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The room's design heat loss, from a room-by-room heat loss calculation.
This is the output the radiator has to deliver on the coldest design day, not the output of the radiator already on the wall. It comes from a room-by-room heat loss calculation — the fabric losses through walls, windows, floor and roof plus the ventilation loss — and a heat pump installation in the UK is required to have one. Entering the existing radiator's rating here answers a different question, and usually the wrong one.
The water temperature entering the radiator at design conditions.
For a heat pump this is the design flow temperature from the heat pump's weather compensation curve at the coldest design day — commonly between 35 and 55 °C (95 and 131 °F). For a boiler it is the flow temperature the system is set to run at, and a condensing boiler only condenses when the return comes back cool enough, which is why lower flow temperatures are worth designing for even without a heat pump.
The water temperature leaving the radiator, back to the heat source.
Heat pumps are usually designed for a small drop between flow and return, often around five Celsius degrees (nine Fahrenheit degrees); boilers for a larger one. The drop is set by how fast water moves through the radiator, so it is a design choice rather than a property of the radiator, and a larger drop at low temperatures lowers the mean water temperature — which is where the output goes.
The air temperature the room is designed to hold.
Use the same design room temperature the heat loss was calculated at — commonly 21 °C (70 °F) for living rooms and 18 °C (64 °F) for bedrooms. A radiator's output depends on the gap between the water and the room air, so a warmer design room needs a noticeably larger radiator at the same water temperatures.
How steeply output falls with temperature — from the radiator's data sheet.
Every radiator catalogue that follows EN 442 publishes this figure beside the output. Steel panel radiators are typically close to 1.3, and that is the default here; column and cast-iron radiators are usually similar, fin-tube and convector types a little higher. A higher exponent means output falls faster as the water cools, so a guessed figure matters most at exactly the low temperatures this calculator is for.
The conditions the output printed in the catalogue was measured at.
The same radiator carries different numbers in different markets because they are quoted at different water temperatures. EN 442 catalogues state output at ΔT50; US hydronic catalogues quote it against average water temperature, most often 180 °F in a 65 °F room, which is a larger excess and so a larger number for the same radiator. Sizing a US-catalogued radiator against ΔT50 orders a radiator that is too small.
Catalogue rating to buy
10,200 BTU/h
At these water and room temperatures a radiator gives 33% of its catalogue rating, so it has to be catalogued at 3.0 times the room's heat loss to meet it. US catalogues usually quote output at an average water temperature — 180 °F in a 65 °F room is the usual row — rather than EN 442's ΔT50. If yours does, choose that basis above, or the radiator ordered will be too small.
- Output at these conditions, as a share of the catalogue rating
- 33.38 %
- Mean water temperature
- 108.5 °F
- Excess temperature — mean water minus room
- 38.7 Δ°F
- Excess temperature the catalogue rating is quoted at
- 90 Δ°F
- The same rating in watts, as European-made panels are catalogued
- 2,995.65 W
They open the calculator with your figures already in it
Radiator Output at Lower Flow Temperature (ΔT Correction) Calculator: 10,222 BTU/h — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- EN 442-2: a radiator's output at other conditions is its rated output × (ΔT ÷ ΔT at rating)^n, where ΔT is the excess of the mean water temperature over the room air and n is the radiator exponent the manufacturer publishes. The standard rating conditions are 75 °C flow, 65 °C return and 20 °C air — a 50 K excess, 'ΔT50'
- EN 442-2: where (return − room) ÷ (flow − room) falls below 0.7, the logarithmic mean excess temperature, (flow − return) ÷ ln((flow − room) ÷ (return − room)), is used in place of the arithmetic mean
- US hydronic catalogues quote output against average water temperature; the row most often read is 180 °F average water in a 65 °F room, an excess of 115 °F-degrees (63.9 K), and the same ratio method applies with that excess in place of 50 K
Inputs used
- Room Heat Loss the Radiator Must Meet
- 3412.14 BTU/hr
- Flow Temperature
- 113 °F
- Return Temperature
- 104 °F
- Room Design Temperature
- 69.8 °F
- Radiator Exponent (n)
- 1.3
- What the Catalogue Rating Is Quoted At
- EN 442 — ΔT50 (UK, Europe, Australia and most catalogues)
- Excess Temperature the Catalogue Rating Uses
- 90 °F
Intermediate steps
- Output at these conditions, as a share of the catalogue rating
- 33.38 %
- Mean water temperature
- 108.5 °F
- Excess temperature — mean water minus room
- 38.7 Δ°F
- Excess temperature the catalogue rating is quoted at
- 90 Δ°F
- The same rating in watts, as European-made panels are catalogued
- 2,995.65 W
Confidence note: At these water and room temperatures a radiator gives 33% of its catalogue rating, so it has to be catalogued at 3.0 times the room's heat loss to meet it. US catalogues usually quote output at an average water temperature — 180 °F in a 65 °F room is the usual row — rather than EN 442's ΔT50. If yours does, choose that basis above, or the radiator ordered will be too small.
What this calculation does not cover
- It sizes the radiator to the heat loss you enter and does not work that heat loss out. The figure has to come from a room-by-room calculation at the design outdoor temperature; an old radiator's rating, or a rule of thumb per unit of floor area, answers a different question.
- The exponent is the manufacturer's, and the answer moves with it. The default is typical of steel panel radiators; a column, cast-iron or fin-tube radiator can differ, and the difference is largest at exactly the low water temperatures this page is for.
- Fan-assisted radiators and trench convectors do not follow a single-exponent curve, so their output at low temperature comes from the manufacturer's own table, not from this ratio.
- It does not check that the flow rate needed to hold the drop you entered is available. A small drop between flow and return needs a much higher flow rate than a large one, and a radiator starved of flow runs cooler than this assumes.
- Covers, shelves over the top, deep window boards and metallic paint all cut output by amounts no catalogue figure includes, and a radiator under a large cold window gives part of its output to the glass.
Add the equipment this sizes
This result is a specification — 10,200 BTU/h — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-21 · v1.0.0
Regulatory standards & verification citations3
- EN 442-2: a radiator's output at other conditions is its rated output × (ΔT ÷ ΔT at rating)^n, where ΔT is the excess of the mean water temperature over the room air and n is the radiator exponent the manufacturer publishes. The standard rating conditions are 75 °C flow, 65 °C return and 20 °C air — a 50 K excess, 'ΔT50'
- EN 442-2: where (return − room) ÷ (flow − room) falls below 0.7, the logarithmic mean excess temperature, (flow − return) ÷ ln((flow − room) ÷ (return − room)), is used in place of the arithmetic mean
- US hydronic catalogues quote output against average water temperature; the row most often read is 180 °F average water in a 65 °F room, an excess of 115 °F-degrees (63.9 K), and the same ratio method applies with that excess in place of 50 K
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