How-to · 4 min read · Updated 6 November 2026
Are my radiators heat pump ready? A room-by-room checklist
Why heat pumps need different radiators
A gas boiler can push water round the system at 70°C or more, so even a small radiator gives out plenty of heat. A heat pump is most efficient when it runs at a much lower flow temperature, often 35°C to 50°C. At those temperatures a radiator gives out a fraction of its catalogue output, so some rooms can end up underheated unless the radiator is larger.
Radiator outputs are usually published at a mean water-to-air temperature difference called Delta T50. At a heat pump's typical Delta T of around 20 to 30, the output can drop to roughly a third to a half of that figure. This is the single most important number to understand before talking to installers.
| Mean water to room difference | Approximate share of Delta T50 output |
|---|---|
| Delta T50 (old boiler system) | 100% |
| Delta T40 | About 75% |
| Delta T30 | About 50% |
| Delta T25 | About 39% |
| Delta T20 | About 29% |
These factors are approximate and vary slightly by manufacturer, but they show why a radiator that was fine with a boiler may struggle at heat pump temperatures.
Step 1: list every radiator and its size
Go room by room and note the radiator type, height, length and number of panels and fins. A type 11 is a single panel with one set of fins, type 21 is double panel with one set of fins, type 22 is double panel with two sets, and type 33 is triple panel. Photograph the label if there is one. This gives a heat pump designer a head start and lets you sense-check their recommendations.
Step 2: work out each room's heat loss
A proper heat pump design should include a room-by-room heat loss calculation, usually following the MCS method. This takes into account wall construction, windows, floor and ceiling types, room height and the design outdoor temperature for your area. If an installer quotes without measuring the rooms, treat that as a warning sign.
Rooms that most often need bigger radiators include living rooms with bay windows, rooms with two or more external walls, ground-floor rooms over suspended timber floors, and rooms in extensions with lots of glazing.
Step 3: compare radiator output with heat loss
For each room, take the radiator's Delta T50 output, apply the factor for the planned flow temperature, and compare it with the room's heat loss. If the reduced output covers the heat loss, the radiator can stay. If not, the options are a larger radiator, a deeper radiator such as a type 22 or 33, an extra radiator, or improving the room's insulation so it loses less heat.
- Radiator output covers heat loss: keep it
- Short by a small margin: consider a deeper model of the same width
- Short by a lot: larger or additional radiator, or underfloor heating
- Room has very high heat loss: look at insulation or glazing first
Step 4: check pipework and valves
Heat pumps move more water than boilers at a smaller temperature difference between flow and return. Very narrow pipework, such as 8mm or 10mm microbore, can restrict flow and may need partial replacement on some runs. Old valves that are seized or partly blocked also limit flow. A good survey will look at pipe sizes, not just radiators.
TRVs can work with heat pumps, but many designers prefer to leave most rooms open and use weather compensation to control output, with TRVs only in rooms such as bedrooms where people like it cooler.
Step 5: think about grants and timing
In England and Wales, the Boiler Upgrade Scheme has offered a grant of £7,500 towards an air source heat pump, paid to the installer and taken off your bill. Check the current terms on the government website, because schemes change. Radiator upgrades needed for the heat pump are often included in the installer's overall price.
If you are not ready for a heat pump yet but need to replace radiators now, ask for replacements sized for low temperature operation. You will pay a little more for larger radiators today but may avoid a second round of work later, and you can already turn your boiler's flow temperature down to save gas.
Questions to ask before a heat pump survey
- Will you carry out a room-by-room heat loss calculation?
- What design flow temperature are you planning?
- Which radiators will be replaced, and why?
- Will any pipework need upsizing?
- Are new radiators and pipework included in the quote?
- How will the system be balanced and commissioned?
Frequently asked questions
Do I need to replace all my radiators for a heat pump?
Usually not. Most UK homes need a few radiators upgrading, often in the living room, kitchen-diner or rooms with large windows, while many others can stay. The answer comes from comparing each room's heat loss with the radiator's output at the planned flow temperature. A proper room-by-room heat loss calculation is the only reliable way to know which ones need changing.
Are aluminium or column radiators better for heat pumps?
Material matters less than output at low temperature. Aluminium heats up quickly, and column radiators can look attractive, but what counts is whether the radiator delivers enough heat at Delta T30 or below. Large steel double or triple panel radiators are often the most cost-effective way to get the output you need. Check the manufacturer's low temperature output figures before buying.
Can I run my boiler at a lower temperature to test this?
Yes. Many modern condensing boilers let you set the flow temperature, and turning it down to around 50°C to 55°C on a mild day shows which rooms struggle. Rooms that stay warm are likely fine. This test is not a full heat loss calculation but is a useful early guide, and running a condensing boiler cooler can also improve its efficiency.
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