UK Heat Pump Glossary

A

Air source heat pump (ASHP) — Heat pump that extracts heat from outside air. Most common UK type. See ASHP vs GSHP.

Anti-vibration mounts — Rubber feet between outdoor unit and concrete base. Cut noise/vibration transmission by 8-12 dB.

Article 4 Direction — Planning rule that removes Permitted Development rights for specific zones. Common in central London, Bath, Edinburgh New Town. See Planning Permission.

B

BS EN 12831 — UK standard for heat loss calculation. Required for MCS heat pump design.

BS EN 14825 — UK standard for measuring SCOP in a controlled environment.

BUS (Boiler Upgrade Scheme) — England's £7,500 heat pump grant. See BUS Guide.

BTU (British Thermal Unit) — Imperial unit for heat output, often used for radiator capacity. 3,412 BTU = 1 kW.

C

Cap (Ofgem price cap) — Maximum unit rate suppliers can charge default-tariff customers. Updated quarterly.

Cavity wall — Two-leaf brick wall with air gap. Standard UK construction 1930-present. Can be insulated.

COP (Coefficient of Performance) — Heat output ÷ electricity input at a single operating point. Manufacturers cherry-pick favourable conditions; always compare SCOP instead.

Compressor — Pump that circulates and pressurises the refrigerant. The most expensive component to replace.

Condensate — Water that drips off the heat pump's evaporator coil during operation. Needs a drain (and frost protection in winter).

Conservation Area — Planning designation protecting historic neighbourhoods. May limit external heat pump siting.

Cylinder — Insulated tank storing hot water for taps and showers. Heat pumps usually need a 200-300L unvented cylinder.

D

DHW (Domestic Hot Water) — Hot water for taps, showers, baths. Separate from heating water.

Defrost cycle — Brief reverse cycle where heat pump melts ice from its outdoor coil in winter. Normal, lasts 5-10 minutes.

Design outdoor temperature — Coldest temperature your heat pump is sized to handle. -2°C London, -5°C Edinburgh, -7°C Highlands.

Design flow temperature — Target water temperature leaving the heat pump at design outdoor temp. Lower = higher SCOP. UK targets 35-50°C ideal.

DESNZ (Department for Energy Security and Net Zero) — UK government department running BUS, F-Gas, Future Homes Standard.

E

EPC (Energy Performance Certificate) — UK home energy rating A-G. Most grants require an in-date EPC.

Evaporator — Outdoor unit coil that absorbs heat from air.

ECO (Energy Company Obligation) — UK scheme requiring energy suppliers to fund energy efficiency for low-income households. Limited heat pump scope.

F

F-Gas — Fluorinated greenhouse gases used as refrigerants. UK F-Gas Regulation requires Cat 1 or Cat 2 certified engineers. See F-Gas Rules UK.

Flare joint — Field-made refrigerant connection (vs factory-brazed). Higher leak risk if poorly made.

Flow temperature — Water temperature leaving the heat pump to radiators/UFH. Lower is better for efficiency.

Future Homes Standard — UK rule (late 2025) requiring new build emissions 75-80% below 2013 baseline. Effectively bans gas boilers in new builds.

G

Gas Safe — UK certification for gas-related work. Different from F-Gas (which is for refrigerants).

GSHP (Ground source heat pump) — Heat pump using buried coils to extract heat from ground. Higher SCOP than ASHP but more expensive install. See ASHP vs GSHP.

GWP (Global Warming Potential) — Climate impact of a refrigerant vs CO₂. Lower = better. R290 = 3, R32 = 675, R410A = 2,088.

H

Heat battery — Compact thermal store using phase-change material (Sunamp, Tepeo). Alternative to cylinder. See Heat Battery vs Cylinder.

Heat loss calculation — Room-by-room assessment per BS EN 12831. Required for proper sizing.

Heritage statement — Document describing impact on a listed building. Required for Listed Building Consent.

HES (Home Energy Scotland) — Scottish heat pump grant + interest-free loan scheme. See HES Guide.

HIES (Home Insulation and Energy Systems) — UK consumer-protection scheme covering MCS installers.

Hybrid heat pump — Heat pump paired with gas boiler. Boiler covers peak demand. See Hybrid Worth It.

I-K

Immersion heater — Electric backup element in the cylinder. Used for weekly Legionella cycle or emergency heating.

Inverter — Electronic control that varies compressor speed. Modern heat pumps modulate output 25-100% via inverter.

