Radiator output at your flow temperature

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Radiator rated output, W

What the room needs, W

The rating is on the box or in the catalogue, and it is quoted at ΔT50 whether or not it says so. The requirement comes from a room-by-room heat loss calculation; this tool will not guess it.

What the system runs at

Heat pump, 45/40. A heat pump gets less efficient the hotter it runs, so it wants to run cool. That is the whole retrofit problem in one line.

Flow, °C

Return, °C

Room, °C

Radiator exponent On the datasheet. Typically 1.3 for a panel radiator, and it varies with design.

At ΔT22.5 35.4% of its rating

531 W

not the 1500W on the box. Mean water 42.5°C into a 20°C room, so 969W less than the catalogue says.

The same radiator on every system

System	ΔT	Output	Of rating
80/60	50	1500 W	100%
75/65	50	1500 W	100%
55/45	30	772 W	51.5%
45/40	22.5	531 W	35.4%
35/30	12.5	247 W	16.5%

531 W is 1,812 BTU/hr, if the quote is in those.

Output correction only. Heat loss is an input and stays one. Nothing uploaded.

Worth knowing

THE NUMBER ON THE BOX IS MEASURED AT ΔT50 AND YOU ARE RUNNING AT ΔT22.5. EN 442 rates every radiator at flow 75, return 65, room 20, and almost nobody runs at that. At your 45/40 into a 20 degree room, this radiator delivers 35.4% of its rating: 531W rather than 1500W, which is 969W less than the catalogue says.
And the fall is steeper than it looks, because output follows the temperature difference raised to about 1.3 rather than in proportion. Halving the difference does not halve the output, it takes off more than half. The exponent varies with radiator design, roughly 1.2 to 1.35, and it is on the datasheet.
Add what the room actually needs and this will tell you whether the radiator meets it, and what it would have to be rated at if not. That figure comes from a room-by-room heat loss calculation, which this tool does not do and neither should any tool that has not seen your walls.
This will not calculate your heat loss, deliberately. That needs fabric U-values room by room, and the volume-times-a-factor tables every other BTU calculator uses are not traceable to anything. An authoritative looking number that is wrong is worse than no number, so the requirement is an input here exactly as it is in the boiler tool.
Two practical notes. Output figures assume the radiator is not boxed in, curtained over or behind a sofa, all of which cut it further. And a bigger radiator at a lower flow temperature is not a compromise: it lets a condensing boiler actually condense, which is where its advertised efficiency comes from and which most installations never reach because they are set up to run at 80 degrees.

Worked out on this device, by this page. Nothing you typed was sent anywhere or stored, and closing the tab loses it.

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Insulation & U-value

With the timber bridging that makes a stud wall a fifth worse than the sum says.

What is a BTU?

The unit on radiator boxes, what it converts to in watts, and what a ton of cooling is.

Every radiator is rated at ΔT50 and almost nobody runs at ΔT50

Under EN 442, radiators are tested at flow 75°C, return 65°C, room 20°C. That gives a mean water temperature 50 degrees above the room, which is where the phrase ΔT50 comes from. Every output figure you see, on the box, in the catalogue, on a comparison site, is measured at that condition.

Run the same radiator cooler and it puts out less. That much is obvious. What is not obvious is how much less, because output does not fall in a straight line: it follows the temperature difference raised to a power of roughly 1.3.

So halving the temperature difference does not halve the output. It takes off appreciably more than half, and the effect compounds as you go cooler.

Which is why a heat pump changes every radiator in the house

Take a radiator rated 1,500W, in a room held at 20°C:

75/65, the rating condition: ΔT50, 1,500W, 100%
55/45, a condensing boiler run properly: ΔT30, about 770W, 51.5%
45/40, a heat pump: ΔT22.5, about 530W, 35%

A third of the number on the box. Nothing is wrong with the radiator and nothing is wrong with the heat pump. A heat pump gets less efficient the hotter it runs, so it is designed to run cool, and a radiator sized for a boiler simply cannot deliver at those temperatures.

