Most homes need radiators rated for roughly 2 to 3 times their current output once they’re running on a heat pump, though the real number depends on the flow temperature your system is designed for. Heat pumps work best at 40 to 55°C, well below a gas boiler’s 70 to 80°C, and radiators sized for a boiler will fall well short of a room’s heat loss at those lower temperatures. Get the sizing right and the room stays warm at a lower, cheaper-to-run flow temperature. Get it wrong and you either end up with a cold room or a heat pump forced to run hotter than it should, which is exactly what you were trying to avoid.
Worth clarifying up front, since it’s easy to mix the two up: sizing your heat pump (its kW output) and sizing your radiators are separate calculations. The heat pump’s capacity comes from a full property heat loss survey that your MCS-certified installer carries out, covering every room’s walls, windows, floor and ventilation, not a figure you can work out from floor area alone. This guide covers the second half of that process, once you know the heat pump’s design flow temperature, working out what your radiators need to be to keep up with it.
Why Heat Pumps Need Different Radiator Sizing
A gas boiler typically runs at a flow temperature of 70 to 80°C. Heat pumps are most efficient at 40 to 55°C, and pushing them hotter than that eats into the efficiency that makes them worth having in the first place. That’s a real problem for your existing radiators, because a radiator’s heat output drops sharply as the water flowing through it gets cooler, and the relationship isn’t a straight line. Halving the temperature difference doesn’t halve the output, it cuts it by a lot more than that.
Radiator manufacturers rate their products at a standard test condition, deltaT50, meaning the average water temperature in the radiator sits 50°C above the room. A gas boiler comfortably delivers that. A heat pump running at a lower flow temperature creates a much smaller deltaT, and the same radiator puts out noticeably less heat as a result.
How Much Bigger Do Radiators Actually Need to Be?
This is the part a lot of online guides oversimplify with a single “multiply by 2.5” rule. The real multiplier depends heavily on the flow temperature your heat pump is actually designed to run at, using the standard industry correction formula (output ratio equals (deltaT actual ÷ 50) to the power of 1.3, the same method radiator manufacturers like Zehnder publish):
| Heat Pump Flow Temperature | Radiator Output vs. Boiler Rating | Multiplier Needed |
|---|---|---|
| 55°C | About 57% | Roughly 1.75x |
| 50°C | About 46% | Roughly 2.2x |
| 45°C | About 35% | Roughly 2.8x |
| 40°C | About 26% | Roughly 3.9x |
(Figures assume a 5°C drop across the radiator and a 20°C room, the standard assumptions used in this kind of calculation.) A real-world example bears this out: heating engineer Graham Hendra has published output figures for a common 700 x 1000mm double-panel radiator, rated at 1,961W at deltaT50. At deltaT30 (roughly a 50°C flow system) it manages about 1,010W, around 51%, and at deltaT20 (closer to a 40°C flow system) it drops to about 596W, around 30%, both in line with the table above.
The practical takeaway: if your installer is designing your system for a 50°C flow temperature, which is common for a straightforward UK retrofit, budget for radiators rated at roughly double their current boiler-era output. If they’re targeting a lower flow temperature for better efficiency, budget for more.
It’s also worth knowing the multiplier isn’t quite identical across radiator types. Single-panel radiators tend to need proportionally more upsizing than double or triple panel ones at the same flow temperature, since they have less surface area to begin with. Where the numbers above put you right on the edge, a double or triple panel radiator gives you more margin than a single panel one of similar rated output.
A Worked Example
Say a bedroom measures 4m by 3m (12m²) with average insulation, so around 65W/m², giving a heat loss of roughly 780W. With the heat pump designed for a 50°C flow temperature, that’s a 2.2x multiplier, so you’d look for a radiator rated at roughly 780 x 2.2, about 1,720W, at the standard deltaT50 rating. A 1000mm x 600mm Type 22 radiator, commonly rated around 2,000W at deltaT50, comfortably covers that, delivering around 920W in real terms at the 50°C flow temperature, above the room’s 780W heat loss with a reasonable margin.
