What do Type 11, Type 21 and Type 22 mean?

The first digit is the number of panels, the second is the number of convector fins. Type 11 = 1 panel + 1 convector (slimmest, lowest output). Type 21 = 2 panels + 1 convector (good output, moderate depth). Type 22 = 2 panels + 2 convectors (highest output, deepest). A Type 21 produces roughly 40-50% more heat than a Type 11 of the same dimensions, and a Type 22 produces about 70-80% more than a Type 11.

Are radiator outputs quoted at standard conditions?

Yes. UK radiator outputs are quoted at Delta T 50 (ΔT50) — meaning a mean water temperature 50°C above room temperature. This equates to flow 75°C, return 65°C, room 20°C. This is standard for gas boiler systems. Heat pump systems typically run at flow 45°C, return 40°C, giving ΔT22.5, at which radiators produce only about 35-40% of their rated output.

How do I calculate radiator output at lower temperatures?

Use the EN 442 correction factor formula: actual output = rated output × (ΔT actual / ΔT 50)^1.3. For a heat pump at ΔT22.5: correction factor = (22.5/50)^1.3 = 0.36. So a radiator rated at 1,000W at ΔT50 would produce only 360W at ΔT22.5. This is why heat pump installations often need larger radiators — typically 2-3 times the rated size.

Which type is best for heat pump systems?

Type 22 radiators are generally best for heat pump systems because they provide the highest output per unit of wall space. Since heat pump flow temperatures are lower, you need maximum radiator surface area. Alternatively, consider oversized Type 21 radiators which are less bulky while still providing adequate output. Underfloor heating is the ideal emitter for heat pumps.

How many BTU do I need per room?

As a rough guide: 40-60 BTU per cubic foot of room volume, depending on insulation level and exposure. A typical 4m x 5m x 2.4m living room (48 m³ / ~1,700 ft³) needs approximately 4,500-6,000 BTU. Use a proper heat loss calculation for accuracy — this accounts for insulation, glazing, external walls and ventilation. A room heat loss calculator gives a much more accurate figure than rules of thumb.

Radiator Output Comparison — Type 11-22

Compare Type 11, Type 21 and Type 22 radiator BTU and watt outputs by size, with correction factors for heat pump flow temperatures.

Output Comparison (600mm High Radiators at ΔT50)

Width	Type 11 (Watts)	Type 21 (Watts)	Type 22 (Watts)
400mm	~350W	~510W	~620W
600mm	~530W	~760W	~930W
800mm	~700W	~1,020W	~1,240W
1000mm	~880W	~1,270W	~1,550W
1200mm	~1,060W	~1,520W	~1,860W
1400mm	~1,230W	~1,780W	~2,170W
1600mm	~1,410W	~2,030W	~2,480W

Correction Factors for Flow Temperature

Flow/Return Temp	Delta T	Correction Factor	System
75°C / 65°C	ΔT50	1.00	Standard gas boiler
65°C / 55°C	ΔT40	0.75	Low-temp gas boiler
55°C / 45°C	ΔT30	0.52	High-temp heat pump
50°C / 40°C	ΔT25	0.41	Heat pump typical
45°C / 40°C	ΔT22.5	0.36	Efficient heat pump

Physical Dimensions

Type 11 — ~65mm deep, slimmest profile, suits tight spaces
Type 21 — ~85mm deep, flat front panel, good balance of output and depth
Type 22 — ~105mm deep, highest output, protrudes furthest from wall

Related Calculators

Use the Radiator Sizing Calculator for room heat loss, or the Radiator BTU Calculator for output requirements.

How We Calculate This

Frequently Asked Questions

Last updated: April 2026

All calculations are estimates. Verify with your supplier.

Radiator Output Comparison — Type 11-22

Output Comparison (600mm High Radiators at ΔT50)

Correction Factors for Flow Temperature

Physical Dimensions

Related Calculators

How We Calculate This

Frequently Asked Questions

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