Combi boiler Condensing mode savings

Enter the annual gas use from your bill and your unit rate. Move the flow-temperature slider; savings are taken from the condensing-efficiency graph (return temperature), assuming return is 20°C below flow. 75°C flow is factory non-condensing; 70°C already saves a little. If you cook with gas, typical cooking use is removed before cost and saving are calculated.

Annual gas usage From your gas bill, in kilowatt hours (kWh)

Do you use gas for cooking?

Estimated space heating usage

Bill kWh minus typical gas cooking (3% of annual use)

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Unit rate per kWh Gas unit rate in pence per kWh, not including standing charge

Heating flow temperature 55°C flow / 35°C return — condensing

Space heating cost

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excluding standing charge

Graph efficiency

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from return temperature

Condensing-mode saving

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versus 75°C factory flow

Formula

Space heating usage = annual kWh × 97% (gas cooking)

Space heating cost = space heating usage × (unit rate p/kWh ÷ 100)

Condensing-mode saving = 1 − η(75°C flow) / η(selected flow)

η comes from the efficiency graph using return = flow − 20°C. Factory 75°C flow is about 55°C return, just above dew point (non-condensing). 70°C flow is about 50°C return, just into condensing, so it still saves. Lower flow keeps climbing the green curve, with smaller extra gains at the coldest returns. HHIC’s 6–8% for 75°C → 55°C flow matches this curve. Standing charge is not included. See how to reduce flow temperature.

Boiler efficiency against return water temperature for condensing boilers, with dew point at 54°C — Efficiency is plotted against **return** temperature. The slider is heating **flow** temperature; return is taken as flow − 20°C. Dew point is 54°C return (about 74°C flow). Gas saving = 1 − η_{75°C flow} / η_selected.