You will probably need some radiators upsized, you will almost certainly not need all of them replaced, and the decision is made by a room-by-room heat loss calculation rather than by the age of your radiators. The reason is arithmetic. MCS's heat emitter guidance sets an oversize factor of 3.1 for a radiator on a system designed to run at 41 to 45°C, against 1.90 at 51 to 55°C, which means the same room needs roughly 60% more radiator when the design flow temperature drops by ten degrees. Every figure on this page comes from the current MCS heat pump design standard, MCS's own reference tables, gov.uk or Energy Saving Trust, read on 10 October 2026.
What the oversize factor actually asks of your radiators
MCS publishes a table of oversize factors in its heat pump reference document, MGD 007, and the instruction is blunt: multiply the room's heat loss in watts by the factor to get the rated output the emitter in that room must have. Rated output here means output at a mean water-to-air temperature difference of 50°C, which is how manufacturers have listed radiators since about the year 2000.
| Design flow temperature | Oversize factor for a radiator | Radiator output needed in a room losing 1,000 W |
|---|---|---|
| Up to 35°C | 6.8 | 6,800 W |
| 36 to 40°C | 4.3 | 4,300 W |
| 41 to 45°C | 3.1 | 3,100 W |
| 46 to 50°C | 2.4 | 2,400 W |
| 51 to 55°C | 1.90 | 1,900 W |
Read down that last column and the whole radiator question answers itself. A design at 55°C asks a 1,000 W room for 1,900 W of rated radiator. The same room on a 45°C design wants 3,100 W, which is roughly 60% more radiator in the same space. Push the design down to 35°C, where a heat pump is at its most efficient, and you are asking for 6,800 W, about three and a half times the 55°C figure. That is why low temperature designs so often end up with underfloor heating or fan-assisted emitters rather than panel radiators.
Two details in MGD 007 catch people out. The table is read at the flow temperature after any blending valve, and if a blending valve is fitted the heat pump has to run about 5°C hotter than that, so emitters sized for a 45°C circuit need the heat pump at about 50°C. And if your radiators are older than about 2000, their printed output is based on a 60°C temperature difference rather than 50°C: MCS's instruction is to multiply those figures by 0.825 before using them. A 1930s semi with original panel radiators and a faded brochure in the loft is therefore working from numbers that are already 17.5% optimistic.
One cold room sets the water temperature for every radiator in the house
Clause 5.9.2 of the MCS heat pump design standard, MIS 3005-D, requires your installer to give you, in writing and before work starts, "the design emitter temperature based on the worst performing room". That phrase is the single most useful thing on this page. A wet heating system runs at one flow temperature, so the room with the least radiator relative to its heat loss dictates how hot the water has to be for the whole house.
The practical consequence is that one skipped radiator costs you efficiency in every other room. If nine rooms are sized for 45°C and the back bedroom is only good for 55°C, the system runs at 55°C all winter. On the manufacturer figures we went through for the Mitsubishi Ecodan range, the same 12kW unit returns a seasonal efficiency of 4.71 at 35°C and 3.51 at 55°C, so about a quarter of the machine's efficiency is decided by the worst room in the house. That is a bigger number than the gap between most brands.
It also tells you how to read a quote. An installer who proposes replacing three radiators and leaving eight is making a defensible engineering choice if the eight are already big enough at the design temperature, and a bad one if the three were chosen because they were easy to get at.
Get the radiator schedule priced, not guessed
Ask MCS-certified installers for a quote that lists each room's heat loss and the emitter going in it. Tell us about your home once and we will pass your details to installers covering your postcode. The form takes about a minute to fill in.
What MCS says you must be given in writing before any work starts
Clause 5.9.2 is a list, and it is worth checking your paperwork against it line by line. Before the installation commences, the customer shall be provided in writing with:
- All specific room heat losses, in watts per square metre.
- The calculated daily hot water demand.
- The type and dimensions of the emitters to be used in the system.
- The design emitter temperature based on the worst performing room.
- The specification of any new hot water cylinder and its recovery rate.
- The design flow temperature of the water leaving the heat pump, before any blending valves.
- The installation's seasonal efficiency figure at that design flow temperature, taken from the MCS website, when supplying space heating at the external design temperature for your location.
The heat loss itself is calculated to BS EN 12831-1:2017, the European standard the MCS design standard adopts, and the heat pump has to cover at least 100% of that calculated load at the chosen flow temperature without help from a supplementary electric heater. So a quote that promises a smaller radiator schedule by quietly leaning on an immersion heater in January is not an MCS design.
There is one more clause worth knowing. Where a design proposes a flow temperature above 55°C, the MCS standard says an alternative design at 55°C or lower should also be provided, and the installer must explain the difference in efficiency and energy consumption so you can choose. In practice, that is the clause that forces the radiator conversation into the open rather than leaving it as a line item.
