heat pump over a year (so both during summer and winter). This system makes sure that the heat pumps that are being installed are efficient enough. By 2023 it is expected that new heat pumps have an HSPF of 8.8. This makes it easy to compare heat pumps and find the right one for your needs.

You can use the HSPF score to see how much energy you are going to save. For example, a system with 9.4 HSPF is 12% more energy efficient than one that has an HSPF of 8.4 (you divide 9.4/8.4 to get this result).

Both HSPF and COP allow you to compare the efficiency of your heat pump. This makes it easier to compare systems based on a standardized method. You can also take a look at the SEER rating if you use your heat pump system to cool your house.

A heat pump can produce a lot of heat for the amount of electricity it uses. It’s also a more efficient option than furnaces that burn biomass or fossil fuel for heating. While the initial investment of getting a heat pump system might be daunting, the long-term savings on fuel costs should make up for it. It’s a cleaner, more eco-friendly option as well.

Are There Other Types of Heat Pumps?

There are other types of heat pumps that might suit your needs other than the standard. One type that doesn’t rely on electricity to run is called an absorption heat pump. It utilizes other energy sources such as natural gas, solar power, propane, and even geothermally-heated water to run its cycle. 

Instead of compressing a refrigerant, this heat pump works by using an ammonia-water absorption cycle to heat and cool the air. It uses a low pump to increase the pressure in the ammonia solution, and then a heat source to boil the ammonia out of the water.

Another type called a reverse cycle chiller (RCC) is similar to the air-source heat pump, but instead of heating or cooling the air it heats and cools an insulated water tank. The water from the tank is then pumped through piping to the places that need temperature adjustment.

The great thing about RCCs is that they eliminate the need for defrosting cycles that normal air-source heat pumps have by simply pumping the warm water around the coils.


Heat pumps are an innovative way to save energy, money, and the planet while you’re heating your home or office. 

Instead of wasting a lot of energy to get sufficient heat, they utilize the refrigeration cycle to get the outdoor heat indoors.

If it is too cold, some heat pumps (especially air-based) might stop working.

The technology is still evolving so it will be interesting to see how it evolves. It is possible that a combination of solar panels and batteries will be used to power electric heaters. This could be an alternative for heat pumps in some locations.