Heat Pump Glossary of UK Terms 2026

Heat pump UK glossary 2026: 40+ key terms defined - SCOP, COP, EER, R290, MCS, BUS, weather compensation, defrost cycle, hydronic, more.

Open reference book representing heat pump glossary terms guide
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By Rob Griffiths17 June 2026 · 9 min read

UK heat pump terminology can feel opaque to newcomers. This glossary defines 40+ key terms in plain English with cross-references to our deeper guides. Bookmark for quick reference when reading installer quotes, manufacturer datasheets, or technical articles.

Efficiency metrics

How heat pump performance is measured.

  • SCOP (Seasonal Coefficient of Performance): Annual kWh of heat delivered divided by annual kWh of electricity consumed. Dimensionless ratio (typical UK 3.0-4.5). The UK + EU standard metric. See SCOP vs HSPF.
  • COP (Coefficient of Performance): Instantaneous heat output divided by electricity input at specific operating conditions. Higher than SCOP because measured at favourable conditions only.
  • EER (Energy Efficiency Ratio): Cooling-mode equivalent of COP (kWh cooling output / kWh electricity input). Less relevant for UK heat pumps used primarily for heating.
  • HSPF (Heating Seasonal Performance Factor): US-standard annual efficiency (BTU/h heat per Wh electricity). Conversion: HSPF / 3.41 = SCOP equivalent.
  • ErP rating (A+++ to G): Energy Related Products directive label - quick-comparison rating based on SCOP at low flow temp. A+++ = SCOP 4.0+, A++ = SCOP 3.4+.
  • SPF (Seasonal Performance Factor): Another term for SCOP; used interchangeably in some manufacturer datasheets.

Regulatory standards + accreditation

Who oversees what.

  • MCS (Microgeneration Certification Scheme): UK quality standard for renewable energy installs. MCS-certified installer required for BUS grant eligibility. See installer questions guide.
  • BUS (Boiler Upgrade Scheme): GBP 7,500 UK government grant for heat pump installation. Available through MCS-certified installer. Runs through 2028 + likely successor.
  • TrustMark: UK Government-endorsed quality scheme. TrustMark registration required for BUS grant eligibility.
  • ECO4 (Energy Company Obligation): Energy supplier funding for heating + insulation upgrades for households on qualifying benefits / EPC D-G. Often covers full install for eligible households.
  • GBIS (Great British Insulation Scheme): Insulation-only successor / parallel to ECO4; broader eligibility (households not on benefits but in EPC D-G properties).
  • HUG2 (Home Upgrade Grant Phase 2): Local-authority-led funding for off-gas-grid properties in EPC D-G. Heat pump + insulation eligible.
  • F-gas regulations: UK regs governing fluorinated greenhouse gas refrigerants. Requires certified technicians for recovery + disposal.
  • Heat Geek: Independent training + accreditation network for installers focused on low-temperature design + best practices.
  • G3: Unvented hot water cylinder competency certificate. Required for engineers installing unvented cylinders (most heat pump installs).
  • Gas Safe: Registration body for gas engineers. Required for any gas-related work including combi boiler decommissioning.
  • CIPHE / CIBSE: Chartered Institute of Plumbing + Heating Engineering / Chartered Institute of Building Services Engineers. Professional body memberships indicating engineering competence.

Components + physical parts

What's in your heat pump system.

  • Compressor: Heart of the heat pump - compresses refrigerant gas to raise temperature. Main wear part; typical lifespan 15-20 years.
  • Heat exchanger: Transfers heat between refrigerant + air (outdoor unit) or refrigerant + water (indoor unit / cylinder coil).
  • Expansion valve: Reduces refrigerant pressure between high + low sides of the cycle. Allows refrigerant to absorb heat.
  • Outdoor unit (or 'monoblock'): Single integrated unit containing compressor, outdoor heat exchanger, fan, controls. Most common UK install type.
  • Cylinder (hot water cylinder): Insulated water tank (200-300L typical) storing hot water heated by heat pump primary circuit. Replaces combi-boiler instant hot water with stored hot water model.
  • Cylinder coil: Heat exchanger inside the cylinder transferring heat from heat pump's primary circuit to the stored water.
  • Buffer tank: Optional small (50-100L) tank providing thermal mass in heat pump primary circuit. Used to prevent short-cycling.
  • Hydronic circuit: Water-based heat distribution loop (radiators, UFH) connected to heat pump.
  • Primary circuit: Loop between heat pump + cylinder coil (separate from radiator loop in some installs).
  • Refrigerant pipework: Insulated copper pipes carrying refrigerant between outdoor unit + indoor (monoblock has minimal indoor refrigerant; split systems more extensive).
  • Immersion heater: Standard electric resistance element in hot water cylinder; backup for heat pump fault or boost.
  • Anti-vibration mounts: Rubber mounts under outdoor unit to reduce vibration transmission to building structure.

