Radiator Sizing for Heat Pumps: Do You Need to Upgrade?
The short answer
- Some radiators, not necessarily all, usually need changing when you switch to a heat pump
- About 3.1 times the room's heat loss is the rated output a radiator needs at 41 to 45°C, against 1.9 times at 51 to 55°C
- About 47% more electricity for the same heat when a heat pump runs at 55°C rather than 35°C
Many homes switching to a heat pump need some radiators changed, though not necessarily all of them. A heat pump runs most efficiently when the water it sends round is cool, and a radiator gives out far less heat at those temperatures. The MCS Heat Emitter Guide says that with water at 41 to 45°C, a radiator's standard rated output needs to be about 3.1 times the room's heat loss, against 1.9 times at 51 to 55°C. This guide explains how radiator sizing works with heat pumps, how to check whether your radiators are big enough, and when underfloor heating is worth considering instead.
Why do heat pumps need different radiators?
A heat pump is most efficient making cool water, and radiators give out less heat at low temperatures. In the trial, running at 55°C instead of 35°C used about 47% more electricity for the same heat.
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Radiators in homes built around a gas boiler were usually sized to run hot. The bigger the gap between the water temperature and the room, the faster a radiator gives out heat, which is why a small radiator can heat a large room from a boiler.
A heat pump works the other way: the cooler the water it has to make, the more heat it gets from each unit of electricity. In the Electrification of Heat trial for DESNZ, the median heat pump ran at 3.50 while producing water at 35°C, 2.38 at 55°C and 2.02 above 65°C. Running at 55°C instead of 35°C used about 47% more electricity for the same heat.
The solution is not to run the heat pump hotter. It is to install radiators with enough surface area to heat each room at the lower flow temperature. Read our running costs guide for how flow temperature affects your annual bills.
How much bigger do radiators need to be?
Rated output of about 3.1 times the room's heat loss at 41 to 45°C, against 1.9 times at 51 to 55°C. So a radiator rated 1,000 W gives only about 320 W at 41 to 45°C.
| Flow temperature | Radiator rating needed, times the room's heat loss | Fan assisted radiator | What a radiator rated 1,000 W gives |
|---|---|---|---|
| Up to 35°C | 6.8 | 4.3 | About 150 W |
| 36 to 40°C | 4.3 | 3.1 | About 230 W |
| 41 to 45°C | 3.1 | 2.4 | About 320 W |
| 46 to 50°C | 2.4 | 2.0 | About 420 W |
| 51 to 55°C | 1.9 | 1.7 | About 530 W |
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The data behind this table
Oversize factors from the MCS Heat Emitter Guide (MCS 021, Issue 2.1): multiply a room's heat loss by the factor to get the output the radiator needs at its standard rating, a mean water to air temperature difference of 50°C. The last column is 1,000 W divided by the radiator factor, rounded.
What radiator changes cost depends on how many rooms fall short and what goes in their place, so ask for them to be itemised in each quote; see our heat pump cost by house type guide for the heat pump itself. The £7,500 Boiler Upgrade Scheme grant in England and Wales comes off the installer's price for the whole job. Reducing heat loss through insulation cuts the number of radiators that need changing.
How do you check your radiators?
Divide each radiator's rated output by the factor for your planned flow temperature and compare it with the room's heat loss. At 45°C a radiator rated at 1,000 watts gives about 320 watts; your MCS installer does this room by room.
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Your MCS installer works out each room's heat loss and compares it with what each radiator gives at the planned flow temperature. This is part of a proper heat pump design and should be in your quote.
As a rough check, find each radiator's rated output, usually on a label or in the manufacturer's data, and divide it by the factor in the table for your planned flow temperature. Older radiators are sometimes rated at a 60°C difference; MCS converts those to the standard rating by multiplying by 0.825. At 45°C, a radiator rated at 1,000 watts gives about 320 watts.
If 320 watts covers the room's heat loss on a cold day, the radiator can stay. If the room needs 700 watts, it needs replacing with a larger one or supplementing.
Which radiators usually need replacing?
Small single-panel radiators in large rooms, bathroom towel rails, and small radiators in kitchens and hallways. A radiator rated at 800 watts gives only about 260 watts with water at 45°C.
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Small single-panel radiators in large rooms. These are the most common candidates. A radiator rated at 800 watts gives only about 260 watts with water at 45°C.
Bathrooms with towel rails. Towel radiators have limited surface area and can struggle at heat pump temperatures. You may need a larger towel radiator or an extra panel radiator.
Kitchens and hallways with small radiators. These rooms often have the smallest radiators because space was limited when the system was originally designed. Check them first.
Which radiators often survive?
Large cast iron radiators, oversized modern ones, double convectors and radiators in well-insulated rooms. Insulating first helps: in the MCS example, a room's heat loss fell from 1,671 W to 976 W and it then worked at 45°C.
