Below is a list of terms used in heat pump systems.  You can sort the table alphabetically or enter a search term to find the information faster.

TermMeaning
Air SourceHeat energy is extracted from ambient air and transferred to another medium. The heat available reduces as the temperature of the air reduces reducing both performance duty and efficiency.
Air to AirA heat pump system where the heat energy is extracted from air and delivered another space via air. This is common in commercial split systems.
Air to WaterA heat pump system where heat is extracted from ambient air and transferred to the water used to heat a space or provide domestic hot water.
AntifreezeA chemical added to water to lower the freezing point. Most common examples in heat pump systems are ethylene or propylene glycol.
Buffer TankA large water cylinder used to improve system efficiency by reducing the number of stop/starts the compressor makes. This is required less with modern variable-speed heat pumps.
Closed LoopHeat energy is collected via pipes placed within the ground or body of water within which a water/glycol mix is circulated. The heat pump extracts the heat energy from the fluid which is always contained within the system.
Coefficient of Performance (CoP)An expression of the output a proportion of input power used to drive the compressor and fans. It is a ratio and therefore does not have a unit of value. See also CoSP and EER.
Coefficient of System Performance (CoSP)The efficiency of a heat pump taking into account the input power from internal control circuits, compressor, fans and pumps of all elements of the heating system, not just the heat pump itself. It is a ratio and therefore does not have a unit of value. CoSP is usually used to determine financial incentives such as the Renewable Heat Incentive. See also CoP, SCoP , SPF, EER.
CompressorThe compressor in a heat pump serves two purposes. The first is to circulate the refrigerant fluid through the circuit, and the second is to compress and raise the pressure and temperature of the refrigerant vapour so that it can easily be condensed back into a liquid as part of the heat transfer process.
CondenserThe condenser is the heat exchanger where hot compressed refrigerant gas is condensed to a liquid, losing energy to the surrounding medium.
DeemedA term used in RHI payment calculations to describe the amount of energy a building is estimated to use over a year. It is calculated by an assessor for use in an Energy Performance Certificate (EPC).
Defrost CycleIf an air source heat pump systems operate at low ambient temperatures, the evaporator in the outdoor unit is likely to be below freezing point and moisture within the air will freeze on it, forming a coating of ice which is removed with an automatic defrost cycle. This defrost cycle is controlled automatically by a combination of time and temperature of the external coil. This defrost cycle has a small effect on the efficiency of the device. (see SPF)
Direct Expansion or DX SystemWhere refrigerant flows directly in ground source collectors. A less common approach.
Domestic Hot Water (DHW)The water that comes out of taps. In a heat pump system, this is always stored in a domestic hot water cylinder.
EfficiencyThe general ratio of useful output energy to energy input. More specific terms are used to describe these exact outputs and inputs to ensure that the purpose of the system is the focus of the efficiency calculation.
EmitterA term used to describe a radiator or underfloor heating system. The emitter emits heat energy to the building. The term is used to allow all types of system to be grouped together descriptively.
Energy Efficiency Ratio (EER)When a heat pump is used in cooling mode, EER is used to describe the efficiency. The rated cooling capacity is divided by the rated total power input. It is a ratio and therefore does not have a unit of value. See also CoP, SEER.
Energy Performance Certificate (EPC)A formal document created by a qualified Energy Assessor that describes the energy performance of a building. This is required to apply for financial incentives and is also a legal requirement for landlords when letting.
EvaporatorThe evaporator is the heat exchanger where the refrigerant fluid is evaporated as it absorbs heat from the surrounding air or water.
Expansion Control DevicesThe flow and evaporation of the refrigerant within the evaporator is controlled by expansion control devices at the entrance to the evaporator. The 3 most common types are capillary tubes, thermostatic expansion valves (TEVs) and electronic expansion valves (EEVs)
Geothermal EnergyGeothermal energy refers to drilling into the earth's crust to extract heat. It is not the same as the collectors used in ground source heat pumps and should not be confused with it.
