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Heat pumpsDo you need to replace your radiators for a heat pump?
The honest answer is that some radiators change and most do not, and the only way to know which is a room by room calculation. Here is the arithmetic behind it, so you can check what a quote is telling you.
Updated 4 August 2026 · written by the Clearline guidance team
The short answer
A radiator does not have a fixed output. It has an output at a stated water temperature, and heat pumps run cooler water than boilers, so every radiator in your house delivers less heat than the number printed on its label. Some rooms will still be fine. Some will not. The calculation is done room by room, and any quote that says all radiators or none is skipping the work.
Why the label number is misleading
Manufacturer radiator outputs are rated at Delta T 50, which means 75C flow, 65C return and a 20C room. That is boiler territory. A heat pump asked to hold a room at the same temperature is typically running a design flow temperature of 45 to 55C on a radiator retrofit under MCS design practice, and lower still where the house allows it.
At 45C flow and 40C return, the same radiator delivers roughly 42 percent of its Delta T 50 rated output. A radiator badged at 1,500 W puts out around 630 W. Nothing has broken. The water is simply cooler, so the radiator surface is cooler, so it moves less heat into the room.
The 2.4x rule
To hold the same room temperature at heat pump flow temperatures, radiators need roughly 2.4 to 2.5 times their Delta T 50 rated output. That sounds alarming until you realise it is achieved by swapping a single panel for a double or triple, or by going up a size, not by covering the whole wall.
Why fewer change than you expect
Two things work in your favour. The first is that a great many UK heating systems are already oversized, because radiators were specified generously and boilers were sized on rules of thumb. An oversized existing radiator already has headroom, and headroom is exactly what lets a heat pump run at a lower flow temperature without replacing anything.
The second is that the requirement is per room, not per house. A living room with a big double panel radiator on an internal wall may pass comfortably while a small single panel in a north facing bedroom fails. Only the failures get changed, and which radiators change is determined room by room in the heat loss calculation rather than by a blanket rule.
| Rated at Delta T 50 | Approx output at 45C flow | Typical verdict |
|---|---|---|
| 600 W single panel | Around 250 W | Usually replaced, commonly with a double panel in the same footprint |
| 1,000 W single panel | Around 420 W | Depends entirely on the room's calculated heat loss |
| 1,500 W double panel | Around 630 W | Often passes in a small or well insulated room |
| 2,400 W double or triple panel | Around 1,000 W | Usually stays, and often lets the whole system run cooler |
Outputs shown use the roughly 42 percent conversion at 45C flow and 40C return. They illustrate the arithmetic and are not a substitute for a room by room heat loss calculation.
The trade-off nobody explains clearly
You have two ways to satisfy any room. Raise the flow temperature so the existing radiator gives more output, or increase the emitter so it gives more output at a lower flow temperature. The first is cheap on day one and expensive for twenty years. The second is the reverse.
The size of that running cost difference is easy to see in manufacturer data. Samsung quotes its EHS Mono R290 5 kW at SCOP up to 4.84 at 35C flow, and its EHS Mono HT Quiet range at SCOP 2.82 to 3.03 at 65C flow. That is roughly 40 percent more electricity for the same heat. Spending on a handful of radiators to hold a lower flow temperature is usually the better long term buy, which is the argument set out in flow temperature explained.
Want an honest read on which of your radiators would actually need changing? That is exactly what the free remote design call is for.
Get your free quote →What a good quote looks like
- A room by room table showing calculated heat loss, existing emitter output at the design flow temperature, and the verdict for that room.
- A single stated design flow temperature for the system, with the reasoning for it.
- A named replacement for each radiator being changed, with its output at the design flow temperature rather than at Delta T 50.
- Pipework noted where it needs upsizing, because bigger emitters can need more flow than the old microbore can carry.
- No line that reads 'replace all radiators' without a room by room justification behind it.
Pipework is the quiet cost
Larger radiators move more water. In houses plumbed in 8mm or 10mm microbore, some circuits may need upsizing to deliver the flow. It is not universal and it is not a reason to abandon a project, but it should appear in the survey rather than as a variation halfway through the install.
Underfloor heating changes the sums entirely
If you have underfloor heating in part of the house, that part needs no output correction at all, because it already runs at 35 to 45C natively and spreads heat over a very large surface. Mixed systems, underfloor downstairs and radiators upstairs, are common and are handled in the room by room design. The detail of how the two circuits are balanced is covered in underfloor heating and heat pumps.
Where to go next
The radiator question is really the survey question in disguise, so read what a proper heat loss survey covers before you take any quote seriously. Every county page on this site carries a local calculator and the current grant position, so check your county for the local picture, and the full heat pumps guide covers the rest of the system. If a battery is also on your list, battery storage and a heat pump interact through your electricity tariff rather than through the plumbing.
Clearline gives independent guidance and never installs. Work is carried out by an MCS-certified installation team for your area.
Radiator questions, answered
Do all my radiators need replacing for a heat pump?
Almost never. The calculation is done room by room against the design flow temperature, and many UK systems already have oversized radiators with enough headroom to pass unchanged. A quote that replaces every radiator without a room by room justification is worth questioning.
How much output does a radiator lose on a heat pump?
At 45C flow and 40C return, a radiator delivers roughly 42 percent of its Delta T 50 rated output. A 1,500 W radiator delivers about 630 W. To hold the same room temperature you need roughly 2.4 to 2.5 times the Delta T 50 rated output, which usually means a deeper panel rather than a much longer one.
Can I keep my radiators if I run the heat pump hotter?
Physically yes, and current R290 units reach 70 to 75C flow, so it is possible. The cost shows up on your electricity bill every day. Samsung's own figures show SCOP up to 4.84 at 35C flow against 2.82 to 3.03 at 65C flow, which is roughly 40 percent more electricity for the same heat.
Will bigger radiators look terrible?
Usually the extra output comes from depth rather than length, so a single panel becomes a double or triple in a similar footprint. Where wall space is genuinely tight, fan assisted radiators are an option worth raising with the designer, though they add a small electrical load and some noise.
Does my pipework need changing too?
Sometimes. Larger radiators need more water flow, so 8mm or 10mm microbore circuits may need upsizing. It should be identified in the survey and priced up front rather than appearing as a mid-install variation.
Who decides which radiators change?
The room by room heat loss calculation does, and under the MCS design standard that calculation has to be done to BS EN 12831 before any certified heat pump installation. It is the document to ask for, because it is the evidence behind every radiator decision in the quote.
See the numbers next: Duracell Dura16, installed, or what a battery costs.
Figures and specifications in this guide are sourced below and were checked on the date shown. Rates and product specifications change; we confirm the current picture on your free design call.
Sources
- APMI, radiator sizing for low temperature systems: Delta T 50 rating basis and typical heat pump flow temperatures apmi.uk
- Squote, radiator sizing for heat pumps: approximately 42 percent of rated output at 45C flow and 40C return squote.app
- Renewable Heating Hub, simple radiator sizing for heat pumps: 2.4 to 2.5 times Delta T 50 rated output renewableheatinghub.co.uk
- Feature Radiators and Staffordshire Training, guidance on existing oversized radiators and room by room determination featureradiators.co.uk
- Reonic, heat loss survey guide: MCS design flow temperatures of 45 to 55C for radiator retrofits reonic.com
- Samsung Climate Solutions, EHS Mono R290 and EHS Mono HT Quiet product pages: SCOP at 35C and 65C flow samsung-climatesolutions.com
- MCS, MIS 3005-D Heat Pump Design Standard 2025 V1.0: BS EN 12831 room by room heat loss calculation requirement mcscertified.com
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