K2 / K3 radiator — Double or triple-panel radiator with higher heat output at low flow temperature. Common in heat pump retrofits.

kW (kilowatt) — Unit of heat output. 1 kW = 3,412 BTU. UK heat pumps typically 5-15 kW.

kWh (kilowatt-hour) — Unit of energy. 1 kWh = 1 kW for 1 hour. UK home heating typically 8,000-15,000 kWh/yr.

L

LBC (Listed Building Consent) — Formal permission required for any alteration to a listed building. Free, 8-week target. See Listed Buildings.

Legionella cycle — Weekly heating of cylinder to 60°C+ to prevent bacterial growth.

Lockshield valve — Adjustable radiator valve used for balancing flow during commissioning.

Lw (sound power) — Total acoustic energy emitted by unit. Manufacturer rating.

Lp (sound pressure) — What you hear at distance. Lp drops ~6 dB per doubling of distance.

M-N

MCS (Microgeneration Certification Scheme) — UK quality mark for renewables. Required for BUS grant. See MCS Explained.

MELCloud / MELCare — Mitsubishi heat pump app and cover plan.

Microbore — 8-10mm copper pipework (1970s-90s common). Often too narrow for heat pumps.

MIS 3005 — MCS install standard for heat pumps.

MIS 020 — MCS noise standard (42 dB(A) at 1m from neighbour window).

MVHR (Mechanical Ventilation with Heat Recovery) — Whole-house ventilation system common in new builds. Reduces peak heat loss.

O-P

Onecta — Daikin heat pump app.

Octopus Cosy — Heat-pump-targeted half-hourly tariff with 3 daily cheap windows. See Cosy vs Standard.

Ofgem — UK energy regulator setting price cap.

PCM (Phase Change Material) — Substance that absorbs heat by melting. Used in heat batteries.

Permitted Development — Right to install heat pump without planning permission (most UK homes since May 2025 reform).

Primary system — The water loop between heat pump and emitters (radiators/UFH).

Q-R

QANW (Quality Assured National Warranties) — MCS consumer-protection scheme.

R290 — Propane refrigerant. GWP 3. Preferred 2026 choice. Mildly flammable; siting rules apply.

R32 — Difluoromethane refrigerant. GWP 675. Still legal but gradually restricted.

R410A — Older refrigerant. GWP 2,088. Banned for new installs from 2020.

Recharge — Adding refrigerant after a leak. Must be done by F-Gas engineer.

RECC (Renewable Energy Consumer Code) — MCS consumer-protection scheme. Trading Standards-approved.

Refcom — UK F-Gas company registration body.

Rural Uplift — £1,500 extra Scottish grant for qualifying rural postcodes.

S

SCOP (Seasonal Coefficient of Performance) — Average COP across a heating season. The most important number. See SCOP Explained.

sensoApp / sensoComfort — Vaillant heat pump app and controller.

SMETS2 — Smart Meter Equipment Technical Specifications v2. Standard UK smart meter, required for half-hourly tariffs.

Solid wall — Single-leaf brick or stone wall (no cavity). Common pre-1930. Higher heat loss.

Standby loss — Heat lost by cylinder/heat battery while sitting full. Cylinder: 1.5-2.0 kWh/day; heat battery: 0.3-0.7 kWh/day.

T-U

T-Cap (True Capacity) — Panasonic technology holding rated output to -15°C.

Tariff — Energy price plan. Heat pumps benefit from half-hourly tariffs like Octopus Cosy.

Trace tape — Self-regulating electric heater wrapped around condensate pipe to prevent freezing.

TRV (Thermostatic Radiator Valve) — Self-regulating valve per radiator. Can hurt heat pump efficiency if cycling too aggressively.

UFH (Underfloor Heating) — Low-temperature heating coils in floor build-up. Highest SCOP combination with heat pumps. See UFH + Heat Pump.

Unvented cylinder — Pressurised cylinder delivering hot water at mains pressure.

U-value — Heat loss rate of a wall, roof, or window. Lower is better. Modern new build wall U = 0.18 W/m²K; Victorian = 1.7-2.1.

V-Z

Vapour compression cycle — Thermodynamic cycle (compressor, condenser, expansion valve, evaporator) underlying all heat pumps.

Warmer Homes Scotland — Scottish low-income full-coverage energy efficiency scheme.

Weather compensation — Control feature that adjusts flow temperature based on outdoor temperature. Adds 0.2-0.4 SCOP for free.

Workmanship warranty — Cover via HIES/RECC/QANW. 2-6 years from MCS install. Separate from manufacturer warranty.