Ground SourceIn a ground source system, heat is extracted from the ground passing water/glycol through tubes buried in the ground either horizontally in loops or straight tubes, or vertically in boreholes.
Heat ExchangerA device that transfers heat energy from one medium to another. A common and easily recognised example would be a household radiator.
Heat PumpA device for transferring energy in the form of low-grade, low-temperature heat from one location to another.
Heat SinkThe element of the system where heat is usefully dissipated. This could be radiators in a room, underfloor heating etc. Other terms are heat load and heat emitter.
Heating CurveIn order to adequately heat a building on cold days, the flow temperature of the water in the heating system needs to be higher. On warm days it can be lower. The heating curve describes what the flow temperature should be at different ambient temperatures.
InverterA sophisticated electrical device that can vary the output of a heat pump by changing the speed of the compressor, and therefore the heat output.
Inverter or Variable Speed Drives (VSD)Older heat pumps were simple on/off devices. When on, they would operate at full output. This is rarely required in real-world solutions, and so the use of an inverter allows the speed of the compressor to be varied. The output of the heat pump can therefore be adjusted, ensuring it is operating at maximum efficiency.
Kilowatt (kW)This is a unit of power and is used to specify the output and input of a Heat Pump. It can refer to the electrical or thermal power and is a Kilojoule of energy per second (kJ/s).
Kilowatt Hour (kWh)This is a unit of energy and is the equivalent one kW of power applied for one hour, or, for example, two kW of power applied for half an hour. A kWh is equivalent to 1 kJ/s for 1 hour. The unit can be applied to both electrical and thermal energy.
Open LoopWhen water is extracted from either the ground or body of water and passed directly through a water source heat pump and then ejected, this is open loop.
RefrigerantThe fluid in a heat pump used to transfer heat, Normally this is a refrigerant with a low-temperature boiling point. All refrigerants now used do no damage the ozone layer, but can contribute significantly to global warming if released and should only be handled by qualified personnel.
Refrigeration CycleIn this cycle, heat energy is extracted from a source, upgraded in temperature and delivered to another location and purpose. The most common method is the vapour compression cycle.
Renewable Heat Incentive (RHI)A financial incentive offered by the UK Government to encourage the uptake of renewable energy systems. More information is available on our RHI page at Technical->Renewable Heat Incentive.
Reverse Cycle Heat PumpA refrigeration system that uses a valve to reverse the flow of the refrigerant and change the operation of the system from heating to cooling. This process can also be used to provide a defrost cycle.
Seasonal Coefficient of Performance (SCoP)The performance of a heat pump over a typical season where the source temperature varies. Commonly applied to air source heat pumps as the source temperature varies considerably over the year. SCoP is dependant on the local climate and the EU has been divided into 3 regions defined in BSEN 14825.
Seasonal Energy Efficiency Ratio (SEER)The performance of the unit over a typical cooling season where the source temperature varies. Commonly applied to air source heat pumps as the source temperature varies considerably over the year. SEER is dependant on the local climate and the EU has been divided into 3 regions defined in BSEN 14825.
Seasonal Performance Factor (SPF)The efficiency of a heat pump described by the ratio of heat output to total energy input (including pumps, fans etc).
SlinkyA type of collector used in horizontal ground source systems. The slinky is a series of coils buried in a trench.
Soft StartAn electrical device that reduces the start "inrush current" surge that is taken by a conventional compressor. They do not save energy, but may improve wear and tear.
Water SourceHeat energy is extracted directly from a body of water. In the UK permission is usually needed from the Environment Agency for extraction of groundwater.
Water to AirA heat pump where the collecting medium (source) is either groundwater or a glycol solution and the destination medium is air. The source side is usually indirect but the delivery side is normally direct.
Water to WaterA heat pump where the collecting medium (source) is either groundwater or a glycol solution and the destination medium is also water or glycol. Normally indirect systems.