Which radiators usually survive, and which do not
There is no official national figure for how many British radiators have to be swapped, and anyone quoting you one without a survey is guessing. What the sizing arithmetic does tell you is where the trouble sits.
- Single-panel radiators in large rooms are the usual casualties. They have the least surface area for their wall space, which is exactly what the oversize factor punishes.
- Rooms that were always slightly cold on the gas boiler were already undersized at 70°C. At 45°C they are hopeless, and they are also the room that will set your whole-house flow temperature.
- Pre-2000 radiators carry outputs rated at a 60°C difference, so apply the 0.825 conversion before believing them.
- Double-panel radiators in bedrooms often pass without change, because bedroom heat losses are small and MCS's internal design temperature for a bedroom is lower than for a living room.
- Towel rails are sized for drying towels rather than heating bathrooms and tend to need a rethink either way.
Pipework gets checked at the same survey. Bigger heat pumps move more water: Mitsubishi, for example, specifies 22 mm water pipework on its 5 and 6kW units and steps up to 28 mm from 8kW, so a larger unit plumbed into whatever was already there will not deliver its rated output. Changing the emitter type can also let the system run cooler, which is MCS's own stated route to a better seasonal efficiency figure, and it is usually cheaper than chasing the last degree out of the heat pump itself. The same logic runs through the hardware choices we cover in our guides to Vaillant's aroTHERM range and Daikin's UK line-up.
Who pays for the radiators
Energy Saving Trust puts a typical air source heat pump installation at around £12,000, and lists "whether you need to upgrade your radiators" as one of the four things that move that figure. EST also quotes its annual saving estimates for an average three-bedroom semi "including any recommended radiator upgrades", which is a quiet admission that upgrades are the normal case rather than the exception. Radiator work is not a separate product you buy later; it is inside the installed price.
Three funding points follow from that, and all three were checked on gov.uk and Home Energy Scotland today.
The Boiler Upgrade Scheme is a fixed amount, not a percentage. In England and Wales gov.uk gives £7,500 towards an air source heat pump, £7,500 towards a ground or water source heat pump, £5,000 towards a biomass boiler and £2,500 towards an air-to-air heat pump, with an additional £1,500 until March 2027 if the property is heated by oil or LPG and has no mains gas connection. One grant per property, nothing for hybrid systems, and no grant to replace a heating system that is already low carbon. Because the grant is a fixed sum, every extra radiator comes off your side of the total rather than the government's.
Scotland pays differently. Home Energy Scotland offers a grant of up to £7,500 for a heat pump plus an optional interest-free loan of up to £7,500, with a £1,500 uplift for rural and island homes. Only heat pumps that provide 100% of a home's heating and hot water qualify, hybrids and air-to-air units do not, and both the installer and the product must be MCS certified. Northern Ireland has no equivalent heat pump grant at the moment.
New radiators fitted as part of the heat pump installation are zero-rated for VAT until 31 March 2027. HMRC's Notice 708/6 treats the job as a single supply where one element is principal and the rest are ancillary, and its worked example is explicit: a central heating system installed with an air, ground or water source heat pump instead of a conventional boiler "would be a single zero-rated supply because the principal elements are zero-rated". Replace radiators on their own, with no heat pump, and the same notice makes that a standard-rated supply of a central heating system at 20%. From 1 April 2027 installations of energy-saving materials revert to the 5% reduced rate. The same cliff edge applies to solar, and we work through it in our guide to VAT on solar panels.
For what the whole job comes to once the grant, the cylinder and the radiator schedule are in, start with our UK heat pump cost guide.
Underfloor heating, listed buildings and the cases where bigger is not an option
Underfloor heating is the easy case and MCS's table shows why: it works at flow temperatures where a radiator would have to be absurd. The guidance sets maximum pipe spacings instead of oversize factors for underfloor systems, and the covering matters as much as the pipe. At the lowest flow temperatures MCS's table stops offering a design at all for some screed-and-covering combinations and says to reduce the heat loss instead, which is the standard's way of telling you insulation comes before emitters.
Mixed systems are common and fine. Underfloor downstairs and radiators upstairs is a sensible British retrofit, as long as you remember clause 5.9.2: the worst performing room still sets the flow temperature, and that is usually a bedroom rather than the kitchen with the warm floor.
The harder case is written into the standard as a note of its own: listed buildings or other design limitations may prevent the use of larger heat emitters. If you cannot fit a bigger radiator on a wall you are not allowed to touch, the options are a higher flow temperature with the efficiency penalty that brings, a different emitter type in that room, or reducing the room's heat loss. Permitted development for the outdoor unit is a separate question again, and it differs in England, Wales, Scotland and Northern Ireland.
