What is the condensing threshold 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.

How does return temperature affect boiler efficiency?

At a 30°C return temperature, a condensing boiler can achieve ~96% gross efficiency. At 50°C, efficiency drops to ~91%. Above 55°C (the dew point), the boiler stops condensing and efficiency drops to 80-85% — similar to a non-condensing boiler. Every 1°C reduction in return temperature improves efficiency.

How can I lower the return temperature?

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.

What is seasonal efficiency?

Seasonal efficiency (SEDBUK or ErP) accounts for the fact that a boiler runs at part load for much of the heating season. At part load, return temperatures are lower and efficiency is higher. The seasonal figure is a weighted average that better represents real-world annual performance than peak-load efficiency.

Is my condensing boiler actually condensing?

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.

TradeCalculator

Condensing Efficiency Calculator — Return Temperature

Calculate boiler efficiency from return water temperature. Determine if the boiler is operating in condensing mode below the 55°C dew point threshold.

Return Temperature

Measured return water temperature

Flow Temperature

Boiler flow temperature setting

Boiler Type

Type of condensing boiler

Rated Output

Boiler rated heating output

How We Calculate This

This calculator maps the return water temperature to the boiler's efficiency curve, showing the impact of operating above or below the 55°C condensing threshold.

Efficiency curve

Below 55°C return: Condensing mode, 89-97% efficiency
Above 55°C return: Non-condensing, 78-86% efficiency
Fuel input: Output kW ÷ (efficiency / 100)
Annual cost: Fuel input × hours × gas price
Dew point: ~55°C for natural gas products of combustion

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.