Heat Pump Glossary: Terms Explained

A B C D E F G H I K L M N O P R S U V W Z

Air source heat pump: An air source heat pump takes heat from the outside air using a refrigerant cycle and moves it into your home for heating and hot water. It works even when the air is well below freezing and runs on electricity.
Air-to-air heat pump: An air-to-air heat pump takes heat from outside air and blows warm air into rooms through indoor units, and many can also cool. It does not provide hot water. In England and Wales homes, a £2,500 Boiler Upgrade Scheme grant applies from 28 April 2026.
Air-to-water heat pump: An air-to-water heat pump takes heat from outside air and transfers it to water for radiators, underfloor heating and a hot water cylinder. It is the most common type for whole-home heating and can qualify for the Boiler Upgrade Scheme in England and Wales.
Anti-freeze valve: An anti-freeze valve automatically opens to drain water from outdoor pipework if the temperature falls close to freezing and the system is not running, such as during a power cut. It protects the heat pump without adding glycol.
Boiler Upgrade Scheme (BUS): The Boiler Upgrade Scheme is a government grant in England and Wales only, giving £7,500 towards an air source heat pump. The MCS-certified installer applies to Ofgem and deducts the grant from your quote. The scheme runs to 2030.
Buffer tank: A buffer tank is a small water store in the heating circuit that adds volume and separates the heat pump's flow from the radiators. It can help with defrost cycles and prevent short cycling, but is not needed in every system.
Cascade system: A cascade system links two or more heat pumps so they work together as one larger system, with controls sharing the load between them. It is used for larger homes and commercial buildings where one unit is not big enough.
Commissioning: Commissioning is the final stage of installation, when the heat pump is filled, tested, set up and adjusted so it runs as designed. It includes checking flow rates, controls and safety devices, and is followed by a handover of how the system works.
Condensate: Condensate is water that forms on the outdoor unit's coil as it draws heat from the air, especially during defrost cycles. It needs to drain away safely, usually to a soakaway or drain, so it does not freeze on paths.
COP (coefficient of performance): COP is the amount of heat a heat pump produces for each unit of electricity it uses at one moment, under set test conditions. A COP of 3 means 3 kWh of heat from 1 kWh of electricity. It changes with outdoor temperature and flow temperature.
Defrost cycle: A defrost cycle is when the heat pump briefly reverses or pauses to melt frost that builds up on the outdoor coil in cold, damp weather. It is normal, usually lasts a few minutes, and a little vapour from the unit is expected.
Delta T: Delta T is the difference between the flow temperature leaving the heat pump and the return temperature coming back. Heat pump systems are usually designed for a small delta T, often around 5°C, which affects pipe and radiator sizing.
Design flow temperature: Design flow temperature is the highest water temperature the heating system is designed to need on the coldest day of the year. A lower design flow temperature improves efficiency and often decides which radiators need upgrading.
Design outdoor temperature: Design outdoor temperature is the cold-day temperature used in the heat loss calculation, based on your home's location and altitude. The heat pump is sized to keep your home warm at this temperature, so colder parts of the UK use a lower figure.
Diverter valve: A diverter valve switches the heat pump's output between the heating circuit and the hot water cylinder. Most heat pumps heat either the house or the cylinder at any moment, and the valve directs water to whichever needs it.
DNO (distribution network operator): The distribution network operator runs the local cables and substations that bring electricity to your home. The installer notifies it about a heat pump where required, so it can confirm the local supply can take the added load.
Draught-proofing: Draught-proofing means sealing unwanted gaps around doors, windows, floorboards, loft hatches and chimneys to stop warm air escaping. It is a low-cost way to reduce heat loss, which can mean a smaller heat pump and lower running costs.
Emitter: An emitter is anything that gives out heat into a room, such as a radiator, an underfloor heating circuit or a fan coil. Emitters must be sized for the heat pump's lower flow temperature to keep each room warm.
EPC (Energy Performance Certificate): An Energy Performance Certificate rates a home's energy efficiency from A to G and suggests improvements. Since 28 April 2026, a valid EPC is no longer required for the Boiler Upgrade Scheme in England and Wales.
F-gas: F-gas refers to fluorinated greenhouse gases, including some refrigerants such as R32, and the rules that control their use. By law, refrigerant work on split system heat pumps must be done by an F-gas certified engineer.
Fan-assisted radiator: A fan-assisted radiator has small built-in fans that push air across the heating element, giving more heat from a compact size at low flow temperatures. It can suit rooms where a larger standard radiator will not fit.
Flow temperature: Flow temperature is the temperature of the water the heat pump sends out to your radiators or underfloor heating. Heat pumps run most efficiently at low flow temperatures, typically around 35–50°C, compared with 60–70°C from many gas boilers.
G3 qualification: G3 refers to Part G3 of the Building Regulations in England and Wales, which covers unvented hot water systems; installers take a G3 unvented qualification to work on them. Fitting an unvented cylinder must be done by someone qualified and competent in this work.
Glycol: Glycol is an anti-freeze fluid that can be added to the water in a heat pump system to protect the outdoor unit and pipes from freezing. It slightly reduces efficiency, so many systems use anti-freeze valves instead.
