Can a heat pump work with my existing radiators? Water temperature, radiator size and a winter test

Often yes, though many homes need some larger radiators first. What decides it is how hot the water in your radiators must be to keep every room warm on the coldest days, and that depends on how big the radiators are and how fast your home loses heat. A heat pump uses more electricity the hotter it has to make the water. A German field test found that radiators large enough for their rooms can run at temperatures close to those of underfloor heating; in a British trial, 93% of homes got new radiators or other heat emitters, such as fan-assisted radiators. A test with your boiler this winter gives a first indication; a room-by-room heat loss calculation settles it.

Why does the water temperature matter so much?

The flow temperature is the temperature of the water leaving the boiler or heat pump for the radiators; it comes back cooler, as the return. A heat pump moves heat from the outdoor air into this water, and the smaller the gap between the two, the more efficient it is. The Verbraucherzentrale Rheinland-Pfalz, a German consumer advice center, says in a February 2026 fact sheet that every degree less makes a heat pump 2 to 3% more efficient, and that 55 °C should be the maximum, less being better.

The British Electrification of Heat trial, funded by the government and run by Energy Systems Catapult, installed 742 heat pump systems and monitored them from 2020 to 2023 (performance report, December 2024). The median coefficient of performance (COP, the units of heat per unit of electricity at a given moment) of the heat pump unit was 3.50 with water at 35 °C, 2.38 at 55 °C and 2.02 above 65 °C: the same heat at 55 °C took about half as much electricity again as at 35 °C. These are half-hour readings; over a whole year the median for the air source units was 2.93.

EU rules reflect this: the energy label (Delegated Regulation (EU) No 811/2013) gives most heat pumps one efficiency class for water at 55 °C and one for 35 °C, and under the ecodesign rules (Regulation (EU) No 813/2013) a “low-temperature heat pump” cannot make water at 52 °C when it is −7 °C outside, so it will not suit radiators that need hotter water. Is a heat pump worth it? explains the label.

How much less heat does a radiator give with cooler water?

A radiator’s output depends on ΔT (“delta T”), the gap between the average temperature of the water inside it and the room air. Its rated output is measured under the European standard EN 442 with water entering at 75 °C and leaving at 65 °C in a 20 °C room: an average of 70 °C, so ΔT 50 K (kelvin; 1 K is a difference of 1 °C). A UK government study, Domestic heat distribution systems (BEIS Research Paper 2021/015, February 2021), describes this rating and gives the equation from EN 442: output = K × ΔT^n, where K depends on the radiator’s size and shape and the exponent n is 1.3 ± 0.1.

ΔT	Example: water in and out, room at 20 °C	Output, share of the rated output	Rated output needed for the same heat
50 K	75 °C in, 65 °C out (the rating)	100%	1 ×
40 K	65 °C in, 55 °C out	about 75% (73 to 77%)	about 1.3 ×
30 K	55 °C in, 45 °C out	about 52% (49 to 54%)	about 1.9 × (1.8 to 2.0)
25 K	50 °C in, 40 °C out	about 41% (38 to 44%)	about 2.5 × (2.3 to 2.6)

Assumptions: the equation above with n = 1.3, and 1.2 to 1.4 for the ranges; room at 20 °C; water cooling by 10 °C across the radiator (example value); a radiator giving its full rating (in practice it may give less, the BEIS study notes).

Left: the example from the table, water at 55 °C in and 45 °C out in a 20 °C room, so ΔT 30 K. Right: the share of its catalogue rating a radiator gives at each ΔT, from the EN 442 equation with n = 1.3.

A worked example. A bedroom loses 1,000 W on the coldest day and its radiator is rated at 1,500 W at ΔT 50 (example values; the rating is on the maker’s datasheet). With water at 55 °C in and 45 °C out it gives about 770 W, almost a quarter short. At that temperature the room needs about 1,940 W of rated output, so a radiator of about 2,000 W, or a smaller heat loss. With n at 1.2 or 1.4 the need is about 1,850 or 2,050 W; keeping the room at 21 °C instead of 20 °C raises it to about 2,030 W.

