What is an Air Source Heat Pump?

Reviewed by David Carmichael, Solution Engineer (May 2024)

Heat pumps are HVAC devices that transfer and exchange heat between areas, allowing both heating and cooling of zones based on need. They use electricity alone and burn no fossil fuels themselves. Air source heat pumps exchange their heat with the exterior atmosphere and are becoming popular for efficiently heating and cooling buildings, businesses, and equipment.

First, what is an air source heat pump? It is an HVAC device that exchanges heat between the zone with cooling or heating needs (such as a building) and the exterior atmosphere (the air outdoors). Ground source heat pumps, however, pump liquid underground in a cycle exchanging a building’s heat with the earth.

Though their form and placement in the system vary, air source heat pump components typically include the following:

• Compressor: Compresses gaseous refrigerant (to increase its temperature) and pumps it throughout the air source heat pump cycle
• Exterior (outdoor) coil: Exchanges heat between the refrigerant and exterior air
• Interior (indoor) coil: Exchanges heat between the refrigerant and either the space’s air or the pumped water used to distribute heat in the space
• Expansion valve: Reduces the refrigerant’s pressure after it releases its heat and liquefies
• Refrigerant hoses/pipes: Transport heat between the coils; insulated to prevent losing heat
• Reversing valve: Switches refrigerant flow direction, allowing toggling between heating and cooling. This valve has provisions for always sending refrigerant through the compressor and expansion valve (one-way devices) in the proper directions
• Fans: The exterior coil’s fan blows air across the refrigerant lines to exchange heat. The interior coil will have its own fan if it exchanges heat with air, but refrigerant-to-water systems use other heat-exchanging technology
• Power cable: Supplies electrical power to the compressor, fans, controls, and instrumentation
• Heating elements: Defrosts exterior components in cold weather

Now that we’ve defined air source heat pump components, just how do air source heat pumps work? They follow the refrigeration cycle seen in air conditioners and refrigerators, though they can reverse the direction of this cycle, thereby achieving either heating or cooling.

When the air source heat pump cycle is heating:

1. The exterior coil’s fan blows exterior cold air across the coil
2. This air does still contain usable heat, and the liquid refrigerant absorbs it and becomes a gas. The exterior coil is thus the evaporator
3. The compressor pumps the gaseous refrigerant and compresses it. The compressing work done to the refrigerant (using electrical energy) increases its temperature and enhances the system’s heat gain
4. The refrigerant exchanges heat with the interior coil, delivering the heat either directly into the indoor blown air or into water pumped throughout the building’s heating system
5. The refrigerant liquefies as it loses heat (making the interior coil the condenser), loses pressure as it passes through the expansion valve, and re-enters the exterior coil to restart the cycle

When the air source heat pump cycle is in cooling mode, the reversing valve sends the refrigerant through the two coils in the opposite direction.

1. The refrigerant instead picks up heat from the interior coil, which is now the evaporator
2. This removal of heat has the effect of cooling the room
3. The fan of the exterior coil (now the condenser) blows this collected heat into the outside air

Air source heat pump benefits start with having a single HVAC unit handling an area’s heating and cooling. Rather than having a heater or boiler and an air conditioner, an air source heat pump system means one unit to buy, make space for, install, and maintain.

Air source heat pump efficiency for heating is greater than in furnaces or boilers since heat pumps move heat around rather than generate it. The electrical energy they consume mainly runs the fans and compressor, allowing a large energy ‘profit’ compared to the energy inputs. This high efficiency means reduced costs and emissions.

Also, these pumps are quieter than furnaces and lack their hazards: carbon monoxide, fire, natural gas, and allergens.

Air source heat pump components have different types with different possible arrangements of the air source heat pump cycle. When configuring your air source heat pump installation, consider the following:

• Ducts: Ductless pumps can be ideal for retrofits since they only need a small wall hole for routing hoses between the coils. Ducted units are good for newly built buildings designed to use an air source heat pump
• Zones: A single-zone system has one interior coil unit blowing heated or cooled air into a room. Air source heat pump installations can also feature one exterior coil serving multiple interior coils (and rooms)
• Interior coil: It’s typically installed inside, ‘split’ from the exterior coil, but air source heat pumps exist with both coils built into one exterior unit. These units send their heated or cooled air or water to the building via ducts or hoses
• Interior fan: Some air source heat pumps are small, simple units meant for cooling equipment. Only their exterior coil has a fan, while the inner coil exchanges refrigerant and inner air heat without forced airflow, similar to standard radiators

The UK government is pushing for more businesses to install heat pumps as part of its goals for complete net zero by 2050. This includes incentives like the Boiler Upgrade Scheme: a grant to assist with the transition cost of switching to greener industrial heating. Some scepticism remains about heat pump effectiveness in the dead of winter, but they have proven effective and popular in cold European countries like Sweden, Switzerland, and Norway. This shows there is the potential for the UK to fully seize air source heat pump benefits.

Browse RS to learn about how our air source heat pumps are compact, efficient, and effective choices for your machinery cooling needs.