Radiator Sizing for Heat Pumps: Do You Need to Upgrade?
Most homes switching to a heat pump need some radiator upgrades, but not all of them. Heat pumps deliver water at 35 to 45 degrees versus the 60 to 70 degrees from a gas boiler. At these lower temperatures, a standard radiator produces roughly half its rated output. This means some rooms will need larger radiators to maintain comfortable temperatures. The typical cost is £2,000 to £4,000 for a 3-bed house. This guide explains how radiator sizing works with heat pumps, how to check whether your existing radiators are adequate, and when underfloor heating is worth considering instead.
Why heat pumps need different radiators
A gas boiler pushes water through radiators at 60 to 70 degrees. The large temperature difference between the radiator surface and the room air drives rapid heat transfer. This is why a relatively small radiator can heat a large room with a gas boiler.
A heat pump operates most efficiently at 35 to 45 degrees flow temperature. Every degree higher reduces the heat pump's COP (efficiency), increasing your electricity bills. At 35 degrees, a well-installed heat pump achieves a COP of 3.0 to 3.5. At 55 degrees (to compensate for undersized radiators), COP drops to 2.0 to 2.5. That 30 to 50% drop in efficiency translates directly to 30 to 50% higher running costs.
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.
Typical upgrade costs
| Property type | Radiators typically needing upgrade | Cost |
|---|---|---|
| 2-bed terrace | 2 to 4 | £1,000 to £2,500 |
| 3-bed semi | 3 to 5 | £2,000 to £4,000 |
| 4-bed detached | 4 to 7 | £3,000 to £5,500 |
| Full house (all radiators) | All rooms | £4,000 to £7,000 |
Costs include radiator units, valves, labour, and any necessary pipework modifications. Not every room typically needs an upgrade.
These costs are in addition to the heat pump installation. See our heat pump cost by house type guide for the full picture. The £7,500 BUS grant does not cover radiator upgrades directly, but reducing your heat demand through insulation reduces the number of radiators that need upgrading.
How to check your existing radiators
Your MCS installer will perform a room-by-room heat loss calculation and compare each room's heat demand against the output of the existing radiator at heat pump flow temperatures. This is part of the standard design process and should be included in your quote.
As a rough guide, you can estimate whether your radiators are large enough by looking at the radiator's rated output (usually printed on a label or available from the manufacturer) and applying a simple rule: at 40 degrees flow temperature, a radiator produces approximately 44% of its output at the standard rating condition (Delta T 50). So a radiator rated at 1,000 watts will produce roughly 440 watts at heat pump temperatures.
If 440 watts is enough to heat the room (based on its size, insulation, and glazing), the radiator can stay. If the room needs 700 watts and the radiator can only deliver 440, it needs replacing with a larger one.
Which radiators usually need replacing
Small single-panel radiators in large rooms. These are the most common upgrade candidates. A single-panel 600mm by 1000mm radiator rated at 800 watts on a gas boiler produces only about 350 watts at heat pump temperatures. If it is heating a 4 by 4 metre bedroom, that is not enough.
Bathrooms with towel rails. Towel radiators have limited surface area and are often undersized even for gas boiler systems. They struggle badly at heat pump temperatures. You may need a larger towel radiator or a supplementary 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. They frequently need upgrading.
Which radiators often survive
Large cast iron radiators in period homes. Victorian and Edwardian cast iron radiators have enormous surface area. Many can deliver adequate heat at 40 degrees without replacement. If your old house has original large radiators, they may be an asset rather than a liability.
Oversized modern radiators. Some plumbers historically fitted radiators larger than strictly necessary. 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 reduces each room's heat demand, which means existing radiators are more likely to be adequate. Loft insulation, cavity wall insulation, and draught-proofing can reduce the number of radiators needing replacement from 5 to 7 down to 2 to 3, saving £1,000 to £3,000 on upgrades.
Replacement options
Larger panel radiators
The most common and cheapest solution. Replace a single-panel radiator with a double-panel convector in the same or a larger size. A double-panel convector produces roughly 2 to 2.5 times the output of a single panel of the same dimensions. This often solves the problem without changing the pipework location.
Budget £150 to £400 per radiator including the unit, TRV (thermostatic radiator valve), and installation labour.
Fan-assisted radiators
These have a built-in fan that actively blows air across the radiator surface, dramatically increasing output at low flow temperatures. A fan-assisted radiator can deliver 2 to 3 times the output of a standard radiator of the same size. They are more expensive (£400 to £800 each) but are the solution when space prevents fitting a physically larger radiator.
The fan uses a small amount of electricity (10 to 20 watts) and produces a gentle hum similar to a computer fan. Most have automatic sensors that switch the fan on only when the radiator is warm.
Underfloor heating
Underfloor heating (UFH) is the ideal heat emitter for a heat pump because it operates at the lowest possible flow temperature (30 to 35 degrees), maximising COP and minimising running costs. It also provides the most even and comfortable heat distribution.
However, retrofitting UFH costs £3,000 to £6,000 per floor, requires lifting existing flooring, and raises floor levels by 15 to 25mm. It is realistically only practical during a major renovation or when floors are being replaced anyway. For new extensions or ground-floor renovations, UFH is worth specifying from the start.
What your installer should provide
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 calculation is mandatory under MCS standards and should be part of any professional heat pump quote. If an installer proposes a heat pump without a room-by-room calculation, that is a red flag. Get quotes from installers listed on the MCS directory.
For optimal heat pump and radiator control, see recommended smart thermostats with room-by-room scheduling and recommended smart TRVs that adjust individual radiator output based on room temperature.
Use our heat pump calculator for a full cost estimate including radiator upgrades, 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?
Most homes need some upgrades but not all radiators. Your MCS installer calculates room by room. Insulating first reduces the number needing replacement.
How much does it cost to upgrade radiators?
£2,000 to £4,000 for a 3-bed semi. £150 to £400 per individual radiator. See our cost by house type guide for the full picture.
Can I keep my existing radiators?
Sometimes. Large cast iron radiators, oversized modern double-panels, and radiators in well-insulated rooms may be adequate. Your installer's heat loss calculation confirms.
Is underfloor heating better than radiators for a heat pump?
Thermally, yes. Practically, it costs £3,000 to £6,000 per floor to retrofit and requires lifting the floor. Only worth it during major renovation. Larger radiators are the pragmatic choice.
What flow temperature should a heat pump run at?
35 to 45 degrees. Lower is better for efficiency. At 35 degrees, COP is 3.0 to 3.5. At 55 degrees, COP drops to 2.0 to 2.5, costing 30 to 50% more to run. See our running costs guide.
Data sources
Radiator output data from manufacturer specifications. Heat pump performance from Energy Saving Trust. MCS installation standards from MCS.