Radiator Sizes and What They Cover
As a rough guide to common Type 22 radiator sizes, here’s their deltaT50 rating alongside a corrected real-world output at a 40°C flow temperature (the more demanding end of typical heat pump operation) and the room size they’d suit at average insulation:
| Size (Width x Height) | Output at deltaT50 | Output at 40°C Flow | Suitable Room Size |
|---|---|---|---|
| 600 x 600mm | 1,200W | ~365W | 4-5m² |
| 1000 x 600mm | 2,000W | ~610W | 7-9m² |
| 1400 x 600mm | 2,800W | ~850W | 9-12m² |
| 1800 x 600mm | 3,600W | ~1,095W | 12-16m² |
These are a starting point for budgeting, not a substitute for a heat loss survey, actual output varies by manufacturer and model.
Working Out Your Room’s Heat Loss
Before you can size a radiator, you need to know how much heat the room actually loses. A proper heat loss survey, the kind an MCS-certified heat pump installer will carry out as standard, measures every wall, window and floor and accounts for insulation levels precisely. For rough initial planning, you can estimate using floor area and a watts-per-square-metre figure based on how well insulated the room is:
| Room Type | Good Insulation (W/m²) | Average Insulation (W/m²) | Poor Insulation (W/m²) |
|---|---|---|---|
| Living room | 45-55 | 70-80 | 100-120 |
| Bedroom | 40-50 | 60-70 | 90-110 |
| Kitchen | 50-60 | 75-85 | 105-125 |
| Bathroom | 55-65 | 80-90 | 110-130 |
Treat this as a starting point for budgeting, not a substitute for a proper survey. Single glazing, solid uninsulated walls, no loft insulation, high ceilings and north-facing rooms all push you toward the higher end; modern cavity wall insulation, triple glazing, standard ceiling heights and good draught-proofing push you toward the lower end. Rooms with two external walls or several large windows tend to need more than the table suggests too.
Which Radiator Type Works Best With a Heat Pump?
Panel radiators are labelled by how many panels and convector fins they have. Type 11 has one of each, Type 22 has two, Type 33 has three. More panels and convector surface means more heat output for the same wall space, which matters more with a heat pump than it did with a boiler.
| Type | Depth | Heat Pump Suitability |
|---|---|---|
| Type 11 (single panel) | About 50mm | Limited, usually needs to be quite large to compensate |
| Type 22 (double panel) | About 100mm | Good balance of output and wall space for most rooms |
| Type 33 (triple panel) | About 155mm | Highest output per metre of wall, useful where wall space is tight |
Purpose-built low-temperature radiators, and fan-assisted models that use a small quiet fan to boost convection, both exist specifically to squeeze more output from a given size at heat pump flow temperatures. They cost more than a standard panel radiator but can be worth it in a room where wall space is genuinely limited. Vertical radiators are also worth considering for narrow walls, since a tall design gets you more surface area without eating up width.
Most UK radiators are steel panels, and that’s the sensible default for a heat pump retrofit too, good value and reliable. Aluminium radiators hold less water and respond to temperature changes faster, which can suit a heat pump’s steadier, longer-running operation, but they cost more. Cast iron works with heat pumps but its high thermal mass makes it slower to respond, better suited to a system that runs continuously at a low temperature rather than cycling on and off.
One practical note before you commit to a size: a large radiator full of water is heavy, an 1800mm x 700mm Type 22 can weigh around 40kg filled. Check the wall can take it, plasterboard on dot-and-dab often needs extra bracketry, and leave clearance around it (roughly 100mm from the wall, 50mm from the floor) so it can actually convect properly.
Do You Need to Replace Every Radiator?
Not necessarily. Some homes, particularly ones where the boiler-era radiators were already oversized, or where good insulation has been added since they were fitted, can get away with upgrading only a handful of rooms rather than the whole house. The way to check is straightforward in principle: compare each radiator’s estimated output at your heat pump’s design flow temperature (using the table above) against that room’s heat loss. If the derated output covers the heat loss, leave it. If it falls meaningfully short, that room needs a bigger radiator.
