Radiator BTU Explained

Radiator BTU usually means BTU per hour: the rate at which the radiator emits heat under specified water and room temperatures. Watts describe the same rate in a metric unit.

Compare output at the relevant temperature condition, not the casing dimensions or a bare BTU number.

Calculate how many BTUs my room needs

BTU/h and watts

One watt is approximately 3.412BTU/h. To convert watts to BTU/h, multiply by 3.412. To convert BTU/h to watts, divide by 3.412. A radiator rated at 1,000W is therefore about 3,412BTU/h at the stated condition.

Rounding is appropriate because the room requirement is itself an estimate.

Required output versus product output

A heat-loss calculation estimates how quickly the room loses heat on a chosen cold design day. The selected emitters need to replace that loss while operating at the designed water temperatures.

A product table reports what the radiator can emit under a defined test condition. Matching the two requires the same units and a suitable temperature basis.

Why ratings need a Delta T

The same radiator emits less heat when its mean water temperature is closer to room temperature. A catalogue value at ΔT50 cannot be used unchanged for a lower-temperature heat-pump design.

Use the manufacturer's correction table or output at the intended flow, return and room temperatures. Read radiator Delta T explained.

Why dimensions mislead

A Type 22 radiator can add convector surface behind the panels and provide more output than a shallower Type 21 of similar height and width. Vertical and designer shapes also vary. The exact model table is the authority.

Purmo's panel radiator data publishes different outputs by type, size and temperature, illustrating why appearance is insufficient.

Use the radiator calculator to estimate the room requirement in both units. Then compare Type 21 and Type 22 radiators and have the final system, pipework and controls checked professionally.

How to read a product table

Find the exact height, length and type, then read across to the output column whose temperature condition matches the design. Check whether the table uses watts, BTU/h or both.

If only a ΔT50 figure is shown for a lower-temperature system, use the manufacturer's own correction data. Keep the corrected number in your notes rather than changing the room requirement.

Example conversion

Suppose a room estimate is 1,250W. Multiplying by 3.412 gives about 4,265BTU/h. A product listed at 4,500BTU/h is about 1,319W, but only at the condition stated beside that rating.

If 4,500BTU/h is a ΔT50 value and the system will operate at ΔT30, it cannot be treated as 1,319W at ΔT30.

Questions to ask about a BTU claim

• Is the figure BTU per hour, and is it heat output rather than boiler input?
• Which flow, return and room temperatures apply?
• Does it describe the exact size and panel type being sold?
• Has a retailer rounded or copied the value from a nearby model?
• Is a lower-temperature output available from the manufacturer?

Adding several radiator outputs is valid only as an arithmetic total at the same temperature condition. It does not prove that the pipework can deliver the required flow or that heat will be well distributed. Controls and balancing still matter.

BTU is therefore not a quality score. It is one unit for a rate of heat transfer. Once all figures use the same units and temperature basis, physical fit, control, installation and verified manufacturer data can decide the shortlist.