What size heat pump do I need for a 3-bedroom house?

A typical 3-bedroom semi-detached house (90-110 m²) with average insulation needs an 8-12 kW air source heat pump. Well-insulated properties may need as little as 6-8 kW. Poorly insulated homes may need 12-16 kW. A full MCS-compliant heat loss survey is essential before installation.

Is an air source or ground source heat pump better?

Air source heat pumps (ASHP) are cheaper to install (typically £8,000-15,000 including BUS grant) and suitable for most UK homes. Ground source heat pumps (GSHP) are more efficient (higher COP) and cheaper to run, but installation costs are higher (£15,000-25,000+) and they need significant garden space. ASHP are the most popular choice in the UK.

Do I need to upgrade my radiators for a heat pump?

Often, yes. Heat pumps operate at lower flow temperatures (35-50°C vs 60-80°C for gas boilers), which means radiators need to be larger to deliver the same heat output. A heat pump installer will calculate the radiator sizes needed — use our Radiator Sizing Calculator to check individual room requirements. In well-insulated homes, existing radiators may be adequate. Underfloor heating is ideal for heat pumps — see our Underfloor Heating Pipe Calculator for pipe quantities.

What is the Boiler Upgrade Scheme (BUS) grant?

The UK Boiler Upgrade Scheme provides grants of £7,500 towards the cost of an air source or ground source heat pump installation. The scheme runs until March 2028. The property must have an EPC certificate and the installation must be done by an MCS-certified installer. The grant is applied directly by the installer, reducing your upfront cost.

What COP can I expect from a heat pump in the UK?

In UK conditions, a well-installed air source heat pump typically achieves a seasonal COP (SCOP) of 2.8-3.5, meaning for every 1 kWh of electricity used, it produces 2.8-3.5 kWh of heat. Ground source heat pumps achieve 3.5-4.5 SCOP. Actual performance depends on flow temperature, installation quality, and climate.

Heat Pump Size Calculator — What kW Heat Pump Do I Need for My Home?

How We Calculate This

Our heat pump sizing calculator estimates the heat loss of your property and recommends an appropriate heat pump capacity in kW. The method follows MCS (Microgeneration Certification Scheme) guidelines, which are required for BUS grant eligibility.

The calculation method

Heat pump size (kW) = (Floor area × Heat loss rate × Property factor) ÷ 1000 + Hot water allowance

Heat loss rates (W/m²)

Poor insulation (pre-1920): 120-180 W/m²
Average insulation (1920-1980): 80-120 W/m²
Good insulation (post-1990): 50-80 W/m²
Excellent (new build / retrofit): 40-60 W/m²

Running cost estimate

Annual running costs are estimated using typical heating hours (2,000 per year) and the heat pump's Coefficient of Performance (COP). ASHP typically achieves COP 3.0, GSHP achieves COP 4.0. The electricity cost is calculated from your entered price per kWh.

Important note

This calculator provides a rough estimate. A full MCS-compliant heat loss survey (room-by-room calculation to MIS 3005) is required before installation and for BUS grant eligibility. The survey accounts for specific building fabric, window areas, ventilation rates, and local design temperatures.

Frequently Asked Questions

Related Calculators

Pipe Sizing Calculator

Calculate the correct pipe diameter for your plumbing installation.

Boiler Size Calculator

Determine the right boiler kW output for your UK property.

Hot Water Cylinder Sizing Calculator

Size your hot water cylinder based on household demand.

Radiator Sizing Calculator

Calculate the BTU output needed for each room.

Central Heating System Volume Calculator

Calculate total water volume in your central heating system.

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Heat Pump Sizing Calculator — What Size Heat Pump Do I Need?