Do I Need New Radiators for a Heat Pump?

Most homes need at least some new radiators for a heat pump, but very few need all of them changed. A heat pump runs most efficiently with water at around 35–50°C, well below the 75°C flow temperature UK radiators are rated at, and a radiator at a lower temperature gives out much less heat. Rooms where the existing radiator is already generous for the room’s heat loss can keep it. Rooms where it is only just big enough on a boiler usually need a larger one.

The only reliable way to know is a room-by-room heat loss calculation, then a check of each radiator’s output at the heat pump’s design temperature. Below we show how that check works, with a worked example, the MCS rules your installer has to follow, what the evidence says about how many radiators change, and what swaps typically cost.

Why Does a Heat Pump Need Bigger Radiators?

A heat pump needs bigger radiators because it heats water to a lower temperature, and a radiator’s output falls sharply as the water gets cooler. To put the same amount of heat into a room at a lower temperature, you need more surface area, more panels and fins, or forced air over the radiator.

The lower temperature is where a heat pump earns its efficiency. The MCS Heat Emitter Guide states that more stars are given for lower emitter temperatures because the heat pump can then work more efficiently. Designing for 45°C instead of 55°C means bigger radiators, but it also means lower running costs every winter for the life of the system.

There is a regulatory side too. In England, Approved Document L says that when a wet heating system is newly installed or fully replaced in an existing home, the pipework and radiators should be sized so the system can run at a flow temperature of 55°C or lower. MCS design rules for heat pumps push further, asking installers who propose anything above 55°C to also offer a 55°C or lower alternative.

What Is Delta T and How Does It Change Radiator Output?

Delta T (ΔT) is the difference between the average water temperature inside the radiator and the air temperature of the room, and a radiator’s output drops much faster than ΔT does. UK radiators are rated at ΔT50, which means water flowing in at 75°C and out at 65°C in a 20°C room. That is a boiler-style test condition. A heat pump system runs well below it.

To find a radiator’s output at a lower ΔT, multiply its ΔT50 rating by a correction factor. The factors below are Stelrad’s published figures, which follow the standard industry curve used by most panel radiator manufacturers.

*Assumes a 5°C drop between flow and return, so the mean water temperature is 2.5°C below the flow temperature. Your installer’s design will set the actual figures.

The key point is in the last column. Halve the ΔT from 50 to 25 and the radiator gives out about 40% of its rated heat, not 50%. That is why a radiator that felt oversized on a boiler can struggle on a heat pump.

How Do You Check Whether an Existing Radiator Is Big Enough?

You check an existing radiator by comparing the room’s design heat loss with the radiator’s output at the heat pump’s design ΔT. If the corrected output meets or beats the heat loss, the radiator can stay. If it falls short, it needs replacing, upsizing or supplementing.

What Does a Worked Example Look Like?

The worked example here is illustrative. Take a living room in a typical 3-bed semi with a design heat loss of 1,200 W, heated to 21°C. It has one 600 x 1,000 mm double panel, double convector radiator (Type 22, often sold as K2), rated at 1,732 W at ΔT50.

Radiator outputs are from Stelrad’s Compact range data. In this example the existing radiator would have been fine on a boiler running near its ΔT50 rating of 1,732 W. On a heat pump it delivers between about 45% and 75% of what the room needs, depending on the design temperature. At 53°C the room needs about 1.35 times the existing radiator’s rating; at 43°C it needs about 2.3 times. That is the trade-off between upfront work and running cost that your installer should explain.

How Does MCS Size Radiators Room by Room?

MCS requires a room-by-room heat loss calculation to BS EN 12831-1, then an emitter in each heated room able to deliver that room’s heat loss at the system’s design flow temperature. The heat pump itself must meet 100% of the whole-house design load. Emitters are sized to the room, not guessed from the old radiator.

Under MIS 3005-D, the MCS heat pump design standard, the installer must also give you written design information. That includes the type and dimensions of each emitter, the design emitter temperature based on the worst performing room, and the design flow temperature leaving the heat pump. The worst performing room sets the flow temperature for the whole house, so one undersized radiator can force the entire system to run hotter. That is why installers sometimes upgrade a single stubborn room, such as a north-facing bedroom with two outside walls, to unlock a lower temperature everywhere else.

What Is the MCS Heat Emitter Guide Star Rating?

