BTU to kW Conversion: Quick Reference for Heating Engineers

Summary

BTU (British Thermal Unit) and kW (kilowatt) are the two standard units for expressing heat output in the UK heating industry. Boilers are typically rated in kW, while radiator catalogues often list output in both BTU/h and kW. All radiator outputs in the UK are tested and rated to BS EN 442 at Delta T 50 (DT50): flow 75 degC, return 65 degC, room 20 degC. Understanding the conversion between these units is essential for correctly sizing radiators, matching them to boiler capacity, and performing heat loss calculations. The exact conversion factor is 1 BTU/h = 0.000293071 kW, but for practical purposes 1 kW = 3,412 BTU/h is the industry-standard figure.

Key Facts

• 1 kW = 3,412 BTU/h
• 1 BTU/h = 0.293 W (0.000293 kW)
• All UK radiator outputs are rated at DT50 (BS EN 442)
• At lower flow temperatures (e.g. heat pumps at 35-50 degC), radiator output drops significantly -- apply manufacturer correction factors
• Boiler kW ratings in the UK refer to heat output, not input
• Combi boilers have two ratings: central heating (CH) output and domestic hot water (DHW) output

Conversion Formulae

Conversion Table

Common Radiator Outputs (at DT50, BS EN 442)

Note: Values are representative. Always check manufacturer datasheets for exact outputs. Outputs vary by brand and model.

Common Appliance Ratings

Note: Combi boiler kW ratings are heat output. CH and DHW ratings differ; DHW is typically higher.

Detailed Guidance

How do I convert BTU to kW?

Formula: kW = BTU/h / 3,412

Worked examples:

1. A radiator catalogue lists a Type 22 (600x1000) at 6,260 BTU/h.

   • 6,260 / 3,412 = 1.83 kW

2. A room heat loss calculation gives 12,500 BTU/h.

   • 12,500 / 3,412 = 3.66 kW

3. Converting back: a 25 kW boiler CH output in BTU.

   • 25 x 3,412 = 85,300 BTU/h

How do I estimate room heat loss?

The simplified cubic volume method gives a reasonable first estimate for radiator sizing. For accurate results on larger projects, use a full room-by-room heat loss calculation to BS EN 12831.

Simplified formula:

Heat requirement (W) = Room volume (m3) x Heat factor (W/m3)

Step 1 — Calculate room volume:

Room volume = Length (m) x Width (m) x Ceiling height (m)

Step 2 — Select heat factor:

Step 3 — Apply room-type adjustment:

Step 4 — Apply exposure adjustment:

Step 5 — Add 15% safety margin, then convert to BTU if needed (x 3.412).

Worked example:

A 1970s semi-detached living room, 5m x 4m, ceiling 2.4m:

• Volume = 5 x 4 x 2.4 = 48 m3
• Heat factor (average insulation) = 36 W/m3
• Base heat = 48 x 36 = 1,728 W
• Living room adjustment: 1,728 x 1.1 = 1,901 W
• Normal exposure: 1,901 x 1.0 = 1,901 W
• With 15% safety margin: 1,901 x 1.15 = 2,186 W
• In BTU: 2,186 x 3.412 = 7,458 BTU/h
• Select a radiator rated at or above 7,458 BTU/h, e.g. a Type 22, 600x1200 (approx. 7,510 BTU/h).

How do I size a radiator for a room?

1. Calculate room heat loss using the method above (or a full BS EN 12831 calculation for accuracy).
2. Identify the BTU/h or kW requirement including the 15% safety margin.
3. Select a radiator from manufacturer catalogues with an output at DT50 that meets or exceeds the requirement.
4. Check wall space — confirm the radiator physically fits the available wall, allowing clearance (min 100mm from floor, 50mm from sill).
5. Consider low-temperature systems — if connecting to a heat pump or low-temperature system, apply the manufacturer's DT correction factor. At DT30 (typical for heat pumps), radiator output is roughly 50% of the DT50 rating.

Quick sizing rule of thumb:

For a typical 1970s-1990s UK home with average insulation and 2.4m ceilings:

• Small bedroom (8-10 m2): 2,500 - 3,500 BTU/h
• Double bedroom (12-15 m2): 4,000 - 5,500 BTU/h
• Living room (16-22 m2): 5,500 - 8,500 BTU/h
• Large living room (22-30 m2): 8,500 - 12,000 BTU/h
• Bathroom (4-6 m2): 2,000 - 3,000 BTU/h (plus towel rail)

Frequently Asked Questions

Are BTU and BTU/h the same thing?

In heating, "BTU" almost always means "BTU per hour" (BTU/h). Strictly, a BTU is a unit of energy (the heat needed to raise 1 lb of water by 1 degF), while BTU/h is a rate of heat output. Radiator and boiler catalogues use BTU to mean BTU/h.

Why do manufacturer outputs differ from my calculation?

Radiator outputs are tested at DT50 (flow 75 degC, return 65 degC, room 20 degC) per BS EN 442. If your system runs at different temperatures, actual output will differ. Modern condensing boilers often run at lower flow temperatures for efficiency, reducing radiator output. Always apply the manufacturer's DT correction factor for non-standard conditions.

Do I need to derate radiators for heat pump systems?

Yes. Heat pumps typically supply water at 35-50 degC (DT15 to DT30), compared to 75 degC for a gas boiler. At DT30, a radiator produces roughly 50% of its DT50 rated output. At DT20, it drops to roughly 30%. You will need significantly larger radiators or additional emitters for heat pump retrofits.

What is the difference between Type 11, Type 21, and Type 22 radiators?

• Type 11 (K1): Single panel, single convector fin. Lowest output, shallowest depth (~65mm).
• Type 21 (P+): Double panel, single convector fin. Mid-range output, moderate depth (~90mm).
• Type 22 (K2): Double panel, double convector fin. Highest output for standard panels, deepest (~100mm).

A Type 22 produces roughly 80-90% more heat than a Type 11 of the same dimensions.

Should I round up or down when selecting a radiator?

Always round up. Select the next radiator size above your calculated heat loss. An undersized radiator will not heat the room to design temperature on the coldest days. A slightly oversized radiator simply means the TRV closes earlier.

Regulations & Standards

• BS EN 442:2014 — Radiators and convectors. Defines testing methods and output ratings at DT50. All UK radiator manufacturers rate output to this standard.

• BS EN 12831:2017 — Energy performance of buildings. Method for calculation of the design heat load. The full standard for room-by-room heat loss calculations.

• Building Regulations Part L — Conservation of fuel and power. Sets maximum heat loss rates for new builds and major renovations. Drives insulation requirements that affect heat loss calculations.

• MCS MIS 3005 — Heat pump system design standard (Microgeneration Certification Scheme). Requires full room-by-room heat loss calculations for heat pump installations.

• Gas Safe Register — All gas boiler installations must be carried out by a Gas Safe registered engineer.

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