What Size Radiator Do I Need? BTU Explained

BTU and watts

Radiator output is given in BTUs per hour (British Thermal Units) or watts. They measure the same thing:

1 watt = 3.412 BTU per hour.

1,000 BTU = about 293 watts.

So a radiator rated at 5,000 BTU gives out about 1,465 watts (1.47kW).

What affects how much heat a room needs

• Room size: length × width × ceiling height. Bigger rooms and high ceilings need more heat.
• Room type: living rooms and bathrooms are usually heated warmer (around 21–22°C) than bedrooms (around 18°C).
• Windows: single glazing and large windows lose far more heat than modern double glazing.
• External walls: a room with two or three outside walls loses more heat than one with only one.
• Insulation: solid walls, uninsulated lofts and suspended timber floors increase heat loss.
• What's above and below: a room over a garage or under an uninsulated loft needs more heat.

A quick example

A typical living room of 4m × 4m with a 2.4m ceiling (38m³), with one external wall, double glazing and average insulation, might need somewhere in the region of 5,000–6,500 BTU (about 1.5–1.9kW). A draughty room with solid walls and big single-glazed windows could need a lot more. The Radiator BTU Calculator takes all these factors into account for your room.

Reading radiator specifications

Radiator outputs are usually quoted at ΔT50 (spoken as “delta T 50”). That's the output when the average radiator temperature is 50°C above the room temperature – typical of a gas boiler running hot. If your system runs cooler, the radiator gives out less heat than the figure on the box.

Heat pumps need bigger radiators

Heat pumps run most efficiently at lower water temperatures. At those temperatures, a radiator gives out a lot less than its ΔT50 rating – at around ΔT30, roughly half. That's why homes switching to a heat pump often need some larger radiators. Heat pump installations should be designed using a proper room-by-room heat loss calculation by the installer, not a rule of thumb.

Radiator types

Type 11: single panel, single convector – slim, lower output.

Type 21: double panel, single convector – a good middle ground.

Type 22: double panel, double convector – the highest output for its size; the most common choice for living rooms.

A Type 22 radiator gives much more heat than a Type 11 of the same height and width, so if wall space is tight, go for a higher type rather than a wider radiator.

Tips

• Put radiators under windows where possible – they counteract cold draughts.
• Don't hide radiators behind sofas or long curtains.
• Fit thermostatic radiator valves (TRVs) so each room can be controlled.
• Bleed radiators and balance the system before deciding a radiator is too small.
• For large rooms, two smaller radiators often heat more evenly than one big one.