Roger Russell

Mains hot-water Combination Boilers, condensing and standard. Compared to a conventional atmospheric hot water storage system.

Condensing boilers have efficiencies of up to 94% by extracting more heat. This has the effect of cooling the flue gasses to around 50C-60C. Non-condensing boilers, which have efficiencies up to 80% to 83%, keep the flue gas temperatures around 220C to prevent condensation occurring in the boiler and flue. Condensing boilers have a greater surface area to extract more heat and the dew point is much lower, causing condensation on the heat exchanger and in the flue system. This water is collected and piped to drain. Condensing boilers are most efficient when they are condensing. Domestic and small commercial heating systems are designed on 82C flow and 71C return temperature, mean primary water temperature is 76.5C = 11C differential. Calculations are based on continuous heating. Night shut down is usual but if the outside temperature is near the design temperature it will be necessary to run 24 hrs/day. For a condensing boiler the system design is 82C flow and 68C return temperature, mean primary water temperature 75C = 14C differential. Therefore when the outside temperature is near the design temperature 0C the boiler thermostat should be set high to maximum.

For a condensing boiler it is more fuel efficient to run for sustained periods of time at a lower temperature rather than running for shorter periods of time at higher temperatures. By trickling heat into the system continuously, so all the radiators are always warm, the burner is kept on a low rate of modulation constantly. The primary water gives up most of its heat as it circulates through the radiators and the return temperature to the boiler is kept low. It is not considered cost effective to oversize radiators to maximise condensing boiler output efficiency. 100% over sizing only improves efficiency by 2.4%. Over sizing will result in reduced system efficiency due to temperature overshoot. Even without entering the condensing mode the boiler will still achieve an annual efficiency of around 86% compared to around 67% for a modern non-condensing boiler. Since it burns less gas than a non-condensing boiler of the same heat output it produces less flue gases to contribute to the atmospheric pollution and ‘greenhouse effect’. Also, some of the contaminants in the flue gases are absorbed by the condensate, instead of being discharged to the atmosphere, and harmlessly neutralised in the domestic drainage system. There is approx. a 25% reduction in NOx and CO2 production with a condensing boiler.

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+44(0)1223 843230
or email R.J.H.C@tesco.net