Heat guide
How Heat Pumps Move Heat
A heat pump uses work to move thermal energy from a colder source to a warmer sink rather than converting electricity directly into heat at the point of use.
Why COP can exceed 1
Coefficient of performance for heating is useful heat delivered divided by work input. Because a heat pump transfers environmental or recovered heat in addition to the electrical work supplied, heating COP can exceed 1 without violating conservation of energy.
The refrigeration cycle
Most vapour-compression heat pumps circulate refrigerant through evaporation, compression, condensation and expansion. The evaporator absorbs heat from the source, while the condenser releases heat to the heated space or water.
Temperature lift affects efficiency
The larger the temperature difference between the heat source and the delivery temperature, the harder the compressor must work. COP therefore tends to fall as outdoor air becomes colder or supply temperatures become higher.
Rated COP is not annual performance
Real seasonal performance depends on weather, defrost cycles, part-load behaviour, controls, distribution temperature and auxiliary resistance heating.
Comparing heat pumps with furnaces
Operating cost comparisons should use useful heat delivered, not appliance input alone. Fuel price, furnace efficiency, electricity price and heat-pump COP all matter.
Defrost and auxiliary heat can affect cold-weather performance
An air-source heat pump operating in cold, humid weather can accumulate frost on the outdoor coil. Periodic defrost cycles temporarily reverse or alter operation to melt that frost, which reduces delivered heating efficiency during those periods. Some systems also use electric resistance or another backup heat source when compressor capacity is insufficient.
Annual performance therefore depends on climate, system sizing, controls and equipment characteristics across the full temperature range. A single rated COP at one outdoor temperature cannot describe the entire heating season.
Source and sink temperatures set the thermodynamic challenge
A heat pump works harder as the required temperature lift between source and sink increases. Milder outdoor conditions or lower heating-water temperatures can improve efficiency, while very cold source temperatures or high delivery temperatures generally reduce COP. This is why emitter sizing and control temperatures matter alongside the heat-pump unit itself.
A heat pump moves heat instead of creating it directly
A heat pump uses work, usually supplied by an electric compressor, to move thermal energy from a lower-temperature region to a higher-temperature region. In heating mode, the useful heat delivered indoors includes both the energy absorbed from the outdoor source and the compressor work added to the refrigerant cycle.
This is why a heat pump can deliver more than one unit of heat for each unit of electrical energy consumed. The coefficient of performance, or COP, is a ratio of useful heating or cooling to work input, not an efficiency in the same sense as a resistive heater.
Outdoor temperature changes heating performance
As the outdoor source becomes colder, an air-source heat pump usually has to operate across a larger temperature lift. Compressor work tends to increase while available heating capacity and COP can fall. Frost formation on the outdoor coil can also require periodic defrost operation under suitable cold and humid conditions.
A single seasonal efficiency label therefore cannot describe every hour of operation. Equipment selection should use performance data for the climate and design conditions that matter for the building.
The refrigeration cycle links pressure and temperature
The compressor raises refrigerant pressure and temperature, the condenser rejects heat as the refrigerant cools and often condenses, the expansion device reduces pressure, and the evaporator absorbs heat as the refrigerant warms and often evaporates. Phase change allows large energy transfer with relatively modest refrigerant mass flow.
Real systems include pressure drops, compressor inefficiency, superheat, subcooling and control strategies. The four-component description is a useful framework, but detailed performance requires refrigerant-property data and manufacturer information.
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Sources and further reading
Use the linked primary or authoritative resources for additional detail, standards and source-specific conditions.