Ground source heat pump: A ground source heat pump takes heat from the ground through pipes buried in trenches or boreholes. It is usually more costly to install than an air source heat pump because of the groundworks, but ground temperatures stay steadier through winter.
Heat loss calculation: A heat loss calculation works out how much heat each room loses through its walls, floors, windows, roof and ventilation on a cold day. MCS installations use a room-by-room method, and the result sets the size of heat pump and radiators.
Heat pump cylinder: A heat pump cylinder is a hot water cylinder designed for a heat pump, with a larger heating coil than a boiler cylinder. Every air-to-water installation needs one, because there is no combi-style option. See hot water cylinders.
Heat pump tariff: A heat pump tariff is an electricity tariff from some suppliers with cheaper rates for heat pump use or at set times of day. Combined with smart controls, it can lower running costs, depending on when your heating and hot water run.
High-temperature heat pump: A high-temperature heat pump can produce water at higher temperatures than a standard unit, which can suit homes where radiators are hard to change. It typically runs less efficiently at high temperatures, so a careful design still matters.
Home Energy Scotland: Home Energy Scotland is the Scottish Government-funded advice service that runs grants and loans for home energy improvements in Scotland. It offers a heat pump grant of up to £7,500, more for eligible rural homes, plus an optional interest-free loan.
Hybrid heat pump: A hybrid heat pump system pairs a heat pump with a boiler, with controls deciding which to use. The heat pump usually covers most heating and the boiler helps on the coldest days. Hybrids are not eligible for the Boiler Upgrade Scheme in England and Wales.
Hydrobox: A hydrobox is the indoor unit of a split system heat pump, where heat from the refrigerant is passed to the water for your heating and cylinder. It usually holds the pump, controls and some valves, and is often wall-mounted near the cylinder.
Immersion heater: An immersion heater is an electric element inside the hot water cylinder that heats water directly. With a heat pump it is mainly used for the legionella cycle or as a backup, as it costs more to run than the heat pump.
Inverter compressor: An inverter compressor can speed up or slow down to match how much heat your home needs, rather than simply switching fully on or off. Most modern heat pumps use one, which helps them run steadily, efficiently and more quietly.
kW (kilowatt): A kilowatt (kW) measures power, the rate at which heat is produced or energy is used at any moment. Heat pumps are sized in kW: a typical three-bedroom semi might need 6–9 kW, depending on its heat loss.
kWh (kilowatt hour): A kilowatt hour (kWh) measures an amount of energy: 1 kW used for one hour. Energy bills are charged per kWh, and heat pump running costs depend on how many kWh of electricity are needed to make each kWh of heat.
Legionella cycle: A legionella cycle, sometimes called pasteurisation, regularly heats the stored hot water to a higher temperature to control legionella bacteria. It is usually scheduled weekly, often using the immersion heater or the heat pump at a higher setting.
Listed building consent: Listed building consent is permission needed before making changes that affect the character of a listed building, inside or out. Fitting a heat pump to a listed building usually needs this consent in each UK nation.
Load compensation: Load compensation is a control that adjusts the flow temperature according to how close each room is to its target temperature, using indoor sensors or thermostats. It is often combined with weather compensation for steady, efficient heating.
Low-loss header: A low-loss header is a short, wide section of pipe that hydraulically separates the heat pump circuit from the heating circuit, so each can run at its own flow rate. It can reduce efficiency if not designed carefully.
MCS (Microgeneration Certification Scheme): MCS is the UK certification scheme setting standards for small-scale renewable heating installers and products. The Boiler Upgrade Scheme in England and Wales requires an MCS-certified installer, as does Home Energy Scotland funding.
MCS 020 (MCS planning standards): MCS 020 is the MCS planning standard for air source heat pumps, setting the noise assessment and other checks an installation must meet. In England, meeting them is one of the conditions for the installation to count as permitted development.
MCS certificate: An MCS certificate is issued when an MCS-certified installer registers a heat pump installation. It confirms the system was installed to MCS standards and is needed for funding such as the Boiler Upgrade Scheme in England and Wales.
Microbore pipework: Microbore pipework is narrow heating pipe, usually 8–10 mm, found in many older systems. It can restrict the higher flow rates a heat pump needs, so some or all of it may need replacing with larger pipe.
Monobloc heat pump: A monobloc heat pump holds all of its refrigerant inside the sealed outdoor unit and connects to the house with water pipes only. No refrigerant work is needed indoors, which keeps installation simpler. See monobloc heat pumps.
Northern Ireland Sustainable Energy Programme: The Northern Ireland Sustainable Energy Programme funds energy efficiency measures in Northern Ireland and has sometimes included heat pumps. There is no dedicated heat pump grant for homeowners in Northern Ireland at present.
Ofgem: Ofgem is the energy regulator for Great Britain. It runs the Boiler Upgrade Scheme in England and Wales, confirms applications with property owners and sets the energy price cap each quarter.
Outdoor unit: The outdoor unit is the part of an air source heat pump that sits outside, containing the fan, coil and, in most systems, the compressor. A typical home unit is roughly 1 m wide and needs free airflow around it.