What to ask for before you sign
- The room-by-room heat loss, in writing, with the specific heat loss in watts per square metre for each room.
- The emitter schedule: type and dimensions for every room, showing which radiators stay and which are replaced.
- The design flow temperature, and the worst performing room that set it.
- The seasonal efficiency figure at that flow temperature, not the headline number from the brochure.
- A 55°C alternative design if the proposed flow temperature is higher, which the MCS standard says should be provided anyway.
Get that from two or three MCS-certified installers and the radiator question stops being a matter of opinion. The schedules will differ, and where they differ you have found the thing worth asking about. EST's own advice is to get quotes from at least three certified installers before committing.
Frequently asked questions
Almost never all of them. A wet heating system runs at one flow temperature, and MCS's design standard requires the installer to calculate each room's heat loss and size the emitter in that room against it, so radiators that are already big enough stay. The ones that go are usually single-panel radiators in large rooms and any room that was slightly cold on the gas boiler, because that room was already undersized at 70°C. There is no reliable national figure for the proportion replaced, so treat any number offered before a survey as a guess.
MCS's reference document MGD 007 gives oversize factors you multiply the room's heat loss by: 1.90 at a design flow temperature of 51 to 55°C, 2.4 at 46 to 50°C, 3.1 at 41 to 45°C, 4.3 at 36 to 40°C and 6.8 at 35°C or below. So a room losing 1,000 W needs a radiator rated at 1,900 W on a 55°C design and 3,100 W on a 45°C design, which is roughly 60% more radiator. Rated output means output at a 50°C mean water-to-air temperature difference. The table is read at the flow temperature after any blending valve; if one is fitted, the heat pump has to run about 5°C hotter than that.
Because a wet system circulates water at a single flow temperature, so the room with the least emitter relative to its heat loss sets the temperature every other room gets. MCS clause 5.9.2 makes the installer state "the design emitter temperature based on the worst performing room" in writing before work begins. The cost of getting that room wrong is efficiency across the whole house: on Mitsubishi's published figures, the same 12kW unit returns a seasonal efficiency of 4.71 at 35°C and 3.51 at 55°C.
Not if they are part of the heat pump installation. HMRC's Notice 708/6 treats a job with a principal zero-rated element and ancillary parts as a single supply, and its own example says a central heating system installed with an air, ground or water source heat pump instead of a conventional boiler is a single zero-rated supply. Radiators replaced on their own, with no heat pump, are a standard-rated supply of a central heating system at 20%. The zero rate on installing energy-saving materials runs until 31 March 2027, and from 1 April 2027 these supplies revert to the 5% reduced rate.
The grant is a fixed amount towards the installation rather than a percentage of it, and gov.uk does not list radiator work as a separate item. In England and Wales it is £7,500 towards an air source heat pump, £7,500 for ground or water source, £5,000 for a biomass boiler and £2,500 for an air-to-air heat pump, with an extra £1,500 until March 2027 for properties heated by oil or LPG with no mains gas connection. One grant per property, nothing for hybrid systems. Because the sum is fixed, extra radiators come off your share of the total. Scotland uses the Home Energy Scotland grant of up to £7,500 plus an optional £7,500 interest-free loan instead.
It works at lower flow temperatures, which is where a heat pump is most efficient, and MCS's guidance reflects that by setting maximum pipe spacings for underfloor systems rather than the oversize factors it applies to radiators. The floor covering matters as much as the pipework, and at the lowest flow temperatures the guidance stops offering a design for some screed and covering combinations and tells you to reduce the heat loss instead. Mixing the two is normal in a British retrofit, but the worst performing room still sets the flow temperature, and with underfloor downstairs that room is usually upstairs.
The MCS design standard anticipates it with a note that listed buildings or other design limitations may prevent the use of larger heat emitters. Your options are a higher design flow temperature and the efficiency penalty that brings, a different emitter type in that room, such as a fan-assisted unit, or reducing the room's heat loss with insulation. Where a design proposes a flow temperature above 55°C, the MCS standard says an alternative design at 55°C or lower should also be supplied, and the installer must explain the difference in efficiency and energy use so you can choose.
Only a room-by-room heat loss calculation answers it, carried out to BS EN 12831-1:2017 as the MCS standard requires, and you are entitled to see it. Ask for the specific heat loss for each room in watts per square metre, the type and dimensions of the emitter proposed for each room, the design flow temperature leaving the heat pump before any blending valve, and the seasonal efficiency figure at that temperature. If your radiators predate about 2000, their printed output is rated at a 60°C temperature difference and MCS's conversion is to multiply by 0.825 before comparing it with anything.