How Hot Do Radiators Get with an Air Source Heat Pump?

Radiator Temperatures with Heat Pumps

With an air source heat pump, radiators typically operate at flow temperatures of 35-55°C, compared to 60-75°C with a gas boiler. This means radiators will feel warm rather than very hot to the touch. While this might initially seem less effective, it actually provides more even and comfortable heating throughout your home.

The lower flow temperature is a key part of how heat pumps achieve their high efficiency. At 35°C flow temperature, a heat pump can achieve a COP of 4.0 or higher, meaning it produces 4 kW of heat for every 1 kW of electricity. At 55°C, the COP drops to around 2.5-3.0 — still efficient, but less so.

Do You Need Bigger Radiators?

Because heat pumps operate at lower flow temperatures, you may need larger radiators to deliver the same amount of heat to each room. As a general rule, radiators may need to be 1.5-2x larger than those used with a gas boiler. However, this depends on your home's insulation level and heat loss.

In many cases, not all radiators need upgrading. Your MCS-certified installer will carry out a detailed heat loss calculation for each room and advise which radiators, if any, need to be replaced. Homes with good insulation may not need any radiator changes at all.

Alternative Heat Distribution Options

Underfloor heating is the ideal partner for a heat pump, as it operates at 30-40°C — the temperature range where heat pumps are most efficient. If you're renovating or building an extension, underfloor heating is well worth considering.

Fan-assisted radiators (also called fan convectors) are another excellent option. These use a small fan to increase heat output at lower flow temperatures, meaning you don't need to replace your radiators with larger ones. They're compact, efficient, and can be retrofitted to existing systems.