Heat Pump Radiator Sizing UK 2026

Radiator heat output depends on:

• Mean water temperature inside the radiator (average of flow + return temperature).
• Room air temperature.
• Radiator surface area + emissivity.

A radiator's quoted output (e.g. '1,500W') is at a standard test condition: 75°C mean water, 20°C room → 55°C delta-T. At heat pump flow temperature 45°C / return 40°C: mean 42.5°C, room 20°C → delta-T 22.5°C. That's roughly 41% of the delta-T, meaning the radiator delivers ~41% of nominal output (output scales roughly with delta-T^1.3).

Result: a radiator quoted at 1,500W on a gas-boiler test condition delivers ~615W at heat pump flow temperature. To still deliver 1,500W, you need a radiator with nominal ~3,600W output at the standard test - roughly 2.4x larger surface area.

UK radiator type codes follow a 2-digit convention: first digit = number of panels, second digit = number of fin convector sets (0, 1, or 2).

• Type 11 (1 panel, 1 convector): lowest output per metre. Decorative use only; rarely sized for primary heating.
• Type 21 (2 panels, 1 convector): low-mid output. Common in 1980s-1990s UK installs.
• Type 22 (2 panels, 2 convectors): the UK gas-boiler default. Most existing UK radiators are Type 22.
• Type 33 (often called K3): 3 panels, 3 convectors. Highest output per metre at any given height + length. Standard heat pump retrofit choice.

K3 radiators look chunkier than standard Type 22 (typically 100mm deep vs 70mm) but most rooms have wall space for the upgrade.

For each room, the calculation is:

1. Room heat loss in watts from MCS heat-loss survey (e.g. lounge 1,800W).
2. Heat pump flow temperature from your system design (e.g. 45°C).
3. Find a radiator with nominal output ≥ room loss × correction factor for your flow temp. Correction factor at flow 45°C: ~2.4x. So a 1,800W room needs a radiator nominal ≥ 4,320W.
4. Check manufacturer datasheet for radiators delivering ≥4,320W at 75°C standard test - typically a K3 type, 600mm height × 1,400-1,600mm length.

This per-room calculation is what your MCS installer should be doing. If their proposal uses existing Type 22 radiators without explicit per-room datasheet verification, push back - they may be assuming a higher flow temperature that reduces SCOP.

Existing radiators may be adequate when:

• Property has lower heat loss than typical (modern insulation, smaller floor area, sheltered location). Some 1990s+ Type 22 installs already have ~2x oversized radiators for the actual heat demand.
• You can accept slightly higher flow temperature (50-55°C rather than 45°C) - SCOP drops 10-15% but radiator upgrade cost is avoided. Trade-off varies by tariff + electricity cost.
• Property has mixed heating (e.g. some rooms underfloor + some radiator). The underfloor rooms can run cooler flow temperatures naturally, allowing the heat pump to optimise for the radiator rooms.
• Installer proposes a hybrid system where the gas boiler covers radiator-rooms peak load + heat pump handles base + hot water. See our hybrid guide.

For most retrofits, however, at least 2-4 radiator upgrades are typical. Budget GBP 200-500 per radiator including fitting.

Underfloor heating (UFH) addresses the radiator-sizing problem fundamentally: the entire floor becomes an emitter, so even at 30-35°C flow temperature the floor delivers comfortable warmth. Benefits:

• SCOP 10-20% higher than radiator-equivalent installs (lower flow temp = higher COP at every operating point).
• More comfortable heat distribution - radiant heat warms occupants directly, no cold spots near windows.
• No visual impact - no radiators on walls.

UFH retrofit costs GBP 60-90 per m² for the heating system + screed/finish work. For new-build or major renovation, UFH is the default heat pump distribution + delivers the best SCOP. For retrofit where UFH would require lifting existing flooring, radiator upgrades are typically the practical choice.