Radiator sizing guide

Heat pumps work differently than traditional gas or oil boilers. While a boiler might send water to your radiators at 70°C, a heat pump is most efficient when it sends water at between 35°C and 45°C.

To keep your home just as warm at these lower temperatures, your radiators often need to be larger to provide enough “surface area” to emit the required heat.

Why size matters

The heat output of a radiator depends on:

  1. Surface area: The physical size and number of panels/fins.
  2. Delta T (ΔT): The difference between the average radiator temperature and the room temperature.

Because the ΔT is much lower with a heat pump, a radiator that was perfectly sized for a gas boiler may only provide 30-40% of its rated output when connected to a heat pump.

Checking your current radiators

A professional MCS-certified installer will perform a room-by-room heat loss calculation to determine exactly what size radiators you need. However, you can do a quick check yourself:

  • Type 11 (K1): Single panel with one set of convection fins.
  • Type 21 (P+): Double panel with one set of fins.
  • Type 22 (K2): Double panel with two sets of fins.
  • Type 33 (K3): Triple panel with three sets of fins.

Upgrading from a Type 11 to a Type 22 of the same dimensions can almost double the heat output without taking up more wall space.

Options for upgrading

If your survey shows that a room needs more heat, you have several options:

1. Larger standard radiators

Replace your existing unit with a taller, wider, or deeper (Type 22 or 33) standard radiator.

2. Low-temperature radiators

Specialised radiators designed specifically for heat pumps. They often include small, silent fans that pull air across the fins to increase heat output significantly.

3. Underfloor heating (UFH)

UFH is the “gold standard” for heat pumps. Because the entire floor becomes a radiator, it can run at very low temperatures (30-35°C), which maximises the efficiency of your heat pump.

4. Smart fans (Add-ons)

You can buy small fan units that clip onto the bottom of your existing radiators. These can boost output by 20-30% and may save you from having to replace the radiator entirely.

Pro Tip: If you are renovating your home or replacing radiators before you install a heat pump, always choose “low-temperature ready” sizes. This “future-proofs” your home and makes the eventual transition to a heat pump much cheaper and easier.

Impact on efficiency

Sizing your radiators correctly doesn’t just keep you warm; it also lowers your bills. Every 1°C you can lower your flow temperature increases the efficiency of your heat pump by around 2% to 3%.

Learn more about heat pump running costs and how flow temperature affects them.