Best radiators for heat pumps

The physics, briefly

A radiator's output depends on the difference between the water in it and the air in the room. A boiler at 75°C gives a large difference and plenty of output from a small radiator. A heat pump at 45°C gives roughly half that difference, and therefore roughly half the output from the same radiator.

So you need either more surface area, or a higher flow temperature. More surface area costs money once. A higher flow temperature costs money every year forever. That is the whole trade-off, and it is why radiator upsizing is not an upsell.

What actually works

Underfloor heating

The ideal partner. Enormous surface area means it works beautifully at 35°C, which is where heat pumps are most efficient. If you are already fitting it, or renovating a floor anyway, take the opportunity.

Type 22 and Type 33 panel radiators

The standard retrofit answer. A Type 11 is a single panel with one convector fin set; a Type 22 is double panel, double convector; a Type 33 is triple. Swapping a Type 11 for a Type 22 of the same physical dimensions can roughly double the output without taking up any more wall. This is the single most common change we make, and it is why upsizing often does not mean bigger-looking radiators.

Fan-assisted radiators

Small radiators with a quiet fan behind them, delivering high output from a compact unit at low flow temperatures. Genuinely useful where wall space is the constraint, such as a kitchen full of units or a narrow hallway. More expensive per unit and they need a power supply, but they solve problems nothing else does.

Old cast iron radiators

Often better than people expect. Their thermal mass is large and their surface area is generous, so many period cast iron radiators work perfectly well at heat pump temperatures. They are slow to respond, which suits the steady continuous operation a heat pump wants anyway. Do not assume they need ripping out.

What does not work well

• Small Type 11 single panels sized for a 75°C boiler. These are the usual culprits and the usual replacements.
• Towel rails as the only heat source in a bathroom. Output is low even at boiler temperatures and derisory at 45°C. Bathrooms often need an additional emitter.
• Decorative column radiators chosen purely on looks without checking the output figure at 45°C rather than the headline figure, which is almost always quoted at 75°C.

How to read radiator output figures

This catches people out constantly. Manufacturers quote output at delta T 50, meaning a 50°C difference between water and room, which corresponds to boiler temperatures. At heat pump temperatures you are typically at delta T 25 or 30.

As a rough conversion, output at delta T 30 is around half the delta T 50 figure. At delta T 25 it is closer to 40%. So a radiator advertised at 1,500 watts gives you roughly 750 at heat pump temperatures. Any manufacturer worth using publishes a full table.

Do all my radiators need changing?

Almost never. In a typical three-bed retrofit we change four to six and leave the rest. The heat loss calculation gives a required output for each room, and we compare that against what the existing radiator delivers at the design flow temperature. Rooms that already have generously sized radiators, which is common in houses where an installer oversized everything years ago, often need nothing at all.

If an installer tells you every radiator needs replacing, ask to see the room-by-room comparison. If one tells you none do, ask the same question.