The MCS Heat Emitter Guide star rating grades a design by its flow temperature, from 6 stars at 35°C or below down to 0 stars above 60°C. More stars means a lower temperature and a more efficient heat pump. The guide also gives an oversize factor: multiply a room’s heat loss by it to find the ΔT50-rated radiator output needed.

This lines up with the worked example. At 41–45°C, a 1,200 W room needs 1,200 x 2.4 = 2,880 W of ΔT50-rated radiator, in the same range as the 2,970 W from the correction factor method. Where the house allows, we usually aim for 4 or 5 stars.

How Many Radiators Usually Need Changing?

Most homes change some radiators, typically the ones in rooms with high heat loss, but the number varies widely. In the government-funded Electrification of Heat trial, which installed heat pumps in 742 homes, 93% of homes needed at least one radiator replaced. The same trial found a median seasonal performance of about 2.8, so those systems still ran efficiently with a mix of new and existing radiators.

Research also suggests more existing radiators are usable than older surveys assumed. A 2025 study by Childs and colleagues used real operating data from around 4,600 boilers in England. It estimated that 31% of homes could run a heat pump at 55°C or below without upgrading radiators or insulation. An earlier government survey-based analysis had put that figure at about 10%. At lower design temperatures, such as 45°C, more rooms will need work.

In our experience, the rooms most likely to need a bigger radiator are living rooms with large windows, bathrooms heated to a higher temperature, and rooms with several outside walls. Kitchens and small bedrooms often get by with what is there.

Which Types of Radiator Work Best With a Heat Pump?

Double and triple panel convector radiators are the standard choice for heat pumps because they pack more output into the same wall space. Special heat pump radiators are not required. Any radiator works if it is sized for the lower temperature. The comparison below uses Stelrad’s ΔT50 figures for a 600 x 1,000 mm radiator and the ΔT25 correction factor.

Fan-assisted radiators suit rooms where wall space is tight. Manufacturers claim some models match a steel panel’s output at around half the size, but they cost more, need wiring and produce some fan noise, so we tend to use them selectively. Always compare radiators on their stated output at your design ΔT, not at ΔT50, because low-temperature performance differs between designs.

Will Microbore Pipework Work With a Heat Pump?

Microbore pipework (8 mm or 10 mm) can often stay for short final connections to radiators, but long microbore runs usually need replacing with larger pipe. A heat pump moves heat with a smaller temperature drop than a boiler, so it needs a higher water flow rate for the same output, and narrow pipes restrict flow.

The Energy Saving Trust’s installer guidance says water velocity should stay within 0.5–1.5 m/s, with around 0.9–1.0 m/s as best practice. Its worked example shows a short 1-metre tail of 10 mm pipe working at about 1.04 m/s. The longer the microbore run, the more likely pressure drop and flow limits mean it has to go. Your installer should check pipe sizes on each circuit during the survey, not assume. A common approach is to upgrade the main runs to larger pipe and keep short microbore tails where the calculation allows.

How Much Does It Cost to Replace Radiators for a Heat Pump?

Replacing a standard radiator typically costs around £150–£300 supplied and fitted, with double panel models at the upper end, according to UK trade price guides updated in 2026. Larger triple panel, vertical or fan-assisted radiators cost more. When radiators are changed as part of a heat pump installation, they are normally priced into the overall quote.

For example, changing three radiators in a 3-bed semi at £200–£300 each would come to roughly £600–£900 before any pipework changes. Treat that as illustrative only. Costs rise if pipework needs upsizing, if floors have to be lifted, or if you choose designer radiators.

Can You Keep Your Existing Radiators With a Heat Pump?

Yes, you can often keep many of your existing radiators, as long as each one can meet its room’s heat loss at the design flow temperature. The goal is to change only the radiators that limit the system, not every radiator in the house.

• Reduce heat loss first. Loft insulation, cavity wall insulation and draught-proofing lower each room’s heat loss, so existing radiators may become big enough.
• Upgrade the worst room only. Because the coldest room sets the flow temperature, fixing that one room can let the rest keep their radiators.
• Accept a slightly higher design temperature. A design at 50°C instead of 45°C needs fewer changes, with some loss of efficiency. Your installer’s MCS performance estimate should show the difference.
• Check the radiators work properly. Sludged or poorly balanced radiators underperform, so a system flush and balance are part of a good installation.

Radiators that stay should be in good condition and free of internal corrosion, since heat pump systems run them for longer hours at lower temperatures. If you would like to know exactly which radiators in your home would need to change, our engineers can carry out a room-by-room survey and show you the figures.