Radiator Output Comparison — Type 11-22

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.

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.

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.

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.

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.