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How Does an Air-Source Heat Pump Work? An air-source heat pump performs as an air conditioner in the summer and a heating system during the winter. They run on electricity and can either work with your existing ductwork or serve as a ductless system.

Related: How noisy are heat pumps?

What Is an Air Source Heat Pump?

It takes in heat from the outside air and transfers it to an indoor space. We can classify air source heat pumps into two categories:

  1. Air-Air Heat Pumps
  2. Air-Water Heat Pumps

Air-Air Heat Pumps are most common in Canada. They heat or cool the air inside your home. Based on the installation type, you can categorize them further:

  • Ducted: Air is heated or cooled by passing over the coil located in a duct. Subsequently, it distributes the treated air through the ductwork to different locations in the home.
  • Ductless: You will find the heat pump’s indoor coil in an indoor unit. Typically installed on the floor or wall, indoor units heat or cool the air in that room. 

Air-air systems operate more efficiently when the gap between inside and outside temperature is relatively small. Consequently, air-air heat pumps optimize their efficiency by heating a higher volume of warm air between 25 and 45°C. 

Air-Water Heat Pumps are far less common in Canada. They heat or cool water in conjunction with hydronic (water-based) distribution systems such as low-temperature radiators, radiant floors, or fan coil units. The heat pump provides thermal energy to the hydronic unit in the winter. In the winter, the process reverses by extracting thermal energy from the hydronic system.

The type of hydronic system is essential when researching air-water heat pumps. For instance, air-water heat pumps run more efficiently when heating the water to lower temperatures, i.e., below 45 to 50°C. As a result, they are a better match for radiant floors or fan coil systems. 

How Do Air Source Heat Pumps Work?

Air-source heat pumps have three cycles:

  • Heating Cycle: Supplies thermal energy to the building
  • Cooling Cycle: Removes thermal energy from the building
  • Defrost Cycle: Removes the buildup of frost on the outdoor coils

The Heating Cycle

The heating cycle involves taking heat from the air outdoors and “pumping” it indoors:

The liquid refrigerant first passes through the expansion device and transforms into a low-pressure liquid/vapour mixture. It then goes to the outdoor coil that acts as the evaporator coil. The heat from the air outdoors is absorbed by the liquid refrigerant, which causes it to boil and become a low-temperature vapour.

The vapour then moves through the reversing valve to the accumulator that collects any liquid that remains before the vapour enters the compressor. Subsequently, the vapour compresses, which reduces its volume and causes it to heat up.

The reversing valve then sends the now-hot gas to the coil, which is the condenser. Subsequently, heat from the hot gas heat transfers to the indoor air, which causes the refrigerant to condense into a liquid. Finally, the fluid returns to the expansion device, and the cycle repeats. You will find the indoor coil in the ductwork near the furnace.

The temperature outdoors affects the ability of the heat pump to transfer heat from the outside air to the air. If the outdoor temperature drops, the heat pump’s ability to absorb heat drops as well.

For many air-source heat pumps, this means that there’s a temperature, which is known as the thermal balance point, where the heating capacity of the heat pump is equal to the house’s heat loss. Below the thermal balance point, the heat pump cannot comfortably heat a living space. Thus, supplementary heat is required.

It is worth noting that most air-source heat pumps have a minimum operating temperature below which they cannot operate. It can range anywhere from -15°C to -25°C for newer models. Below this temperature, you would have to use a supplemental system to provide heating to the building.

The Cooling Cycle

The heat pump reverses the process described above and cools the house during summer by removing the indoor air and depositing it outside.

The liquid refrigerant passes via the expansion device in the heating cycle and transforms into a low-pressure vapour/liquid mixture. It then goes to the indoor coil that acts as the evaporator. The heat from the indoor air is absorbed by the liquid refrigerant causing it to boil and become a low-temperature vapour.

The vapour moves through the reversing valve to the accumulator that collects any remaining liquid and then goes to the compressor. The heat pump compresses the vapour, which reduces its volume and causes it to heat up.

Finally, the gas that’s now hot moves through the reversing valve to the outdoor coil that acts as the condenser. Subsequently, the heat emanating from the hot gas transfers to the outdoor air, which causes the refrigerant to condense into a liquid. After that, the fluid returns to the expansion device, where the cycle repeats.

The heat pump dehumidifies the indoor air during the cooling cycle. Moisture in the air condenses on the coil’s surface while passing over it and collects in a pan at its bottom. This pan connects to the house drain via a condensate drain.

The Defrost Cycle

If the outdoor temperature drops to near or below the freezing point when the heat pump operates in heating mode, the moisture present in the air coil will condense and freeze while passing over the outside coil. The level of frost buildup will depend on the level of humidity in the air and the outdoor temperature.

The buildup of frost reduces the coil’s efficiency since it reduces its ability to transfer heat to the refrigerant. The heat pump has to get rid of the frost by switching into defrost mode at some point.

The most common approach is:

Once the device switches to cooling mode, the heat pump sends hot gas to the outdoor coil, thus melting the frost in cooling mode. Simultaneously, the outdoor fan, which typically blows cold air over the coil, shuts off, thus reducing the level of heat needed to melt the frost.

The heat pump is cooling the air in the ductwork while all this is happening. The heating system would usually warm this air while it circulates throughout the house.

One of two methods determines when the unit enter defrost mode:

  • Demand-frost controls monitor refrigerant pressure, airflow, air or coil temperature pressure differential across the outdoor coil to detect frost accumulation.
  • Time-temperature defrost initiated and terminated by either temperature sensor or a preset interval timer on the outside coil. The cycle commences every 30, 60, or 90 minutes depending on the system’s design and climate.

The seasonal performance of the heat pump is reduced by unnecessary defrost cycles. Therefore, the demand-frost method is generally more efficient because the defrosts cycle only initiates when required.

Supplementary Heat Sources

The minimum outdoor operating temperature for air-source heat pumps is -15°C to -25°C, which is why it is crucial to consider a supplemental heating source for air-source heat pumps. Supplementary heating may also be needed when the heat pump is defrosting.

Various options are available:

All Electric: The ductwork’s electric baseboards or electric resistance elements supplement the heat pump in this configuration. These resistance elements tend to be less efficient than the heat pump, but their ability to provide heating doesn’t depend on the outdoor temperature.

Hybrid Systems: The air-source heat pump uses a supplemental system such as a boiler or furnace in a hybrid system. Hybrids work well for new installations and when adding a heat pump to an existing system, such as when a heat pump replaces a central air-conditioner.

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