Operation modes + control concepts

How the heat pump runs.

  • Weather compensation (WC): Automatic adjustment of flow temperature based on outdoor temperature. Highest-impact tunable. See WC setup guide.
  • WC curve (or heat curve): Graph plotting flow temp vs outdoor temp - linear or piecewise line.
  • Flow temperature: Temperature of water leaving the heat pump for distribution to radiators/UFH. Lower flow temp = higher SCOP.
  • Return temperature: Temperature of water returning to heat pump from radiators. Difference between flow + return = system 'delta-T'.
  • Defrost cycle: Brief reverse-cycle operation clearing frost off outdoor heat exchanger. Every 30-90 min in damp +0-5C weather. Normal.
  • Frost protection mode: Heat pump runs circulator pump continuously when outdoor temp drops below ~+3C to prevent water freezing.
  • Aux heater (auxiliary): Built-in electric resistance heater (3-9 kW typical) that fires automatically during extreme cold or defrost cycles.
  • Anti-cycle delay: Controller setting enforcing minimum time between heat pump cycles (typically 5-15 min). See anti-cycling guide.
  • Modulation: Heat pump's ability to vary output continuously rather than on-off operation. Minimum modulation typically 30-40% of nameplate.
  • DHW priority: Controller setting prioritising hot water cylinder reheat over space heating during simultaneous demand.
  • Setback / setpoint: Lower target temperature during specific schedule periods (e.g. nighttime, daytime when empty).

Refrigerants

The working fluid.

  • R290 (propane): Modern UK standard refrigerant. GWP 3. Highest efficiency potential. Mildly flammable (A3 safety class) but practical install safe in outdoor monoblock configuration. See refrigerant guide.
  • R32: Previous UK standard (2015-2024). GWP 675. Phased down 2025-2030 per F-gas regs but still installed in some scenarios.
  • R454B: Mid-GWP refrigerant (GWP 466). Mainly commercial / hybrid units.
  • R410A: Older refrigerant (GWP 2,088). Largely phased out from new UK installs.
  • R134a, R407C: Older refrigerants found in legacy installs.
  • GWP (Global Warming Potential): Multiplier of CO2-equivalent warming impact. Lower = better environmental profile.
  • Charge / refrigerant charge: Mass of refrigerant in the system (kg). F-gas regs require leak inspection for systems >5 kg CO2-equivalent.
  • A1/A2L/A3 safety class: ASHRAE refrigerant safety classification. A1 = non-flammable (R32 is A2L, R290 is A3 mildly flammable).

Install + commissioning terms

Process vocabulary.

  • Heat-loss calc: MCS-certified calculation of property's heat demand at design conditions (typically -3 to -5C outdoor). Drives heat pump sizing.
  • Design temperature: Coldest outdoor temperature the heat pump should comfortably handle. Standard UK is -3 to -5C; exposed sites -10 to -15C.
  • Commissioning: Final install setup + tuning - electrical connection, refrigerant charge, WC curve, schedule, controller settings.
  • MCS 020 standard: Planning sound calculation for outdoor unit acoustic compliance (42 dB(A) at nearest noise-sensitive receptor).
  • Permitted Development (PD): UK planning rules allowing ASHP installs without planning permission subject to: under 0.6m3 volume, 1m+ from boundary, not on principal elevation, not in conservation area.
  • Listed Building Consent (LBC): Required for any heat pump install on a listed property (Grade II / II* / I).
  • G98 / G99 notification: Distribution Network Operator (DNO) notification for electrical connection - heat pump triggers G98 (single-phase up to 11.04 kW).
  • DNO supply upgrade: Increasing mains supply capacity (60A to 100A typical) when existing supply insufficient for combined load.

Building + envelope terms

Property characteristics affecting install.

