Heat Pump Radiator Calculator

Radiators are rated at a standard ΔT of 50K (BS EN 442 reference condition: flow 75°C, return 65°C, room 20°C). Heat pumps operate at much lower flow temperatures (35–45°C), which dramatically reduces radiator output. At 45°C flow with a typical 5K heat-pump return spread and a 20°C room, a radiator produces approximately 35% of its BS EN 442 rated output (mean water 42.5°C, so ΔTm ≈ 22.5K, factor = (22.5/50)^1.3 ≈ 0.35).

The de-rating factor is the proportion of a radiator’s rated output available at the heat pump flow temperature. It is computed live from (ΔTm/50)^1.3 per BS EN 442, with a default 5K return spread and a 20°C room. Typical values at those defaults: 35°C flow ≈ 0.16, 40°C ≈ 0.26, 45°C ≈ 0.35, 50°C ≈ 0.46, 55°C ≈ 0.57. So a 1000W rated radiator produces only about 354W at 45°C flow.

Not always. If your existing radiators were originally oversized for the rooms (common in older installations), they may still provide adequate output at lower flow temperatures. Radiators in well-insulated rooms with low heat loss are also more likely to be adequate.

For heat pump systems, Type 22 (double panel, double convector) or Type 33 (triple panel) radiators provide the most output for their footprint. Fan-assisted convectors are also excellent as they force air across the fins, significantly boosting output at low flow temperatures.

Yes, but higher flow temperatures reduce heat pump COP (coefficient of performance). As a rough guide, an ASHP loses roughly 0.4–0.5 COP points for every 10°C rise in flow temperature, so running cooler is always more efficient. For example, MCS product-database testing of one model showed a COP of about 4.1 at 35°C flow falling to about 3.3 at 55°C; exact figures vary widely by model and outdoor temperature, so check the manufacturer’s SCOP/COP data for your unit. It is generally better to upsize radiators than to increase flow temperature.