An air source heat pump uses electricity to move heat between outdoor air and your home. In winter, it takes heat from the outside air and transfers it indoors. In summer, it reverses the process and removes heat from your home, working like an air conditioner. ENERGY STAR cold-climate testing includes operation at 5°F.
Air Source Heat Pumps at a Glance
| Question | Answer |
|---|---|
| What is the heat source? | Outdoor air |
| What powers the system? | Electricity |
| How does it transfer heat? | A refrigerant, compressor, heat exchangers and an expansion valve |
| How does it heat a home? | It moves heat from outside to inside |
| How does it cool a home? | It moves heat from inside to outside |
| What does it deliver? | Warm air through ducts or indoor units, or hot water for radiators and underfloor heating |
| Can it work in cold weather? | Yes. Heating capacity and efficiency usually fall as outdoor temperatures decrease |
How Does an Air Source Heat Pump Heat Your Home?
An air source heat pump heats your home by using a vapor-compression cycle to absorb heat outdoors and release it indoors. The system changes the refrigerant's pressure and temperature as the refrigerant moves through the cycle.
1. The Outdoor Coil Absorbs Heat
An outdoor fan draws air across a heat exchanger called the evaporator coil. A cold, low-pressure refrigerant flows through the coil.
The refrigerant evaporates at a low temperature. As it changes from a liquid into a vapor, it absorbs heat from the outdoor air, even when the air is below freezing.
2. The Compressor Raises the Refrigerant Temperature
The compressor squeezes the refrigerant vapor. This raises its pressure and temperature, creating a hot, high-pressure gas.
The compressor uses most of the system's electricity. The outdoor fan, indoor fan, controls and, in air-to-water systems, water pumps also use electricity.
3. The Indoor Heat Exchanger Releases Heat
The hot refrigerant travels to the indoor heat exchanger, also called the condenser coil.
In an air-to-air heat pump, the indoor fan blows air over the hot coil. The system then sends warm air through ductwork or an indoor mini-split unit.
In an air-to-water heat pump, the heat exchanger transfers heat into water. The warm water can circulate through radiators, underfloor heating or a hot-water cylinder.
4. The Expansion Valve Lowers the Pressure
After releasing heat indoors, the refrigerant passes through an expansion valve. The valve lowers the refrigerant's pressure and temperature.
The cold refrigerant then returns to the outdoor coil and absorbs more heat. The cycle continues until the home reaches the thermostat setting.
How Does an Air Source Heat Pump Provide Cooling?
An air source heat pump provides cooling when a reversing valve changes the direction of refrigerant flow.
The indoor coil becomes the evaporator and absorbs heat from the indoor air. The outdoor coil releases that heat outside. The indoor unit then supplies cool air, much like a conventional air conditioner.
A ducted air source heat pump can therefore provide both heating and cooling through the same system. A ductless mini-split heat pump uses one or more indoor units to do the same job.
What Happens When an Air Source Heat Pump Gets Cold Outside?
An air source heat pump can operate in cold weather, although lower temperatures usually reduce the heat available from outdoor air and the system's heating capacity.
Cold-climate heat pumps use technologies such as advanced compressors and refrigerants to maintain performance at lower temperatures. The right system depends on the local climate, the building's heat loss and the equipment design.
The Defrost Cycle
Frost can form on the outdoor coil when the air is cold and damp. Frost restricts airflow and reduces heat transfer.
The heat pump periodically enters a defrost cycle. It may temporarily reverse the refrigeration cycle or send warm refrigerant through the outdoor coil to melt the frost. During defrost, the outdoor unit may produce visible steam and the indoor fan may pause briefly.
A backup heating system may be included in very cold climates. Options include electric resistance heat, a gas furnace in a dual-fuel system or another heating source. Whether backup heat is needed depends on the heat pump's low-temperature capacity and the home's heating requirements.
What Are the Main Parts of an Air Source Heat Pump?
An air source heat pump normally includes these components:
- Outdoor heat exchanger: Absorbs heat during heating and releases heat during cooling.
- Indoor heat exchanger: Releases heat indoors during heating and absorbs heat during cooling.
- Compressor: Raises the refrigerant's pressure and temperature.
- Expansion valve: Lowers the refrigerant's pressure and temperature.
- Reversing valve: Changes the direction of refrigerant flow so the system can heat or cool.
- Fans or water pumps: Move air or heated water around the property.
- Refrigerant pipework: Carries refrigerant between the outdoor and indoor components.
- Controls and thermostat: Adjust operation based on indoor and outdoor conditions.
What Is the Difference Between Air-to-Air and Air-to-Water Heat Pumps?
Air-to-air heat pumps transfer heat into indoor air, while air-to-water heat pumps transfer heat into water for a heating system or hot-water cylinder.
| Type | How it distributes heat | Typical applications |
|---|---|---|
| Air-to-air heat pump | Transfers heat directly into indoor air | Ducted systems and ductless mini-splits |
| Air-to-water heat pump | Transfers heat into water | Radiators, underfloor heating and domestic hot water |
| Air-to-air and air-to-water system | Some systems provide heating and cooling | Homes requiring both space heating and cooling |
Air-to-air systems usually connect to ducts or indoor fan units. Air-to-water systems connect to a hydronic heating system and may also connect to a hot-water tank.
Why Can Heat Pumps Produce More Heat Than the Electricity They Use?
A heat pump moves existing heat instead of creating heat through combustion or electrical resistance.
A heat pump with a coefficient of performance, or COP, of 3 delivers three units of heat for every unit of electricity used under a particular set of operating conditions. The COP changes with outdoor temperature, the required indoor temperature and the system's design.
ENERGY STAR states that certified air source heat pumps can deliver up to three times more heat energy than the electrical energy they consume. Seasonal performance depends on the climate, equipment, installation and controls.
What Affects an Air Source Heat Pump's Performance?
The main factors are:
- Outdoor temperature: Lower outdoor temperatures usually reduce heating capacity and efficiency.
- System sizing: An oversized or undersized system can reduce comfort and efficiency.
- Building insulation: Better insulation reduces the amount of heat the system must supply.
- Ductwork: Leaky or poorly insulated ducts can waste heated or cooled air.
- Indoor delivery temperature: Air-to-water systems generally operate more efficiently when they can provide heat at a lower water temperature.
- Installation quality: Incorrect refrigerant charge, poor airflow and unsuitable controls can reduce performance and shorten equipment life.
For a replacement system, the installer should calculate the home's heating and cooling requirements rather than sizing the heat pump from the old furnace or air conditioner. In the United States, installers commonly use an ACCA Manual J load calculation for this work.
How Do You Remember How an Air Source Heat Pump Works?
An air source heat pump moves heat rather than generating it:
- In heating mode, it moves heat from outside into the home.
- In cooling mode, it moves heat from inside the home to outside.
- The compressor drives the cycle.
- The refrigerant carries the heat.
- The reversing valve changes the direction of heat flow.
Correct sizing, suitable low-temperature performance and professional installation determine how well the system heats a particular home.