Bedrooms are often the easiest to leave alone, since people generally want them a degree or two cooler than living spaces anyway, which reduces the heat loss you need to cover. Living rooms, with their bigger windows and longer occupied hours, are the rooms most likely to need upgrading.
Typical Room Temperatures, for Reference
How warm you want a room affects its heat loss and, in turn, its radiator size. As a general guide: living rooms are usually kept around 20-21°C, bedrooms a bit cooler at 18-19°C, bathrooms warmer at around 22°C, and hallways or landings can run at 18°C since they’re not occupied for long. Use your own preferences rather than these as gospel, but they’re a reasonable starting point if you haven’t thought about it room by room before.
Where you can, position radiators under windows rather than on internal walls, it counters the cold draught windows create and gives more even warmth across the room. Bathrooms are worth a specific mention: a standard heated towel rail often doesn’t have enough surface area to heat the room properly at heat pump temperatures, it’s fine as a towel warmer but may need a proper panel radiator alongside it, or a larger towel rail sized for the room’s actual heat loss rather than just picked for looks.
What Does It Cost?
BeFix’s own radiator installation pricing (exclusive of VAT unless stated otherwise) starts from £150 to £250 per standard radiator, with labour charged at £75 to £150 an hour depending on complexity, more if pipework needs moving or a radiator is going somewhere new. A whole-room or whole-house upgrade is priced properly once an engineer has seen the rooms involved and confirmed sizes, since it depends entirely on how many radiators need changing and how much pipework is involved.
That price is for the radiators and their installation only. The heat pump unit itself, and the heat loss survey and system design that goes with it, needs an MCS-certified heat pump installer, which is a separate qualification to Gas Safe registration. BeFix offers radiator installation in London and can supply and fit the radiators your heat pump installer specifies, working alongside them or after their survey, but doesn’t install the heat pump unit itself.
Does the Boiler Upgrade Scheme Grant Cover Radiators?
No. The Boiler Upgrade Scheme gives £7,500 toward an air or ground source heat pump for most UK homes, and there’s currently a temporary increase to £9,000 for homes not on the mains gas grid (typically oil or LPG heated), running from 21 July 2026 to 31 March 2027. The grant is deducted from your heat pump installer’s quote upfront, but it’s designed to support the heat pump itself, not radiator or pipework upgrades that go alongside it. Budget for radiator work as a separate cost. Local council schemes and energy company obligation funding sometimes offer further help depending on your circumstances and where you live, worth a quick check before you budget for the full cost yourself.
Pipework and an Alternative to Bigger Radiators
Bigger radiators need adequate flow to actually deliver their rated output. Older homes, especially ones plumbed in the 1970s to 90s, sometimes have microbore pipework (thin, around 6-10mm) that was fine for a boiler but restricts flow too much for a heat pump running larger radiators. It’s worth having this checked as part of your heat loss survey rather than discovering it after the radiators are in.
If a room genuinely doesn’t have the wall space for an adequately sized radiator, or you’re in a listed building where visible changes are restricted, underfloor heating installation in London is worth considering as an alternative. It runs at even lower flow temperatures than radiators (typically 35-45°C) and uses the whole floor as the heat emitter, so it doesn’t need the same kind of oversizing calculation at all. BeFix installs underfloor heating as a standard service, so it’s a genuine option alongside, or instead of, a radiator upgrade in the rooms where it makes sense.
Choosing a Heat Pump Installer
Since this is the part BeFix doesn’t do, a few pointers on getting it right. Confirm MCS certification, it’s what makes you eligible for the Boiler Upgrade Scheme grant and it’s the accreditation that actually covers heat pump design and installation competence. Ask specifically about heat pump experience rather than general heating experience, a boiler engineer and a heat pump installer aren’t automatically the same skill set. Get more than one quote, and expect a proper heat loss survey to be part of it, not just a number given over the phone.