Part L (England): Part L is the section of the Building Regulations in England covering the conservation of fuel and power. It sets efficiency standards for new and replacement heating systems, including heat pumps. Wales, Scotland and Northern Ireland each have their own equivalent rules.
Permitted development: Permitted development lets certain building work go ahead without a planning application. In England, it allows up to two air source heat pumps on a house since 29 May 2025. Wales, Scotland and Northern Ireland have their own rules, which we check at survey.
Price cap: The price cap is the maximum Ofgem allows suppliers to charge per kWh and per day on standard variable tariffs in Great Britain. For October to December 2026, the average electricity unit rate is 26.32p per kWh on Direct Debit.
R290 (propane): R290 is propane used as a refrigerant. It has a very low global warming potential and lets heat pumps reach useful flow temperatures efficiently. Units using it are usually monobloc, with the refrigerant sealed in the outdoor unit.
R32: R32 is a common heat pump refrigerant with a lower global warming potential than many older refrigerants. It is used in both monobloc and split system units, and handling it on split systems requires an F-gas certified engineer.
Radiator output: Radiator output is the amount of heat a radiator gives off, usually stated in watts at a set temperature difference. Output falls at lower flow temperatures, which is why some radiators are upsized for a heat pump.
Refrigerant: Refrigerant is the fluid inside a heat pump that absorbs heat from outside air as it evaporates and releases it as it is compressed. It circulates in a sealed circuit and is not used up in normal operation.
Return temperature: Return temperature is the temperature of the water coming back to the heat pump after it has passed through your radiators or underfloor heating. The gap between flow and return temperature is called the delta T.
SCOP (seasonal coefficient of performance): SCOP is a heat pump's average efficiency across a whole heating season, rather than at a single test point. A SCOP of about 3–4 is common in well-designed systems, and it is the most useful guide to running costs.
Screed: Screed is a layer of sand and cement, or a flowing alternative, laid over underfloor heating pipes to spread heat evenly across the floor. It takes time to cure before the heating can be fully used.
Short cycling: Short cycling is when a heat pump switches on and off too often, usually because it is oversized, the flow rate is too low or the controls are set poorly. It reduces efficiency and adds wear, and good design and commissioning help prevent it.
Smart controls: Smart controls are heating controls you can adjust from an app, often with scheduling, room sensors and tariff-aware timing. With a heat pump, they can heat the cylinder during cheaper periods and work alongside weather compensation.
Sound power level: Sound power level is the total sound energy a heat pump gives out, measured in decibels and fixed for each model at a given output. It is the figure used to start the MCS noise assessment.
Sound pressure level: Sound pressure level is how loud a sound actually is at a particular spot, such as a neighbour's window. It depends on the unit's sound power, the distance and nearby walls, which is why siting matters.
SPF (seasonal performance factor): SPF is the efficiency a heat pump actually achieves in a real home over a season or a year, worked out from measured heat output and electricity use. It reflects the whole system's design, controls and settings, not just the unit.
Split system heat pump: A split system heat pump has an outdoor unit linked to an indoor unit by refrigerant pipes, with the heat passed to water indoors. The outdoor unit can be smaller, but refrigerant work must be done by an F-gas certified engineer.
Standing charge: A standing charge is a fixed daily amount on an energy bill, paid whatever you use. If a heat pump replaces gas and the gas meter is removed, you no longer pay the gas standing charge, about £108 a year at current cap rates.
U-value: A U-value measures how quickly heat passes through a part of a building, such as a wall, roof or window. The lower the U-value, the better the insulation. U-values are used in heat loss calculations and in Building Regulations.
Underfloor heating: Underfloor heating uses pipes laid under the floor to warm the room from below, using a large surface at low water temperatures. It suits heat pumps well. See underfloor heating with heat pumps.
Unvented cylinder: An unvented cylinder is a hot water cylinder fed directly from the mains, giving good pressure at taps and showers without a loft tank. Heat pump cylinders have a large internal coil to transfer heat efficiently at lower temperatures.
VAT zero rate: Across the UK, including Northern Ireland, the installation of heat pumps is zero-rated for VAT until 31 March 2027, so no VAT is added to a qualifying installation. Guide prices on this site are shown including VAT.
Volumiser: A volumiser is a small vessel added to the pipework to increase the amount of water in a heat pump system. It helps the unit run steadily and defrost properly, without separating the circuits as a buffer tank can.
Weather compensation: Weather compensation is a control that uses an outdoor sensor to raise the flow temperature as the weather gets colder and lower it when it is milder. It keeps rooms comfortable while helping the heat pump run at its most efficient.
Zoning: Zoning divides a home's heating into areas, such as upstairs and downstairs, each with its own timing and temperature. With heat pumps, zones are kept simple, because too many closing zones can cause short cycling.

Heat pump glossary

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Heat pump glossary

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