Radiator Sizing Guide (Heat Loss & BTU)
A radiator's catalogue output is only true at one flow temperature. Size from the room's heat loss and correct for your actual delta-T, and every room reaches temperature — guess, and one always disappoints.
Radiator sizing looks simple until you realise the output on the box assumes a flow temperature you may never run. Size a radiator without correcting for your actual system temperatures and it can fall well short — most visibly on heat pump and low-temperature systems, where the correction is large.
This guide covers sizing radiators the reliable way: work out the room's heat loss, understand BTU and watt output, then apply the delta-T correction for the temperature your system actually runs at.
Key takeaways
- Start from the room's heat loss — the radiator has to replace exactly the heat the room loses.
- Radiator outputs are quoted at a stated delta-T; run cooler than that and the real output drops.
- Delta-T is the difference between the average radiator temperature and the room temperature.
- On low-temperature and heat pump systems the correction is large — a radiator may give half its catalogue figure.
- Size to hit temperature at your design flow temperature, then record the numbers against the job.
Start with room heat loss
A radiator exists to replace the heat a room loses to the outside, so the sizing starts with the room's heat loss — its dimensions, construction, glazing, insulation and the temperature you want to maintain. That figure, in watts or BTU, is the output the radiator (or radiators) in that room must deliver.
Getting the heat loss right is the foundation. Oversize every room 'to be safe' and you inflate the system, hurt efficiency and, on a heat pump, undermine the whole low-temperature design. Undersize it and the room never gets warm on a cold day.
BTU, watts and radiator output
Radiator outputs are quoted in watts or BTU per hour (the two convert directly). The catalogue figure, though, is measured at a specified temperature difference between the radiator and the room — commonly a delta-T of 50°C in older listings, or 60°C in some, and increasingly lower figures for heat pump ranges.
That headline number is only achieved if your system runs at the temperature the test assumed. Buy a radiator on its delta-T 50 figure and then run the system cooler, and it will quietly under-deliver in the actual installation.
Correcting for delta-T
Delta-T is the difference between the mean water temperature in the radiator and the room air temperature. If your flow and return average, say, 60°C and the room is 21°C, your delta-T is about 39°C — lower than the delta-T 50 the catalogue assumed, so the real output is less than the printed figure and you apply a correction factor to find it.
On a heat pump system running a low flow temperature, the mean water temperature might be barely above the room, giving a small delta-T and a large correction — a radiator can deliver roughly half its catalogue output. That is precisely why heat pump installs so often need bigger radiators or underfloor heating.
Always size at your real delta-T
Pick the radiator from its corrected output at your system's actual delta-T, not the headline catalogue figure. On low-temperature systems this is the difference between a warm room and a cold callback.
From sizing to the job
Work room by room: heat loss, then the corrected radiator output at your design delta-T, then the radiator that meets it with a little headroom for the coldest days. Free heating calculators make the arithmetic quick and consistent so you are not doing it on the back of an envelope.
Record the sizing against the job so the numbers are there if a room is ever questioned, and so a quote for the work reflects the emitters the design actually needs — not an optimistic guess that comes back to bite you.
Keep the numbers on the job
Logging the sizing and design figures against the job in TradePlanr means the calculations, the quote and the eventual certificate all live in one place — handy the next time that customer calls.
Frequently asked questions
What is delta-T in radiator sizing?
Delta-T is the difference between the average water temperature in the radiator and the room air temperature. Catalogue outputs are quoted at a stated delta-T, so if you run cooler than that, the real output is lower and you apply a correction factor.
Why do heat pump systems need bigger radiators?
Because heat pumps run at low flow temperatures, the delta-T is small and a radiator delivers far less than its catalogue figure — often around half. To meet the room's heat loss at that low temperature you need larger radiators or underfloor heating.
Should I oversize radiators to be safe?
Size to the room's heat loss at your design delta-T with modest headroom, not wholesale oversizing. Over-sizing hurts efficiency and, on a heat pump, undermines the low-temperature design the whole system depends on.
From guidance to action
Related guides
Air Source Heat Pump Installation Guide
System design, siting, commissioning and the Benchmark and MCS paperwork for an ASHP install.
Underfloor Heating Installation Guide
Wet vs electric UFH, screed and pipe spacing, commissioning and handover documentation.
Expansion Vessel Sizing for Sealed Systems
How to size an expansion vessel from system volume, charge pressure and acceptance factor.