Radiators are often bigger than the room strictly needs, but usually not by enough: in the UK homes the BEIS study surveyed (431 of its 515 had enough data), their rated output was on average 1.46 times the heat needed on a peak winter day, less than the margin the table shows for 55 °C. It estimated that only 10% of UK homes with radiators could meet peak-day demand at 55 °C with no changes, and 53% on an average day of the coldest month. Pumping water faster helps little: in its model, tripling the flow rate at 55 °C added about 10% of output.

What did field tests in real homes find?

Germany. Fraunhofer ISE monitored 77 heat pumps in one- to three-family houses. The final report (25 November 2025) linked efficiency to the heating water temperature, not to the age of the house, and found that radiators large enough for their rooms can run at similarly low temperatures to underfloor heating. In the 42 systems where radiators set the temperature, the yearly average flow temperature in 2024 was 34 to 45 °C in 30 and 45 to 55 °C in 12. Limits: temperatures below −5 °C were rare, and many older houses had been partly renovated (in those built up to 1979, 82% of the roofs and 56% of the outside walls).
Britain. New radiators or other heat emitters came with 93% of the heat pumps in the Electrification of Heat trial “as the existing radiators would have been too small”, says its installation report (December 2022), which also saw “a trend of ‘over caution’ in heat pump design”, with more radiators than required. They cost on average about £2,800 per home, about £300 each (installed in 2020 and 2021; fitting included for two of the three contractors).

Can you test your radiators this winter with your boiler?

Yes, as a first indication. Zukunft Altbau, the information program of the Baden-Württemberg state energy agency, suggests setting the boiler’s flow temperature to 55 °C at most and heating as usual: if the rooms are warm enough even on the coldest days, the house can be heated with a heat pump.

Wait for a cold spell. A mild week proves little.
Find the heating temperature setting on the boiler (for the radiators, not the hot water). If you cannot, or the boiler only has a heating curve that follows the outdoor temperature, ask a heating engineer.
Set it to 55 °C or lower and pause any night-time lowering of the heating, as Zukunft Altbau advises.
Note each room: the ones that do not reach your usual temperature point to the radiators to look at.
Afterwards you can keep the lower setting if the house stays comfortable: condensing boilers also work more efficiently at lower temperatures, notes the BEIS study.

The limits. If the boiler heats a hot water cylinder through the same setting, do not lower it: the UK Health and Safety Executive advises storing hot water at 60 °C at least to kill legionella bacteria. The test does not size the heat pump: it is a check, not a design.

What can you change if some rooms stay cold?

Replace only the radiators that are too small, with larger or deeper ones: sometimes a few are enough, says Zukunft Altbau, and Fraunhofer ISE recommends checking sizes room by room. In the British trial 57% of installations got low-temperature radiators, and fan-assisted ones saved space in some small rooms.
Insulation. Insulating the roof, outside walls or cellar ceiling, and new windows, lower the heat loss and so the water temperature needed (Zukunft Altbau).
Balancing. Without hydraulic balancing, radiators near the boiler get too much water and distant ones too little, says the Verbraucherzentrale; the fix is setting the flow on each radiator valve.
A hybrid, a heat pump with a gas boiler for the coldest days, can often keep the existing radiators while the heat pump covers most of the year’s heat, the BEIS study suggests. Replacing a gas boiler compares the options; in England and Wales hybrids get no Boiler Upgrade Scheme grant.
A high-temperature heat pump. In the British trial these reached a yearly efficiency similar to standard models, largely because most of the time they ran at similar temperatures, and 99% of them still came with new radiators or other heat emitters.
Narrow pipes. Replacing “microbore” pipework stretched British trial installations from the usual 2 to 4 days to around 8 to 10, and its cost kept some homes out.

What should the installer do, and what should you ask?

