UK Heat Pump Glossary 2026

All terms

Air Source Heat Pump (ASHP)(also: ASHP)

A heating system that extracts heat from outside air and concentrates it to warm a home. The most common heat pump type installed in UK homes — typically 4–14 kW capacity. Works down to roughly -20 °C outside temperature.

Context: ASHPs receive the £7,500 Boiler Upgrade Scheme grant in England & Wales and qualify for the Home Energy Scotland Loan + Cashback in Scotland.

Ground Source Heat Pump (GSHP)(also: GSHP)

A heat pump that extracts heat from underground via buried pipework (slinky loop or borehole). More efficient than ASHP because ground temperature is more stable, but typically 2–3× more expensive to install.

Context: Less common in UK retrofits because most homes lack the garden space for slinky loops. Boreholes (vertical, 100m+ deep) cost £3,000–£8,000 per borehole on top of the heat pump itself.

SCOP (Seasonal Coefficient of Performance)(also: SCOP)

A weighted-average measure of how efficiently a heat pump runs across a full UK heating season. SCOP 3.0 means each kWh of electricity produces 3 kWh of heat. Most UK ASHPs deliver SCOP 2.8–3.4.

Context: MCS-certified installs publish a calculated SCOP in the post-install certificate. The Energy Saving Trust 2024 trial of 750 UK heat pumps measured an average SCOP of 2.94.

COP (Coefficient of Performance)(also: COP)

Instantaneous efficiency at a single moment — for example COP 4 at 7 °C outside means 1 kW electricity → 4 kW heat right now. COP varies with outside temperature; SCOP averages COP across the year.

Boiler Upgrade Scheme (BUS)(also: BUS)

The UK government grant scheme that pays £7,500 toward an air source heat pump installation in England & Wales. Administered by Ofgem, paid directly to the installer, no upfront cost to the homeowner.

Context: Runs to March 2028 with a £450M total budget. Around 75,000 vouchers issued by start of 2026.

Home Energy Scotland Loan + Cashback(also: HES)

Scotland's equivalent of the BUS grant — an interest-free loan with a cashback element of up to £7,500 (no repayment) for an air source heat pump.

Context: Administered by the Energy Saving Trust on behalf of the Scottish Government. Subject to scheme changes — current figures at homeenergyscotland.org.

MCS (Microgeneration Certification Scheme)(also: MCS)

The UK quality assurance certification scheme for renewable energy products and installers. Only MCS-certified installers can apply for the BUS grant, HES Cashback, or Smart Export Guarantee.

Context: Each install receives an MCS certificate, which homeowners need to retain for warranty and grant verification. Register: mcscertified.com

Heat-loss survey

The pre-install calculation that determines what kilowatt heat pump capacity, what flow temperature, and what radiator changes a home needs. Done by an MCS-certified installer or surveyor.

Context: A correctly-sized heat pump matches the heat loss at the design outdoor temperature (-2 °C in most of England, -3 to -5 °C in Scotland). Oversizing wastes money; undersizing leaves you cold.

Flow temperature

The temperature of the water leaving the heat pump and entering the radiator circuit — typically 35–55 °C for heat pumps, vs 70–80 °C for traditional gas boilers. Lower flow temperature = higher SCOP.

Monobloc heat pump

A heat pump where the refrigerant circuit is fully contained in the outdoor unit. Water (not refrigerant) is pumped through the home. Easier to install, F-gas regulation simpler, slightly less efficient than split systems.

Context: The most common UK heat pump type — Daikin Altherma, Mitsubishi Ecodan, Vaillant aroTHERM are all monobloc.

Split heat pump

A heat pump where refrigerant flows between an outdoor unit and an indoor unit. Slightly more efficient than monobloc but requires F-gas-certified installation of the refrigerant connection.

Refrigerant

The fluid inside the heat pump that absorbs and releases heat as it circulates. Common UK heat pump refrigerants: R32 (most common), R290 (propane, lowest GWP), R454B (newer alternative).

Context: F-gas regulations apply to refrigerant handling. R290 is the most environmentally friendly but requires extra ventilation considerations because it's flammable.

Buffer tank

A small water tank between the heat pump and the radiator circuit. Prevents short-cycling, smooths defrost cycles, and provides a thermal buffer during sudden demand changes.

Context: Typical size 30–80 litres for a 3-bed home. Some heat-pump systems can omit a buffer tank if radiator volume is sufficient — 'volumiser' is a common alternative term.

Domestic Hot Water cylinder (DHW)(also: DHW cylinder)

A separate insulated tank where the heat pump heats hot water for taps and showers. Required for any heat pump except in flats with shared water systems. Typical size 180–300 litres.

