Air Source Heat Pump (ASHP) Heat Loss Sizing

Air Source Heat Pump Size Calculator

Calculate total heat loss (kW and BTU/hr) across every room in your property using CIBSE fabric and ventilation standards. Sizes outdoor ASHP units with domestic hot water allowances and UK cold weather buffers.

Last reviewed 28 July 2026
Metric & Imperial
Room-by-Room CIBSE
DHW Cylinder Sizing
2026 UK Running Costs
Rule of thumb: UK domestic air source heat pumps are sized between 5 kW and 12 kW for standard homes. Heat loss is calculated at -3°C outdoor design temperature. Add 2.5 kW for hot water (DHW) cylinder heating and a 15% defrost buffer. Avoid oversizing to prevent short-cycling and high running costs.
Property & Heat Loss Settings
Configure property age, hot water demand, and room dimensions
Unit System

Sets default wall and roof thermal transmittance (U-values) based on UK building era.

Adds hot water cylinder re-heat capacity (kW) to the space heating heat loss.

Add all heated rooms
Sizing follows CIBSE fabric & ventilation heat-loss methods at -3°C UK design temperature, with domestic hot water cylinder allowance and 15% cold-weather defrost buffer.

Technical Guide & Standards

How UK Heat Pump Sizing Works

Understand CIBSE heat-loss calculations, flow temperature differences, and why proper kW sizing is essential for efficiency.

1. The CIBSE Heat Loss Calculation Method

Unlike gas boilers that are routinely oversized (e.g. 24kW–30kW) to rapidly heat domestic hot water on demand, heat pumps must be sized precisely to match the property's peak heat loss on the coldest day of the year. In the UK, CIBSE (Chartered Institution of Building Services Engineers) standards specify a design outdoor temperature of -3°C.

Total Room Heat Loss Equation
Total Heat Loss (W) = [Fabric Loss] + [Ventilation Loss] × North Factor × Design Margin
• Fabric Loss (W) = ∑ (Area × U-value × ΔT)
• Ventilation Loss (W) = 0.33 × Air Changes/hr × Volume (m³) × ΔT

Where ΔT is the difference between target indoor temperature (e.g. 21°C living room) and the -3°C outdoor design minimum (ΔT = 24°C).

Heat Pump Size Reference Table (UK Domestic)
Typical nominal ASHP kW ratings based on property size and construction era
Property Type & Era Floor Area Peak Heat Loss Recommended HP Size
1–2 Bed Flat / Modern Apartment 40–60 m² 2.0–3.5 kW 4–5 kW
2–3 Bed Semi (Post-2000 Insulation) 70–90 m² 3.5–5.0 kW 6–7 kW
3–4 Bed Semi / Detached (1970–2000) 100–130 m² 5.5–8.0 kW 8–10 kW
4–5 Bed Detached (Pre-1970 Uninsulated) 140–180 m² 9.0–12.5 kW 12–14 kW
Large Victorian / Edwardian Solid Wall 200+ m² 13.0–16.0+ kW 14–16 kW (or Cascade)

2. Why Oversizing Kills Efficiency

When an installer fits a 12 kW heat pump to a property that only needs 6 kW of peak heat loss, the unit will short-cycle during autumn and spring when heating demand drops to 2–3 kW. Short-cycling causes:

  • Lower COP (Coefficient of Performance): Efficiency drops from ~3.8 down to 2.2 because inverter compressors operate poorly when rapidly switching on and off.
  • Higher Electricity Bills: Compressor inrush currents spike power consumption every time the unit restarts.
  • Premature Failure: Excessive thermal expansion cycles shorten compressor lifespan.

Always size to the calculated room-by-room heat loss plus standard DHW allowance and cold-weather defrost buffer — never add arbitrary safety margins.

3. UK Boiler Upgrade Scheme (£7,500 BUS Grant)

Under the UK Government's Boiler Upgrade Scheme (BUS), homeowners in England and Wales can claim an upfront grant of £7,500 towards installing an air source heat pump.

To qualify for the grant, the system must be installed by an MCS-certified installer who performs a formal room-by-room CIBSE heat-loss calculation matching the standards used in this tool.

Common Questions

Heat Pump Sizing FAQs

An air source heat pump (ASHP) is sized to match total fabric and ventilation heat loss on the coldest day of the year (typically -3°C in the UK according to CIBSE design standards). Unlike a gas boiler which can cycle on and off at 24kW–30kW for quick hot water, a heat pump runs continuously at lower temperatures. You must calculate room-by-room heat loss in Watts, sum them for the property, add an allowance for hot water cylinder heating (typically 2.0kW–3.5kW), and apply a cold-weather defrost buffer (15%).
Because heat pumps run at lower flow temperatures (35°C–55°C compared to 65°C–70°C for gas boilers), radiator surface area must be matched to each room's exact heat loss. Sizing room-by-room ensures no individual room is left cold and identifies where radiator upgrades or micro-bore pipe replacements are required before installation.
No. Oversizing an air source heat pump is harmful to performance and efficiency. An oversized heat pump will short-cycle during mild weather (autumn/spring), causing premature compressor wear, lower COP (Coefficient of Performance), higher electricity bills, and uncomfortable temperature swings. Always size to the calculated design heat loss with a standard 15% buffer.
Yes. An air source heat pump heats an unvented hot water cylinder. Because hot water cylinder reheating requires extra thermal capacity alongside space heating, an additional allowance of 2.0 kW to 3.5 kW is added to the space heating heat loss depending on household size and cylinder capacity.
Air source heat pumps extract heat from outside air. In cold winter conditions (-3°C design outdoor temperature), frost forms on the outdoor evaporator coil, requiring automated defrost cycles. The 15% buffer ensures the heat pump maintains full house warmth during defrost operation.
Yes, provided the radiators are large enough to deliver the required room heat loss at a lower flow temperature (typically ΔT30 at 45°C–50°C flow, rather than ΔT50 at 65°C gas boiler flow). This calculator gives per-room heat loss in Watts so you can check your existing radiator ratings.

More Heating & Energy Tools

You May Also Need

Energy
Radiator BTU Calculator
Size room radiators with ΔT30 heat-pump low flow temperature corrections.
Open
Energy
Air Conditioner Calculator
Calculate AC cooling sizing in BTU, kW and Tons, with efficiency and UK running costs.
Open
Energy
Loft Insulation Calculator
Rolls needed to reach the recommended 270mm depth to current Building Regs.
Open
Energy
Wall Insulation Calculator
Insulated plasterboard or stud and slab quantities for internal wall lining.
Open
HP Size
—
Heat Loss
—
Hot Water
—
Rooms
—