Advanced options
COMBI BOILERS ONLY for the simple version: on a combi, flow temperature affects radiators only: hot water taps are separate and unaffected. System boilers with a hot water cylinder need the cylinder kept at 60C+ (legionella): adjust heating flow only, or get advice. The setting is the radiator symbol dial/menu on the boiler front: it's user-adjustable by design and completely reversible: turn it back up if the house struggles in a cold snap.
What The Boiler Flow Temperature Calculator Works Out
- Saving from dropping to 60C: your annual heating gas bill multiplied by the percentage a condensing boiler saves when it moves from your current flow temperature to 60C: about 9% from the 80C factory default, 8% from 75C and 6% from 70C. On a GBP 900 bill that is GBP 50 to GBP 80 a year for a 30-second dial change.
- Why it works: condensing boilers have been mandatory in new installations since 2005, but they only recover the extra 10% or so of flue-gas heat when return water is below about 55C. At 75C to 80C flow they run in permanent non-condensing mode; at 60C they run as certified.
- The trade-off: radiators run warm rather than hot and cycle longer, so rooms heat from cold more slowly. Timer-scheduled households rarely notice; poorly insulated homes with undersized radiators may need 65C to 70C, and the dial is reversible in a cold snap.
- Target flow temperature (advanced): 60C captures the full saving; 65C is a compromise that keeps roughly 60% of it for colder, harder-to-heat homes; 55C in a well-insulated house with generous radiators squeezes out a little more and is also the dress rehearsal for the 35C to 45C regime heat pumps run.
- Share of the bill that is heating (advanced): if you only know your total gas bill, enter the heating share here. Hot water from a combi is unaffected by the flow setting; a typical home spends 70% to 85% of its gas on space heating.
- Gas unit rate and gas saved (advanced): the money saved divided by your unit rate, defaulting to the current Ofgem price cap, gives the kilowatt-hours of gas you stop burning each year.
- CO2 saved (advanced): the gas saved multiplied by 0.183 kg of CO2 per kWh. A typical 8% saving on a GBP 900 bill avoids around 160 kg a year.
- Saving over the years and price change (advanced): the annual saving multiplied by the years you choose, with an optional percentage change to gas prices applied to every year. Higher prices make the free fix worth more, not less.
- Per month in the heating season (advanced): the yearly saving spread over a seven-month October to April season, the figure you would actually see on winter bills.
- Versus a smart thermostat (advanced): payback on a smart thermostat at a typical 8% saving, for comparison with the flow dial's zero outlay. The two measures stack, as do draught-proofing, radiator balancing and the combi's separate hot-water dial, which often ships scalding hot.