Defrost cycle

When the outdoor unit briefly reverses to melt ice from its evaporator coils — typically once every 30–60 minutes during cold, damp weather. Brief reduction in heating output, automatically managed.

Permitted Development

The set of UK planning rules that allow a heat pump installation without a separate planning application, provided certain criteria are met (size, distance from boundary, noise level).

Context: England's permitted development for heat pumps was expanded in 2024 — most domestic ASHP installations no longer require planning consent unless in a conservation area or listed building. Scotland's rules are tighter.

Conservation area

An area designated by a local planning authority for special architectural or historic interest. Heat pump installations in conservation areas usually require a separate planning application even where permitted development would otherwise apply.

Listed building

A property formally listed as having special architectural or historic interest. Listed-building consent is required for almost any external alteration including heat pump installations. Approval times vary 8–16 weeks.

F-gas regulation

EU and UK rules covering fluorinated refrigerants. F-gas-certified installers are required for any installation using more than 5 kg of refrigerant or where refrigerant is handled (mostly applies to split heat pumps).

Smart Export Guarantee (SEG)(also: SEG)

The UK scheme that pays you for surplus electricity exported back to the grid. Relevant if your heat pump is paired with solar PV — surplus solar can be sold rather than wasted.

Octopus Cosy / Octopus Heat Pump

Octopus Energy's heat-pump-specific time-of-use electricity tariffs. Cheaper electricity during off-peak hours (typically 04:00–07:00 and 13:00–16:00), letting heat pumps preheat water and home cheaply.

Context: Switching to a heat-pump-friendly tariff can save £200–£400/year on running costs for a typical 3-bed home.

EDF Heat Pump tariff

EDF Energy's heat-pump-specific tariff — alternative to Octopus Cosy. Different time-of-use windows and unit prices but similar overall economics.

Microbore plumbing

8–10 mm internal-diameter pipework, common in 1960s–1990s UK homes. Heat pumps require larger pipework (15–22 mm) to deliver heat at lower flow temperatures. Microbore replacement adds £500–£2,000 to a typical install.

Cavity wall

A two-leaf brick wall with a gap (cavity) between leaves, common in UK homes built post-1930. The cavity can be filled with insulation, dramatically improving thermal performance and heat-pump efficiency.

Context: BUS grant eligibility requires no outstanding cavity-wall insulation recommendations on your EPC. Filling the cavity first is often a prerequisite.

Solid wall

A single-leaf brick or stone wall, common in UK homes built pre-1919. Lower thermal performance than cavity walls (U-value typically 1.7–2.0 vs 0.5 for filled cavity). Heat pumps still work but require larger units and radiator upgrades.

Internal Wall Insulation (IWI) / External Wall Insulation (EWI)(also: IWI / EWI)

Retrofits applied to solid walls to improve thermal performance. EWI sits outside the original wall and is more effective; IWI sits inside and reduces internal floor area but doesn't change building appearance.

EPC (Energy Performance Certificate)(also: EPC)

A UK-mandated document grading a home's energy efficiency from A (best) to G (worst). Required for property sales and rentals, and for BUS grant eligibility.

Context: Average UK EPC is band D. Heat pumps work in any band but achieve best SCOP in C+ homes.

Tenements (Scotland) Act 2004

The Scottish law governing tenement properties — flats sharing a common stair (close), roof, and external walls. Affects heat pump installations because outdoor unit placement on shared external walls requires consent from co-proprietors.

Boiler Plus regulations

UK building regulations requiring new gas boilers to meet specific efficiency criteria. Doesn't apply to heat pumps directly but creates pressure to switch when replacement is due.

Smart heating control

A heating control system that learns home occupancy and weather patterns to optimise heat pump operation. Required by the BUS grant scheme — installer-supplied controllers like Honeywell, Tado, or Hive's heat-pump variants are common.

Weather compensation

A control feature that adjusts heat pump flow temperature based on outside temperature — colder outside means warmer flow. Automatic and built into modern heat pumps. Improves SCOP by 5–10%.

Zonal heating

A control approach where different rooms or floors run on different schedules. Heat pumps can do zonal heating with the right manifold and controls — useful for large homes where some rooms are unused most of the day.

Underfloor Heating (UFH)(also: UFH)

A heating circuit embedded in the floor (concrete screed or joist void). Operates at low flow temperatures (35–45 °C), making it a heat pump's ideal partner. Higher SCOP than radiator-based systems by ~5%.