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Large cast iron radiators in period homes. Big old radiators can have a lot of surface area for the room they heat, so check their output before replacing them. If your old house has original large radiators, they may be an asset rather than a liability.
Oversized modern radiators. Some homes already have radiators larger than the room strictly needs. If your existing radiators are double-panel convectors (the thick type with fins between two panels), they may have enough output at lower temperatures.
Rooms in well-insulated parts of the house. If a room has new double glazing, insulated walls, and minimal draughts, its heat demand is low enough that even a modest radiator may cope. This is another reason to insulate before installing a heat pump.
Insulate first to reduce radiator upgrades
Better insulation cuts each room's heat loss, so existing radiators are more likely to be big enough. In the MCS Heat Emitter Guide's worked example, cavity wall insulation, double glazing, underfloor insulation and draught-proofing cut a room's heat loss from 1,671 W to 976 W; with a double convector radiator of the same size, the room went from needing water above 60°C to working at 45°C. Start with loft insulation, cavity wall insulation and draught-proofing.
What can replace a radiator that is too small?
A larger panel radiator in the same space, a fan-assisted radiator, or underfloor heating. In the MCS example a same-size double convector gave about 70% more output; underfloor heating usually only suits a major renovation.
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Larger panel radiators
The most common solution is a radiator with more output in the same space. In the MCS worked example, swapping a 1,600 by 700 mm double panel radiator for a double convector with the same face size raised its rated output from 1,938 W to 3,269 W, about 70% more.
Fan-assisted radiators
These have a built-in fan that blows air across the heat exchanger, which raises output at low flow temperatures. In the MCS guide, a fan assisted radiator needs to be rated at 2.4 times the room's heat loss at 41 to 45°C against 3.1 for an ordinary radiator, so it can be about a quarter smaller, or about a third smaller at 35°C. They suit rooms where there is no space for a larger radiator.
The fan uses some electricity and makes some noise, so check the specification before fitting one in a bedroom.
Underfloor heating
Underfloor heating (UFH) can run at the lowest flow temperatures, where a heat pump is most efficient, and the MCS guide covers underfloor systems at 35°C and below. It also spreads heat evenly across the room.
However, retrofitting UFH means lifting the existing floor and usually raises floor levels, so it is realistically only practical during a major renovation or when floors are being replaced anyway. For new extensions or ground floor renovations, it is worth specifying from the start.
What should your installer provide?
A room by room heat loss calculation, each radiator's output at the planned flow temperature, and which need upgrading, with costs. If an installer proposes a heat pump without one, that is a red flag.
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A competent MCS installer will provide a room-by-room heat loss calculation showing the heat demand of each room in watts, the output of each existing radiator at the proposed flow temperature, identification of which radiators need upgrading, and recommended replacements with costs.
This room by room calculation should be part of any professional heat pump quote. If an installer proposes a heat pump without one, that is a red flag. Find certified installers on the MCS directory.
Use our heat pump calculator for a cost estimate for your home, and our boiler vs heat pump tool to compare total costs against sticking with gas.
Frequently asked questions
Do I need new radiators for a heat pump?
Many homes need some radiators changed, though not necessarily every one. MCS guidance says that with water at 41 to 45°C a radiator's rated output needs to be about 3.1 times the room's heat loss. Your MCS installer works it out room by room. Insulating first reduces the number needing replacement.
How much does it cost to upgrade radiators?
It depends on how many rooms fall short and what goes in their place, so ask for radiator work to be itemised in each quote. See our cost by house type guide for the full picture.
Can I keep my existing radiators?
Sometimes. Large radiators, double convectors and radiators in well-insulated rooms may be big enough already. Your installer's room by room calculation confirms it.
Is underfloor heating better than radiators for a heat pump?
Thermally, yes: it can run at the lowest flow temperatures, where heat pumps are most efficient. Practically, retrofitting it means lifting floors, so it is usually only worth it during a major renovation. Larger radiators are the practical choice for most homes.
What flow temperature should a heat pump run at?
As low as your radiators allow. In the Electrification of Heat trial, the median heat pump ran at 3.50 units of heat per unit of electricity when producing water at 35°C, 2.38 at 55°C and 2.02 above 65°C, so running at 55°C rather than 35°C used about 47% more electricity for the same heat. See our running costs guide.
Data sources
Radiator sizing comes from the MCS Heat Emitter Guide and efficiency by flow temperature from the Electrification of Heat trial, with the grant rules from GOV.UK.
Show every source
Radiator oversize factors and worked examples from the MCS Heat Emitter Guide (MCS 021, Issue 2.1). Efficiency by flow temperature from the Electrification of Heat Demonstration Project, Heat Pump Performance Data Analysis Report (Energy Systems Catapult for DESNZ, December 2024), section 1.4.4. Grant from GOV.UK. All checked on 25 September 2026.