What size air source heat pump do I need?

ASHP capacity should match your property heat loss (typically 6-12 kW for UK homes) plus hot water demand (typically 2-3 kW). A common mistake is oversizing, which leads to short-cycling and reduced efficiency. A properly insulated 3-bed semi-detached house typically needs an 8-10 kW ASHP.

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

Not necessarily. Heat pumps run at lower flow temperatures (35-45°C vs 70-80°C for gas boilers), so radiators produce less heat. If your radiators are already oversized for the rooms they heat, they may work fine. Otherwise, you may need to upgrade some radiators to larger models or add fan convectors for rooms with the highest heat loss.

What COP should I expect from an air source heat pump?

Seasonal COP (SCOP) for a well-designed ASHP system is typically 2.8-3.5. This means for every 1 kWh of electricity, the heat pump produces 2.8-3.5 kWh of heat. COP is higher in milder weather and with lower flow temperatures. At -3°C outside with 45°C flow, expect a COP of around 2.5-3.0.

Is an air source heat pump cheaper to run than a gas boiler?

At current UK energy prices (gas 6.5p/kWh, electricity 24.5p/kWh), an ASHP with a COP of 3.0 effectively costs 8.2p per kWh of heat, compared to 7.6p for a 85% efficient gas boiler. With a higher COP or lower electricity tariff, the ASHP becomes cheaper. The £7,500 BUS grant and rising gas prices improve the economics further.

How big a hot water cylinder do I need with a heat pump?

A minimum 200-litre cylinder is recommended for heat pump systems. Heat pumps heat water more slowly than gas boilers, so a larger thermal store is needed. For 4+ occupants, a 250-300 litre cylinder is advisable. The cylinder must have a large coil surface area designed for heat pump temperatures.

TradeCalculator.co.uk

Air Source Heat Pump Calculator — ASHP Sizing

Size an air source heat pump for your existing property. Assess radiator compatibility, COP, running costs, and savings versus gas heating.

Property Heat Loss

From heat loss survey or EPC

Insulation Level

Overall property insulation standard

Number of Radiators

Total radiators in the property

Typical Radiator Size

Most common size in your home

Hot Water Cylinder

Existing or planned cylinder size

How We Calculate This

This calculator sizes an air source heat pump based on your property heat loss and assesses whether your existing radiators can deliver sufficient heat at the lower flow temperatures used by heat pumps.

Radiator assessment

Standard rating: Radiators are rated at delta T 50°C (flow 75°C, return 65°C, room 20°C)
Heat pump flow: At 45°C flow (delta T 25°C), radiator output drops to approximately 52% of rated output
Assessment: If total radiator output at the required flow temperature exceeds the heat loss, existing radiators are adequate

COP estimation

The Coefficient of Performance (COP) is estimated based on the flow temperature required and outside design temperature. Lower flow temperatures and milder outside conditions give higher COP. The SCOP (seasonal average) is typically better than the design-day COP because most heating hours occur in milder weather.