The Short Answer
Probably some, and we'd replace only the ones a room-by-room heat loss survey says fall short. A heat pump heats the water to around 35C to 55C instead of a boiler's 70C to 80C, so each radiator gives out much less heat, and rooms that are already borderline won't keep up. In a well-insulated home, many existing radiators cope. In a draughty one, it can be most of them.
On our estimate, a typical three-bed semi might need five to seven replaced, at about £150 to £400 each fitted, so £750 to £2,800. Only the survey tells you which, so be wary of any quote that replaces every radiator without one. Check your own rooms below. The checker assumes the heat pump sends water to the radiators at about 52C, a middle-of-the-road setting between the 45C and 55C figures in the sizing section.
Radiator Size Checker
Enter your room details to see what size radiator you need for a heat pump.
The honest answer is that it depends on your home. Some houses can use every existing radiator without spending a penny on upgrades. Others need most of them replaced (up to about twelve in a big, poorly insulated house, on our estimate) before a heat pump will heat the home comfortably.
Why Heat Pumps Need Larger Radiators
A gas boiler typically heats water to 70C to 80C before circulating it through your radiators (British Gas's figures). A heat pump, by contrast, works most efficiently at much lower flow temperatures, usually 35C to 55C.
The amount of heat a radiator emits depends on the difference between the water inside it and the air in the room, which the industry calls Delta T. Radiator catalogues quote output at Delta T50, which is boiler territory. We've used Stelrad's published correction factors, assuming a 20C room with water cooling by 5C across the radiator. On those, the same radiator gives out only about 57% of its catalogue output with the heat pump at 55C, and about 35% at 45C. Drop from boiler temperatures to 45C and you lose roughly two-thirds of the heat.
To compensate, you have two options: run the heat pump at a higher flow temperature (reducing its efficiency), or fit larger radiators that produce the same total heat output at lower temperatures. The second option is nearly always better for your running costs.
When Your Existing Radiators May Be Fine
Not every home needs radiator upgrades. Your existing radiators may be perfectly adequate if:
- Your home is well insulated: loft insulation, cavity wall insulation and double glazing reduce the heat demand so much that smaller radiators can cope
- Your radiators were already oversized when installed. If a room is comfortably warm now with the radiator turned down, there's a buffer there.
- You are happy to run the heat pump at a higher flow temperature, accepting higher running costs in exchange for avoiding radiator replacement costs
A good MCS-certified heat pump installer will carry out a room-by-room heat loss survey. This calculates how much heat each room needs at the design outdoor temperature (a cold winter day for your area) and checks whether your existing radiators can deliver it at the intended flow temperature. Don't accept a quote from any installer who skips this step.
When You Will Need to Upgrade
Radiator upgrades are likely if you have:
- Cold rooms that are hard to heat even now, which may have undersized radiators already
- Small, decorative radiators that were chosen for appearance rather than output
- Poor insulation: solid walls, an uninsulated loft or single glazing mean heat demand is too high for standard radiators at low flow temperatures
- A large older property with high ceilings, where heat loss is substantial
How to Size Replacement Radiators
To give the heat the old one gave on the boiler, a replacement needs a Delta T50 rating about 1.8 times the old radiator's if the heat pump runs at 55C, and nearer 2.8 times at 45C (Stelrad's correction factors). It needs more if the old one was already struggling. In practice, your installer calculates the required output (in watts) for each room from its heat loss, then picks a radiator big enough to deliver that at the system's design flow temperature. To size a radiator room by room, try our radiator BTU calculator.
For example, a room with a heat loss of 1,000W at design conditions needs a radiator that gives at least 1,000W at the flow temperature you'll actually run, not 1,000W in the catalogue. Catalogues quote output at Delta T50, so ask your installer to show each radiator's output at your design flow temperature, or the correction factor they've applied to get there.
What Radiator Upgrades Cost
| Scenario | Radiators Replaced | Approximate Cost (our estimate) |
|---|---|---|
| Small terraced house, well insulated | 0 to 3 | £0 to £1,200 |
| Average semi-detached, mixed insulation | 5 to 7 | £750 to £2,800 |
| Larger detached or poorly insulated home | 7 to 12 | £1,050 to £4,800 |
On our estimates, replacing a radiator costs about £150 to £400 supplied and fitted: the lower end for a small one swapped onto the existing pipework, the top end for a large one or one in a new position. That's a sense-check against trade prices, not a quote.
A typical three-bedroom semi might need five to seven radiators replaced, so £750 to £2,800. A larger or poorly insulated home needing ten or twelve costs more, as the table above shows. These costs sit on top of the heat pump installation itself, so factor them into your overall budget from the start.
Alternatives to Replacing Radiators
Underfloor Heating
Underfloor heating (UFH) gives out its heat across the whole floor rather than from one panel. For the reasons above, that lets it run at lower water temperatures than radiators, which is what a heat pump wants. If you're renovating or building an extension, adding UFH at that point is well worth considering. Retrofitting UFH into an existing home with solid floors is more disruptive and expensive, but it is feasible in major refurbishments.
Fan Convectors
Fan convectors look similar to radiators but contain a small fan that blows room air over a heat exchanger. Myson, which makes them, pitches its fan convectors at low-temperature heating systems, heat pumps included. They're worth asking about for a room where there isn't space for a bigger radiator; ask for the unit's output at your flow temperature, as you would for a radiator.
Get free heat pump quotes
Match with MCS-certified installers who take the Boiler Upgrade Scheme grant off the price in England and Wales, and get personalised quotes for your home. Free, no obligation, takes about 60 seconds.
The Practical Approach
If you're getting heat pump quotes, ask each installer to include a detailed room-by-room assessment. A good one replaces only the radiators that fall short. A quote that replaces every radiator without a survey behind it is a warning sign, so get at least three proposals and compare the radiator upgrade recommendations carefully.
For more on heat pump running costs after installation, see our guide to heat pump running costs. If you're weighing the full cost picture, our heat pump costs guide covers everything from unit prices to grant funding. If you do need to upgrade, our best radiators for heat pumps guide covers what to buy. If you add smart TRVs, we'd set them up with your installer and not shut rooms right down, because a heat pump runs best low and steady.
