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Quick answer
Radiator output is measured in watts or BTU per hour (1 watt is about 3.41 BTU/h). The heat a room needs depends on its size, insulation, windows, external walls and the temperature you want. Radiator outputs are usually quoted at Delta T 50, meaning the radiator is 50°C hotter on average than the room. If you run the boiler at lower flow temperatures for efficiency, radiators give out considerably less heat, so a radiator that was fine at high temperatures may struggle. Online BTU calculators give a rough guide; a proper heat loss calculation is more accurate.
Key takeaways
- 1 watt of output is roughly 3.41 BTU per hour.
- Room heat demand depends on volume, insulation, glazing, walls and target temperature.
- Manufacturer outputs are normally quoted at Delta T 50.
- At lower flow temperatures, output falls significantly, roughly half at Delta T 30.
- Undersized radiators lead to cold rooms and pressure to raise flow temperatures.
Watts, BTU and Delta T
Radiator catalogues list output at a specified Delta T, typically 50. Delta T is the difference between the average water temperature in the radiator and the room temperature. At Delta T 50, a room at 20°C needs an average radiator temperature of 70°C. If you lower the flow temperature so the average radiator temperature is 50°C, that is Delta T 30, and output drops to around half of the Delta T 50 figure.
What affects how much heat a room needs
- Room size (floor area and ceiling height).
- Number of external walls and whether walls are solid or cavity.
- Insulation in loft, walls and floors.
- Window size and glazing type.
- Target temperature: living rooms need more than bedrooms.
- Draughts and air changes.
Using a BTU calculator
Online calculators ask for dimensions, wall type and window area and estimate the required output. They are useful for a rough check. For heat pumps or low-temperature systems, a room-by-room heat loss calculation is much better.
Signs a radiator is too small
- Room never reaches temperature on cold days.
- You need the boiler flow temperature high to stay warm.
- The radiator is hot but the room still feels cold.
Check it is not simply a stuck valve or sludge first. See one radiator not heating up.
Upsizing radiators
Replacing a small radiator with a larger or double-panel model increases output without raising flow temperature, which helps efficiency. See radiator types explained and keeping old radiators with a new boiler.
Book with Gasmaster
Rooms that never get warm enough? Gasmaster Plumbing & Heating, Gas Safe registered (220932), sizes and replaces radiators across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. See our central heating services.
Frequently asked questions
How many BTU do I need for a bedroom?
It depends on size and insulation. Use a calculator as a guide, then adjust for your home.
Is a bigger radiator more expensive to run?
Not necessarily. A larger radiator can heat the room at a lower water temperature, which can be more efficient.
What does double panel mean?
Two panels increase surface area and output. Adding convector fins increases it further.
Sources
- BS EN 442, radiators and convectors (bsigroup.com)
- CIBSE, heat loss guidance (cibse.org)
- Energy Saving Trust (energysavingtrust.org.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
