Heating · Radiators sized right
If the delivery layer can't take the load, nothing upstream matters.
Every radiator in your house has a fixed job on the coldest day of the year; deliver that room's design heat loss at the system's flow temperature. Sized for a boiler running at 65 to 70°C, most existing radiators do it comfortably. Sized for a heat pump running at 45°C, roughly two-thirds of them fail. This is the rung most retrofits quietly skip, and the reason so many heat pumps disappoint their owners.
January morning, existing rads on 45°C flow
A pump running perfectly and a house running cold; the fabric is fine, the flow is right, but the K1 rads simply cannot deliver enough watts per minute at 45°C.
What the emitter check actually is.
MIS 3005 requires the heat pump installer to size each existing radiator against its room's calculated design heat loss at the proposed flow temperature. A K1 double-panel rated at 1600 W at ΔT50 delivers roughly 900 W at ΔT30, which is what you get when a 20°C room is heated by 45°C flow with a 5°C drop across the rad. The maths is fixed; the physics is fixed; the check either passes or it does not.
Roughly two-thirds of UK radiators fail. Not because they were badly specified, but because they were specified for a hotter system. The room got the right output at 70°C flow; at 45°C it delivers about 45 per cent of that, and the room runs cold on design day.
Same morning, K3 rads sized for 45°C flow
Same house, same flow, K3 triple-panel radiators on the same walls; the emitter finally hands over the watts the room actually needs.
How to satisfy the emitter check.
Three options in descending order of elegance in an occupied home.
Oversized radiators (K2 or K3 panels)
- Strength
- Every UK manufacturer sells them; typically £180 to £450 fitted per room on a like-for-like wall position.
- Trade-off
- Roughly twice the surface area of the boiler-era rad they replace; you notice the change on the wall.
- Best for
- Occupied homes where you want the change done in a day per room with no floor lift.
Retrofit wet underfloor
- Strength
- Runs at 30 to 40°C flow, which is exactly where a heat pump wants to live; large emitter area delivers steady low-grade heat.
- Trade-off
- Expensive and disruptive in an occupied home; best done during a renovation with the floor already up.
- Best for
- Rooms being renovated to studs or extensions being built alongside the heating change.
Design the heat pump at 55°C instead of 45°C
- Strength
- Keeps most of the existing radiators; avoids visible rad upgrades in every room.
- Trade-off
- Loses roughly a size band of seasonal COP for the life of the system; running cost sits closer to a gas boiler.
- Best for
- Solid-wall homes where the radiator upgrade cost and disruption outweigh the running-cost penalty.
Verdict
Radiators sized right, honestly assessed.
Ask any heat pump installer for the emitter schedule before you sign; a professional installer will produce one showing each rad's required output at design flow, and any refusal to produce one is the answer.
A K3 triple-panel replacement on the coldest three or four rooms is often the whole job; the remaining rads carry through untouched. Expect £2,500 to £6,000 across a typical three-bed semi if the whole house is upgraded, or a fraction of that if only the failing rooms are touched.
Where nothing fits, the honest answer is wet underfloor in that room and matched flow temperature across the system. Where nothing fits and nothing lifts, design the whole system at 55°C and accept the running-cost penalty as a conscious trade-off, not an accident.
What it gives you
- Every room actually reaches temperature on the coldest morning of the year.
- Lower flow temperature lifts seasonal COP for the life of the system.
- The emitter schedule is a permanent design record; it protects you at resale.
What it costs you
- K3 panels are visibly larger than the K1s they replace.
- Retrofit underfloor in an occupied home is expensive per square metre.
- Rad upgrade cost sits on top of the heat pump quote; some homeowners are surprised on the day.
Why we think thisOpenClose
Reasoning
The 45 per cent output ratio between ΔT50 and ΔT30 is standard radiator manufacturer performance data (BS EN 442) and applies to any panel radiator. The two-thirds failure rate is drawn from published MCS installer heat loss surveys of the UK's typical semi and terrace stock, where flow temperatures below 55°C are the design target.
Cost bands reflect installer quotes in 2026; £180 to £450 per K3 radiator upgrade on a like-for-like wall, £1,500 to £3,500 per room for retrofit wet underfloor with the boards up, and £2,500 to £6,000 for a whole-house rad upgrade on a typical three-bed semi.
Assumptions
- The heat loss calculation is genuinely room-by-room, not a whole-house rule of thumb.
- Existing pipework is 15mm copper capable of the required flow rate; microbore runs may need re-plumbing separately.
- The house has already had the fabric work that makes the lower design flow viable.
If this were our house
If this were our house, the heat pump quote would not be signed until the emitter schedule was on the table with three numbers per room.
- 1Ask for the room-by-room heat loss and the emitter schedule as part of every heat pump quote; walk away from anyone who cannot produce one.
- 2Upgrade the three or four coldest rooms first with K2 or K3 panels; leave the rest untouched if they pass at design flow.
- 3In rooms being renovated anyway, install wet underfloor at 30 to 40°C; it will outperform any radiator upgrade for felt comfort.
This order rests on the current HouseState above; a solid-wall Victorian without fabric work may honestly need a 55°C design instead, at a known running-cost cost.