This is the single biggest reason heat pump retrofits disappoint, and it is arithmetic rather than opinion. It is also why a heat pump quote includes so much radiator work, which reads like an upsell until you have seen the numbers.

The multiplier is the honest answer

If a room needs 1,000W and you are running at 45/40, the radiator has to be rated at around 2,800W to deliver it. That is nearly three times the room's actual requirement, and it is the number worth having in your head before anybody talks to you about radiator sizes.

It also explains why the honest answer is sometimes not a bigger panel but a different emitter altogether. Underfloor heating is designed for 35/30 and is happy there; radiators at that temperature have to be enormous.

This is not a heat pump problem, it is a flow temperature problem

Worth being clear about, because the framing usually blames the heat pump. Running cooler is what lets a condensing boiler actually condense, which is where its advertised efficiency comes from.

Most condensing boilers in the UK are installed and left running at around 80°C, at which they never condense and never reach the efficiency on their label. Turning the flow temperature down is free and it works, provided the radiators are big enough, which is exactly the calculation on this page.

So bigger radiators at a lower flow temperature is not a concession made for heat pumps. It is how a wet heating system should have been set up all along, and the heat pump just makes ignoring it impossible.

What this tool will not do

It will not calculate your heat loss, and that is deliberate rather than an omission.

A real heat loss calculation needs fabric U-values, room by room, taking account of construction, insulation, glazing and exposure. The room volume times a factor tables that every other BTU calculator on the internet uses are not traceable to any standard, and they produce a number that looks authoritative and is not. That is worse than no number, because people order against it.

So the requirement is an input here, exactly as it is in the boiler sizing tool, and what this page does is the correction that nobody else does.

Two practical things that make it worse

Published outputs assume the radiator is open to the room. Boxing it in, hanging long curtains over it or putting a sofa in front of it all cut the real output further, and none of that is in any manufacturer's figure.

And the exponent varies. Around 1.3 is typical for a panel radiator, but it runs from roughly 1.2 to 1.35 depending on the design, and a column radiator behaves differently from a modern double panel. It is on the datasheet, and the tool takes it as an input for that reason rather than pretending it is a universal constant.

Common questions

What does ΔT50 mean on a radiator?

It is the test condition every radiator is rated at under EN 442: flow 75°C, return 65°C, room 20°C, giving a mean water temperature 50 degrees above the room. The output printed on the box, in the catalogue and on comparison sites is measured there. Almost nobody runs their heating at those temperatures, so almost nobody gets that output.

How much output do I lose at a lower flow temperature?

More than proportionally, because output follows the temperature difference raised to a power of about 1.3 rather than in a straight line. A condensing boiler at 55/45 gives about 51.5% of the rated figure. A heat pump at 45/40 gives about 35%. So a 1,500W radiator is roughly 530W on a heat pump.

Why does a heat pump need bigger radiators?

Because a heat pump gets less efficient the hotter it runs, so it is designed to run cool, and a radiator sized for a boiler cannot deliver its rated output at those temperatures. Nothing is wrong with either the radiator or the heat pump. It is arithmetic, and it is the single biggest reason retrofits disappoint people who were not told about it.

What is the radiator exponent?

The power the temperature difference is raised to when working out output. It is typically around 1.3 for a panel radiator and varies roughly between 1.2 and 1.35 depending on design, so it is on the datasheet rather than being a universal constant. The tool defaults to 1.3 and lets you put the real one in.

Why will this not calculate my heat loss?

Because a real heat loss needs fabric U-values room by room, and the room-volume-times-a-factor tables that every other BTU calculator uses are not traceable to anything. An authoritative looking number that is wrong is worse than no number, so the requirement is an input here, exactly as it is in the boiler sizing tool.

Is a bigger radiator at a lower temperature a compromise?

No, it is usually the better system. Running cooler is what lets a condensing boiler actually condense, which is where its advertised efficiency comes from and which most installations never reach because they are set up at 80 degrees. Bigger radiators at a lower flow temperature is not a heat pump concession, it is how a boiler should have been run all along.