Radiator upsizing

Replacing existing radiators with larger ones to deliver the same room heat output at heat-pump flow temperatures (45–55 °C vs gas boiler 70–80 °C). A typical UK semi requires 2–4 radiator upgrades.

K1, K2, K3 radiators

Radiator panel-and-fin classifications. K1 = single panel, K2 = double panel single fin, K3 = double panel double fin. Heat pump retrofits typically require K2 or K3 radiators where K1 was original.

Hybrid heat pump

A system combining a heat pump with a gas boiler — heat pump runs at moderate temperatures, gas boiler kicks in for very cold weather and hot water. Doesn't qualify for the BUS grant.

Context: Sometimes recommended for hard-to-retrofit solid-wall homes but increasingly outdated as heat pump capacity improves.

Air-to-air heat pump

A heat pump that delivers heat directly via warm air (not water through radiators). Common in apartments and small flats. Does NOT qualify for the BUS grant (which requires a wet heating system).

Heat pump tariff

An electricity tariff designed for heat-pump homes — typically lower unit price during overnight or off-peak hours when the heat pump can pre-heat water and home. Switching can save £200–£400/year.

Inverter compressor

A modern compressor type that varies its speed to match heat demand — more efficient than older fixed-speed compressors. All current UK heat pumps use inverter compressors.

Defra noise rules (MCS 020)

MCS 020 is the noise standard that heat pump installations must meet — typically <42 dB at neighbouring properties. Modern monobloc units run at 40–48 dB at 1m, well within rules with correct siting.

Coastal-rated unit

A heat pump unit specified for sea-air corrosion resistance — additional galvanisation, corrosion-resistant fins. Required for properties within ~500m of the coast. Adds £200–£500 to standard quotes.

Open-vented vs sealed system

Old UK heating systems were typically open-vented (with a header tank in the loft). Heat pumps usually require a sealed system. Conversion is part of a typical heat pump install.

Volumiser

A small water vessel inline with the radiator circuit — alternative to a buffer tank. Provides hydraulic separation and thermal mass without taking as much physical space.

Manifold

A distribution block where multiple heating circuits meet. Used in zonal heating and underfloor systems. Allows individual zone control.

Heat pump cylinder

An insulated hot-water cylinder optimised for heat-pump use — larger heat exchanger coil (so the heat pump can heat water at lower flow temperatures), thicker insulation, and typically Megaflo / Telford / OSO branding.

Megaflo / Telford / OSO

Three of the most common UK hot-water cylinder brands. Megaflo is owned by Heatrae Sadia (Glow-worm parent). Telford is a smaller specialist. OSO is Norwegian. All offer heat-pump-optimised cylinders.

Daikin / Mitsubishi / Vaillant / Samsung / Panasonic / LG

The six most common UK heat pump brands. Daikin Altherma and Mitsubishi Ecodan dominate the UK market by install count; Vaillant aroTHERM is European-built (closer to compliance with new UK refrigerant rules); Samsung, Panasonic and LG are growing in market share.

BS EN 14511

The European standard for testing and rating heat pump performance. SCOP and COP figures published in product datasheets are derived from BS EN 14511 testing.

Section 6 (Scottish Building Standards)

Scotland's energy-efficiency building regulations — equivalent of England's Part L. Sets minimum performance standards for new build and major retrofits, including heat pumps.

TPS (Telephone Preference Service)(also: TPS)

The UK's free opt-out register for unwanted phone marketing calls. Lead-generation form processes must respect TPS — buyers are statutorily responsible for screening before placing live calls.

PECR (Privacy and Electronic Communications Regulations)(also: PECR)

The UK regulations covering electronic marketing — sets rules on consent, opt-out, and TPS screening. Apply UK-wide and are enforced by the ICO.

DPO (Data Protection Officer)(also: DPO)

A designated person responsible for an organisation's UK GDPR compliance. Required for some organisations; recommended for any operation processing personal data at scale.

BUS voucher

The £7,500 voucher issued by Ofgem to a homeowner's MCS-certified installer once an application is approved. Redeemed by the installer at install time — homeowner only ever pays the post-grant amount.

Cashback (HES context)

The non-repayable portion of the Home Energy Scotland Loan + Cashback scheme. Up to £7,500 for an ASHP. Distinct from the interest-free loan portion (which is repaid over time).

Off-gas-grid (OGG)(also: OGG)

A property not connected to mains gas — typically using oil, LPG, electric storage, or solid fuel for heating. Heat pump payback is fastest for OGG homes (typical 5–7 years vs 10–14 years for mains-gas swaps).

Context: Around 15% of UK homes are OGG. The Highlands of Scotland reach ~70% OGG concentration.