Moving heat instead of making it
Most heating systems create heat, either by burning a fuel or by passing electricity through a resistive element. A heat pump works differently: it moves existing heat from one place to another. Even cold outdoor air contains thermal energy, and a heat pump can collect some of it and deliver it indoors.
A familiar example is the refrigerator. It pulls heat out of the food compartment and releases it at the back. A heat pump does something similar, but the direction is aimed at warming a home. Many models can also run in reverse in summer, taking heat from the indoor air and releasing it outside, like an air conditioner.
The cycle in four steps
A heat pump circulates a refrigerant, a fluid that boils and condenses at low temperatures. In heating mode the cycle is typically described like this:
- Evaporator. The refrigerant, which is colder than the outdoor air, passes through a coil outside. It absorbs heat from the air and evaporates into a gas.
- Compressor. The gas is compressed, which raises its pressure and temperature. This is the main step that uses electricity.
- Condenser. The hot gas passes through a coil inside, or a heat exchanger connected to the home’s heating system. It releases heat and condenses back to liquid.
- Expansion valve. The liquid pressure drops, cooling it so it can absorb heat again, and the cycle repeats.
The key idea is that compressing a gas makes it hotter than the space you want to warm, so heat flows into the house even though the source was cold.
Why the output can exceed the input
Because most of the delivered heat is moved rather than generated, a heat pump can deliver several units of heat for each unit of electricity. The ratio is called the coefficient of performance, or COP. A COP of 3 means roughly three units of heat for every one unit of electricity used, at the conditions tested. This is not creating energy from nothing; the extra comes from the outdoor environment.
Compare this with a simple electric resistance heater, where one unit of electricity gives about one unit of heat. Seasonal ratings combine performance across a range of temperatures, and the names of the metrics differ between regions. When comparing models, check which test conditions a rating assumes and whether it applies to your climate.
Performance falls as the temperature difference between outside and inside grows. On a very cold day the same machine is less efficient than on a mild one.
Common types
- Air-source heat pumps take heat from outdoor air. They are the most common and usually the least expensive to install.
- Ground-source (geothermal) heat pumps use pipes buried in the ground or in a water source. Ground temperatures are steadier than air temperatures, which can improve performance, but installation tends to cost more and needs space or drilling. This is distinct from the deep geothermal energy used for power generation.
- Ducted and ductless systems. Ducted units use existing duct work. Ductless “mini-split” units serve individual rooms or zones and are often used in homes without ducts.
- Air-to-water systems heat water for radiators, underfloor heating or hot water tanks. These are also relevant to water heating, since some models supply domestic hot water.
Where they tend to suit
Heat pumps often suit homes that:
- are reasonably well insulated, so the heating demand is modest;
- have heat emitters designed for lower water temperatures, such as large radiators or underfloor heating, if using an air-to-water system;
- are in climates where winters are mild to moderately cold, although modern cold-climate models are designed to work in much lower temperatures;
- are being fitted when an old furnace, boiler or air conditioner needs replacing.
They may be less straightforward in homes with poor insulation, very high heat loss or older systems that depend on high water temperatures. A professional assessment should check heat loss and sizing, because an undersized unit struggles on cold days and an oversized one cycles on and off inefficiently. Better home insulation and attention to air leaks can improve the case for a heat pump.
Running costs depend on prices
Whether a heat pump saves money compared with another fuel depends on local energy prices, especially the relative price of electricity and natural gas or oil. Where electricity is expensive relative to gas, savings may be small or absent even with a good COP. Where electricity is cheaper or the old system uses costly fuel, savings can be larger. Prices change, so a comparison based on today’s rates may not hold later. The electricity itself also has a source, so emissions depend on the local energy mix and vary from one grid to another.
Practical considerations
- Noise and placement. The outdoor unit has a fan and compressor and needs space, airflow and clear drainage.
- Defrost cycles. In cold, humid weather the outdoor coil can frost up, and the system periodically defrosts it, which uses some energy.
- Backup heat. Some systems include resistance heating or keep an existing furnace for the coldest periods. Ask how this is controlled, since heavy use of backup reduces savings.
- Refrigerants. Refrigerants are greenhouse gases if they leak, and rules about which ones are used differ by region. Proper installation and servicing matter.
Installation and safety
Refrigerant handling and electrical connections are regulated work. In many places they require licensed or certified technicians, and this is not a task for amateurs. Electrical capacity at the house may also need to be checked, since a new heat pump adds load to the panel. Quotes from more than one qualified installer, including the heat-loss calculation they used, give a better basis for a decision than headline efficiency figures.