The installer should calculate the heat loss of every room, the heat it loses on the coldest design day through walls, roof, floor, windows and drafts, with the European method EN 12831-1. In Britain the design standard of MCS, the UK certification scheme for heat pump installers, MIS 3005-D: 2025 (issue 2.0, 5 December 2025), requires that method. Before work starts, the standard requires the installer to be given in writing each room’s heat loss, the type and size of each radiator, the radiator temperature set by the worst room, the design flow temperature (the water temperature leaving the heat pump on the coldest design day) and the SCOP at that temperature (the efficiency over a whole heating season); ask for a copy. A design above 55 °C should come with an alternative at 55 °C or lower, and the difference in efficiency and energy use must be explained to you so you can choose.

Ask for:

Which radiators change, with each new one’s output at the design temperature, not only at ΔT 50.
Whether pipes or balancing need work.
At least three written quotes for the same design.

Country notes, checked on 1 November 2026; rules and grants can change:

Italy. Eligible costs for the ecobonus (the Italian tax deduction for energy-saving work) on heat pumps include work on the “sistemi di emissione”, the radiators, says the ENEA guide of 23 September 2026; the new heat pump may exceed the old generator’s power by more than 10% only if a sizing check under the EN 12831 standard justifies it.
Germany. For the federal heating grant (KfW 458), the funding guideline of 17 August 2026 requires sizing the system with a room-by-room heat load calculation under DIN EN 12831, and hydraulic balancing.
United Kingdom. The Boiler Upgrade Scheme (England and Wales) gives £7,500 towards an air source heat pump (plus £1,500 until March 2027 for homes heated by oil or LPG with no mains gas); gov.uk says to get quotes from MCS-certified installers, and the installer applies for you.

This page explains how it works and what to ask; it is not financial advice or a design. Electrical work and paperwork are for a qualified installer or advisor.

Where to go next

Heat pump vs gas boiler calculator: your running cost.
Is a heat pump worth it compared with a gas boiler?: SCOP and the label.
My gas boiler is old: new boiler, hybrid or heat pump?: the three choices compared.
Home energy incentives by country: grants and tax breaks.

Common questions

Can a heat pump work with existing radiators?

Often yes, though many homes need some larger radiators first. It depends on whether your radiators can keep each room warm on the coldest days with water at about 55 °C or less. A German field test found that radiators large enough for their rooms can run at temperatures close to those of underfloor heating; in a British trial, most homes had new radiators fitted. A room-by-room heat loss calculation tells which radiators, if any, must change.

Is a heat pump worth it with radiators?

It can be, as long as the water does not have to be very hot. In a British trial the median efficiency in half-hour readings was 3.5 units of heat per unit of electricity with water at 35 °C and 2.4 at 55 °C, so cooler water means a smaller bill. Whether that beats gas depends on your prices and on the efficiency over a whole season (the SCOP): the heat pump vs gas boiler calculator works it out.

Do you need underfloor heating for a heat pump?

No. Underfloor heating works with cooler water than most radiators, but a Fraunhofer ISE field test in German homes found that radiators large enough for their rooms can run at similarly low temperatures. If some rooms stay cold, larger radiators in those rooms, insulation or a hybrid system are the usual ways forward.

Why are my radiators not hot with a heat pump?

A heat pump sends cooler water to the radiators than a boiler usually does: across the heat pumps in a large British trial the average was about 40 °C, while radiators are rated with water averaging 70 °C. So the radiators feel warm rather than hot, which is normal as long as each room reaches the temperature you set. If a room does not, ask the installer to check the design flow temperature, the heating curve settings and the balancing of the radiators.

Do you need a high-temperature heat pump for old radiators?

Not always. High-temperature models can make water above 65 °C, but in a British trial they reached a yearly efficiency similar to standard models largely because most of the time they ran at similar temperatures, and 99% of the homes that got one still had new radiators fitted. The British MCS design standard says they should be avoided unless the home needs water hotter than 55 °C.

Topics: heat pumps, radiators, heating, flow temperature, insulation