How a Heat Pump Works

How a Heat Pump Works

A heat pump transfers heat from one place to another.  It sounds simple, but where is the heat coming from if fuel isn’t being burned?

An air source heat pump system uses advanced technology and the refrigeration cycle to heat and cool your home. This allows a heat pump to provide year-round indoor comfort – no matter what the season is.

In the warmer months, the heat pump can act as an air conditioner - drawing out interior heat and humidity, and redirecting it to the outside. During colder months, heat from the outdoor air is extracted and transferred to the interior of your home. Believe it or not, even a 32°F day produces enough heat to warm your home. Science is amazing!

For example, when there is a temperature difference such as your 98.6°F body and 32°F air, heat is transferred from the warmer object to the cooler air. This is why you start to feel cold! So when you're trying to pull heat from 32°F air, you have to put it in contact with something even colder. That's the job of the refrigerant in a heat pump.

Colder months: Heat pumps pulls heat from the outside air and transfer the heat to your home.

Warmer months: Heat pumps pulls warm air and humidity from inside your home and transfer it outside, leaving cooler air indoors.

Air Conditioning Mode

When properly installed and functioning, a heat pump can help maintain cool, comfortable temperatures while reducing humidity levels inside your home.

  1. Warm air from the inside of your house is pulled into ductwork by a motorized fan.

  2. A compressor circulates refrigerant between the indoor evaporator and outdoor condensing units.

  3. The warm air indoor air then travels to the air handler while refrigerant is pumped from the exterior condenser coil to the interior evaporator coil. The refrigerant absorbs the heat as it passes over the indoor air.

  4. This cooled and dehumidified air is then pushed through connecting indoor ducts to air vents throughout the home, lowering the interior temperature.

  5. The refrigeration cycle continues again, providing a consistent method to keep you cool.

Heat Mode


Heat pumps have been used for many years in locations that typically experience milder winters. However, air-source heat pump technology has advanced over the past five years, enabling these systems to be used in areas with extended periods of subfreezing temperatures.1

  1. A heat pump can switch from air condition mode to heat mode by reversing the refrigeration cycle, making the outside coil function as the evaporator and the indoor coil as the condenser.

  2. The refrigerant flows through a closed system of refrigeration lines between the outdoor and the indoor unit.

  3. Although outdoor temperatures are cold, enough heat energy is absorbed from the outside air by the condenser coil and release inside by the evaporator coil.

  4. Air from the inside of your house is pulled into ductwork by a motorized fan.

  5. The refrigerant is pumped from the interior coil to the exterior coil, where it absorbs the heat from the air.

  6. This warmed air is then pushed through connecting ducts to air vents throughout the home, increasing the interior temperature.

  7. The refrigeration cycle continues again, providing a consistent method to keep you warm.

Parts of a Heat Pump


To get a better idea of how your air is heated or cooled, it helps to know a little bit about the parts that make up the heat pump system. A typical air-source heat pump system is a split or two-part system that uses electricity as its power source. The system contains an outdoor unit that looks similar to an air conditioner and an indoor air handler. The heat pump works in conjunction with the air handler to distribute the warm or cool air to interior spaces. In addition to the electrical components and a fan, a heat pump system includes: 

Compressor: Moves the refrigerant through the system.

Control board: Controls whether the heat pump system should be in cooling, heating or defrost mode. 

Coils:  The condenser and evaporating coil heat or cool the air depending on the directional flow of refrigerant. 

Refrigerant:  The substance in the refrigeration lines that circulates through the indoor and outdoor unit.

Reversing valves: Change the flow of refrigerant which determines if your interior space is cooled or heated. 

Thermostatic expansion valves:  Regulate the flow of refrigerant just like a faucet valve regulates the flow of water. 

The accumulator: A reservoir that adjusts the refrigerant charge depending on seasonal needs.

Refrigeration lines and pipes:  Connect the inside and outside equipment.

Heat strips: An electric heat element is used for auxiliary heat. This added component is used to add additional heat on cold days or to recover from lower set back temperatures rapidly.

Ducts: Serve as air tunnels to the various spaces inside your home.

Thermostat or control system: Sets your desired temperature.

Why is There Ice on My Heat Pump?

Don’t panic! It is quite common to see frost or even ice on your heat pump. The process of transferring heat to the refrigerant can cause excess moisture to build up on your coil. This excess moisture can freeze during extremely cold temperatures. The good news is that your heat pump was designed for this!

A properly functioning heat pump has a defrost mode that kicks in when it detects ice buildup.  The unit simply reverses the refrigerant cycle, and the heat is directed to the outdoor coil. While this is happening, the backup or auxiliary heat strips are used to heat your home until the ice is melted.  

However, if your heat pump does not thaw the ice buildup, it may be an indication that something isn’t working properly. If this occurs, call your local licensed profession HVAC dealer to have the unit inspected.

To learn even more on how a heat pump works, check out our "How a Heat Pump Works" video.


1 Air-Source Heat Pumps. (n.d.). Retrieved from