On a combi, set the radiator (flow) temperature to around 60C and the hot water to around 50C. Most boilers are installed at 70 to 80C, which is far too high: at that temperature a condensing boiler cannot actually condense, and you lose a chunk of the efficiency you paid for. Turning it down costs nothing and saves gas immediately.
Most combis have two controls on the front. One sets the temperature of the water going to the radiators, called the flow temperature. The other sets the temperature of the hot water at the taps.
They do completely different jobs, and the flow temperature is the one that matters for your gas bill.
Every boiler sold in the UK for the last twenty years is a condensing boiler. The clever bit is that it extracts extra heat from the flue gases by cooling them until the water vapour in them condenses back into liquid.
But that only works if the water coming back from your radiators is cold enough to condense those gases, which means below roughly 55C.
Set the flow to 80C and the water coming back is far too hot to condense anything. The boiler still works. It just runs as an ordinary, less efficient boiler, and the condensing feature you paid for does nothing at all.
This is genuinely one of the most common and most expensive misconfigurations in British homes, and almost nobody knows about it.
Start at 60C on the heating flow. If the house heats up fine, try 55C.
Your rooms will take a bit longer to come up to temperature, because the radiators are running cooler. The house still gets just as warm, it just gets there more gently, and it uses less gas doing it.
If you have a room thermostat, it will simply run the boiler for longer rather than blasting and stopping. That is what you want.
Around 50C at the taps on a combi is plenty for washing and showering, and it reduces the risk of scalding.
One important exception: if you have a hot water cylinder rather than a combi, the stored water needs to reach at least 60C to kill legionella bacteria. Do not turn a cylinder down to 50C. That rule is about stored water, and it does not apply to a combi heating water on demand.
Only that a house that is very poorly insulated, or has radiators that were undersized to begin with, may struggle to get warm on a lower flow temperature. If you turn it down and the house will not heat, turn it back up and get someone to look at the radiators.
That is the same problem that makes a house a poor candidate for a heat pump, incidentally. A house that heats happily at 55C is a house that would suit a heat pump.
Around 60C for the radiators on a condensing combi, and you can often go as low as 55C. Most boilers are left at 70 to 80C, which prevents the boiler from condensing and wastes gas.
Because a condensing boiler only recovers extra heat from the flue gases when the returning water is cool enough to condense them, which means below roughly 55C. Run it at 80C and the condensing feature does nothing, so you are running an ordinary boiler and paying for an efficient one.
On a combi, 50C at the taps is fine and reduces scalding risk. If you have a hot water cylinder, the stored water must reach at least 60C to control legionella bacteria. The distinction matters: it is about stored water, not water heated on demand.
No, it will just heat up more gradually. The house reaches the same temperature, it takes a little longer to get there, and it uses less gas. If the house genuinely will not get warm at 60C, the radiators may be undersized and that is worth looking at.
We diagnose the actual fault before quoting to fix it. No jargon, no scare tactics.