  • U-value: Heat transfer coefficient (W/m2K). Lower = better insulation. Modern wall U-value 0.18; uninsulated cavity wall 1.5-1.8.
  • Heat-loss demand: Total heat output needed at design conditions to maintain comfort (kW). Typical UK 3-bed: 5-9 kW depending on insulation.
  • Thermal envelope: Combined insulating barrier of walls, roof, floor, glazing - determines heat loss.
  • EPC (Energy Performance Certificate): A-G rating of property's energy efficiency. Required for sale/rental. Heat pump install typically lifts EPC by 1-2 bands.
  • Cavity wall: Wall construction with two leaves of brick + air gap. Filled cavity insulation = significantly improved U-value.
  • Solid wall: Single-leaf brick or stone wall (no cavity). Typical Victorian / Edwardian / pre-1930 UK construction.
  • Building Regs Part L: UK building regulations governing energy efficiency.
  • Future Homes Standard: 2025+ UK new build standard mandating low-carbon heating + tighter U-values.
  • MEES (Minimum Energy Efficiency Standards): UK regulation requiring EPC C minimum for rentals from 2028.
  • SAP (Standard Assessment Procedure): UK methodology for calculating EPC ratings.

Distribution + emitters

How heat reaches rooms.

  • Radiator (wet): Water-filled steel panel attached to wall, sized for specific heat output at designed flow temp.
  • K1 / K2 radiator: Single-panel (K1) or double-panel with fins (K2). K2 delivers ~30% more output for same footprint.
  • Underfloor heating (UFH) / wet UFH: Water pipes embedded in floor screed or insulation. Lower flow temp + larger emitter area = highest SCOP.
  • Fan coil unit (FCU): Indoor unit with fan blowing air across water coil. Used for cooling + heating; alternative to radiators for hydronic distribution.
  • TRV (Thermostatic Radiator Valve): Valve on radiator inlet enabling per-room temperature offset. Smart TRVs (Tado, Drayton Wiser, Honeywell Evohome) add scheduling + app control.
  • Zone / multi-zone: Independent thermostat areas. Single zone = whole-house common; multi-zone = per-floor or per-room.
  • Manifold: Distribution hub for UFH circuits; allows individual loop control + balancing.
  • Hydraulic separator (low-loss header): Decouples heat pump's primary circuit from distribution circuit. Used in multi-zone or low-volume systems.

Common acronyms

Quick-reference list.

  • ASHP: Air-Source Heat Pump.
  • GSHP: Ground-Source Heat Pump.
  • WSHP: Water-Source Heat Pump.
  • DHW: Domestic Hot Water.
  • UFH: Underfloor Heating.
  • WC: Weather Compensation.
  • SCOP: Seasonal Coefficient of Performance.
  • COP: Coefficient of Performance.
  • MCS: Microgeneration Certification Scheme.
  • BUS: Boiler Upgrade Scheme.
  • ErP: Energy Related Products (directive).
  • F-gas: Fluorinated greenhouse gas (refrigerant regulations).
  • GWP: Global Warming Potential.
  • DNO: Distribution Network Operator.
  • OZEV: Office for Zero Emission Vehicles (grants).
  • EPC: Energy Performance Certificate.
  • MEES: Minimum Energy Efficiency Standards.
  • HUG2: Home Upgrade Grant Phase 2.
  • ECO4: Energy Company Obligation Phase 4.
  • GBIS: Great British Insulation Scheme.
  • HMO: House in Multiple Occupation.
  • FHL: Furnished Holiday Let.
  • FCU: Fan Coil Unit.
  • TRV: Thermostatic Radiator Valve.
  • HIU: Heat Interface Unit (for communal heat networks).
  • EWI: External Wall Insulation.
  • IWI: Internal Wall Insulation.
  • LBC: Listed Building Consent.
  • PD: Permitted Development.
  • SAP: Standard Assessment Procedure.
Q01What's the difference between SCOP and COP?
COP is instantaneous efficiency at specific test conditions; SCOP is annual average across the heating season. SCOP is always lower than peak COP because it includes cold-weather + defrost cycles + aux-heater contribution. For UK heat pump shopping, focus on SCOP.
Q02What does MCS mean for heat pumps?
MCS = Microgeneration Certification Scheme. UK quality standard for renewable energy installs. MCS-certified installer required for BUS grant eligibility. Lookup at mcscertified.com to verify installer certification.
Q03What's a Heat Geek?
Heat Geek = independent training + accreditation network for UK heat pump installers focused on low-temperature design + best practices. Subset of MCS installers with higher-quality design + commissioning approach. Heat Geek installs typically deliver better SCOP than average MCS install.
Q04What's the difference between ASHP and GSHP?
ASHP (Air-Source Heat Pump) extracts heat from outdoor air via heat exchanger + fan. GSHP (Ground-Source Heat Pump) extracts heat from stable ground temperature via borehole or ground loop. GSHP higher SCOP (4.0-5.0 vs ASHP 3.0-4.5) but higher install cost (GBP 25-40k vs GBP 10-14k pre-BUS).