Common Sizing Mistakes
The single most common mistake is sizing radiators as if the heat pump will run at a boiler-style flow temperature, then finding rooms won’t reach temperature once it’s actually installed and running at 45 or 50°C. Using deltaT50 ratings directly, without converting them to your system’s real flow temperature, guarantees an undersized result.
The second is skipping the heat loss side of the equation and going straight to “bigger radiators everywhere.” If a room is poorly insulated, a bigger radiator is treating the symptom. Draught-proofing, loft insulation or cavity wall insulation, where it’s missing, often reduces the heat loss enough to change what radiator size the room actually needs, and it’s worth doing before finalising radiator sizes rather than after.
The third is leaving a new system unbalanced. Lockshield valves need adjusting so each radiator gets its fair share of flow. An unbalanced system means some rooms run hot while others struggle, regardless of how well the radiators were sized on paper.
The fourth is placement rather than size. A correctly sized radiator boxed in behind a sofa, or with curtains hanging over it, loses a lot of its effective output. Radiators on internal walls or crammed into corners also tend to heat unevenly compared with one positioned under a window on an external wall.
FAQs
Is sizing my heat pump the same as sizing my radiators?
No, they’re two different calculations. Your heat pump’s kW capacity comes from a whole-property heat loss survey carried out by an MCS-certified installer, covering every room. Radiator sizing is the follow-on step, working out what each radiator needs to be once you know the heat pump’s design flow temperature, which is what this guide covers.
How much bigger do radiators need to be for a heat pump?
It depends on the flow temperature your system is designed for, roughly 1.75 times at 55°C, up to around 4 times at 40°C. There’s no single fixed multiplier that applies to every home, which is why a proper heat loss survey and flow temperature design matter more than a rule of thumb.
Can I keep my existing radiators with a heat pump?
Some of them, potentially. If a radiator was already oversized for the boiler it replaced, or the room is well insulated, it may still cover the heat loss at a heat pump flow temperature. Check each room individually rather than assuming either way.
Will the £7,500 heat pump grant pay for my radiators?
No, the Boiler Upgrade Scheme grant is for the heat pump unit itself, not radiator or pipework upgrades. Budget for radiator work separately.
What’s the best radiator type for a heat pump?
Type 22 double-panel radiators offer a good balance of output and wall space for most rooms. Type 33 gives more output from the same wall length where space is tight, and purpose-built low-temperature or fan-assisted radiators exist for rooms where neither fits comfortably.
Does BeFix install air source heat pumps?
No, heat pump installation needs MCS certification, which is separate from Gas Safe registration. BeFix’s engineers can supply and fit the radiators your MCS-certified heat pump installer specifies as part of the retrofit.
What happens if radiators are left too small?
Rooms won’t reach the target temperature at the heat pump’s designed flow temperature, and the system may need to be run hotter to compensate, which reduces efficiency and increases running costs, working against the reason for installing a heat pump in the first place.
How long does a radiator upgrade take?
A straightforward swap is usually a few hours per radiator. A whole-house upgrade with several radiators, especially if any are moving to a new position and need new pipework, typically takes one to three days depending on how many rooms are involved.
Are vertical radiators suitable for a heat pump system?
Yes, as long as the total surface area is enough to cover the room’s heat loss at your system’s flow temperature. They’re a good option for narrow walls, beside doors or patio doors, where a wide horizontal radiator wouldn’t fit.
Are low-temperature radiators worth the extra cost?
They cost more than a standard panel radiator of the same physical size, but deliver more output at heat pump flow temperatures, so they can let you fit a smaller unit in a room where wall space is tight. In a room with plenty of wall space, a larger standard radiator is usually the cheaper way to get the same output.
Sizing Your Radiators the Right Way
Get an MCS-certified installer to carry out a proper heat loss survey and confirm your heat pump’s design flow temperature, that’s the number the radiator sizing actually depends on. Once you know what’s needed room by room, BeFix’s Gas Safe registered engineers can supply and fit the radiators. Call BeFix on 020 8058 4241 to book a radiator installation.
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