Condensing Efficiency Calculator — Return Temperature

The dew point of natural gas flue products is approximately 55°C. When the return water temperature drops below this point, water vapour in the flue gases condenses on the heat exchanger, releasing latent heat. This latent heat recovery is what makes condensing boilers more efficient than non-condensing ones.

At a 30°C return temperature, a condensing boiler can achieve ~96% gross efficiency. At 50°C, efficiency drops to ~91%. At or above 55°C (the flue-gas dew point) the boiler stops condensing and gross efficiency falls to 78-86% — similar to a non-condensing boiler. Every 1°C reduction in return temperature improves efficiency.

Lower the boiler flow temperature (50–55 °C instead of 70–80 °C), ensure radiators are correctly sized for the lower flow temperature, balance the system properly (ΔT 11–12 °C), use weather compensation, and ensure adequate insulation to reduce heat demand. Use our Heating Balancing Calculator to get the correct valve settings, and our Radiator Sizing Calculator to check radiator adequacy at lower flow temperatures.

Seasonal efficiency (SEDBUK / ErP) accounts for a boiler running at part load for much of the heating season. The SEDBUK method (BRE SAP technical papers CONSP:02 and S10TP-12) averages a full-load test at a 60°C return temperature with a part-load test at a 30°C return temperature, reflecting an average firing return of about 45°C across the season. Following that approach, this calculator blends the gross efficiency at your entered (full-load) return temperature with the gross efficiency at the 30°C part-load condition, then caps the result at the boiler’s peak gross efficiency so it can never exceed it. Because part-load operation runs cooler and condenses more, this seasonal estimate better represents real-world annual performance than peak-load efficiency. It is an estimate from a typical efficiency curve, not a certified SEDBUK rating.

Check the condensate drain — if water is flowing during heating operation, the boiler is condensing. Alternatively, measure the return pipe temperature with a clip-on thermometer. If it's below 55 °C, the boiler should be condensing. Many modern boilers also show return temperature on the display. Use our Condensate Pipe Calculator if you need to size or check the condensate run.