Boiler Optimiser

Two settings do most of the work: the highest heating output the boiler is allowed to fire at, and the heating flow temperature. Hot water on a combi stays at its own higher output.

Boiler

System

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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Estimated house heat loss The low end is a milder year and the high end a colder day, from the space heating usage. Enter a figure if you know the heat loss.

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

Flow temperature now 75°C flow, about 55°C return

Fully condensing, up to 60°C Partially condensing, 61–73°C Non-condensing, 74°C and above

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 a 75°C flow

How to set the flow temperature and power on a particular boiler

Which brand

Power level

A boiler left on its full heating rating often puts out more heat than the house can lose. It reaches temperature, shuts off, then starts again. Those short runs use more gas on each ignition, make more noise, and wear the boiler.

A lower central-heating maximum keeps the burner on longer at a lower rate. That is steadier heat, and on a condensing boiler it spends more time with a cool enough return to condense. On a combi, leave the hot-water output at its higher rating so the taps stay hot. A Gas Safe engineer sets this.

Boiler heating output The central-heating kW, or Auto if the maximum is not capped.

Set flow to

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Radiator output

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of the ΔT 50 catalogue figure

Gas saving

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versus a 75°C flow, heating only

Return is taken as flow minus 20°C. Condensing starts once return is below about 54°C. Radiator output uses the EN 442 exponent 1.3 against a 20°C room. The saving uses the same efficiency curve as the condensing-mode calculator. Standing charge is not included. Changing boiler parameters is normally an engineer’s job. See how to lower flow temperature, maximum output mode, and OpenTherm controls.