Boiler 2 September 2026

What Should Your Boiler Flow Temperature Be?

For most homes with a modern condensing combi and ordinary radiators, a heating flow temperature of 60°C is the sensible starting point, and plenty of systems will run comfortably a little below that. If yours is still on the setting it left the factory with — often 75°C or higher — the boiler is almost certainly not condensing for much of the time it is running, which is the whole reason it was fitted.

It is the single most useful dial on the front of a boiler and the one almost nobody has been shown. When our engineers open up a boiler in Kent for a service or a repair, the flow temperature is very often exactly where the installer left it on the day it went in.

What the boiler flow temperature actually is

The flow temperature is how hot the boiler heats the water it sends out to your radiators. That water travels round the circuit, gives up its heat into the rooms, and comes back cooler. The temperature it comes back at is the return temperature — and that is the number the boiler really cares about.

A condensing boiler gets its efficiency from pulling the last of the heat out of its own flue gases, which only happens when the water returning from the radiators is cool enough to condense the moisture in those gases. Broadly, the return needs to be under about 55°C for that to happen properly. You cannot set the return temperature directly. You set the flow, and the return follows it down.

Run the flow at 80°C and the water coming back is usually still too warm for the boiler to condense. It will still heat your house perfectly well. It just does it while behaving like a much older, simpler appliance, and the efficiency figure on the badge quietly stops applying.

Three controls that get confused constantly

This is where most of the confusion on the phone comes from, so it is worth separating them plainly.

The heating flow dial sets how hot the water going to the radiators is. On most combis it is marked with a radiator symbol, and on newer boilers it is a menu item on a digital display rather than a physical knob.

The hot water dial sets the temperature of the tap water. On a combi it is a separate control, usually marked with a tap symbol, and it has no effect on your radiators at all. If you have a hot water cylinder instead, the rules are different again — we have covered that in hot water cylinder temperature, and the short version is that a stored cylinder must be kept hot enough to be safe, so do not turn that one down.

The room thermostat on the wall decides when the boiler runs. The flow dial decides how hot the water is while it runs. Turning the flow temperature down does not mean turning the house down — the thermostat still holds the room at whatever you have asked for. It simply gets there more gently.

Does lowering the flow temperature save money?

It lowers the amount of gas the boiler burns to deliver the same heat, because the appliance spends more of its running time condensing rather than throwing heat up the flue. That is a genuine mechanical gain, not a trick.

What we will not do is put a number on it. The saving depends on your boiler, your radiators, how well the house holds heat and how you run the heating, and any figure quoted as though it applied to every home is made up. What we can tell you is that it costs nothing to try, it is reversible in seconds, and if the house still gets warm you have gained something for free.

The trade-off is honest and worth stating: at a lower flow temperature the radiators feel warm rather than hot, and the house takes longer to come up from cold. That suits a home where the heating runs steadily through the day far better than one where it is blasted on for an hour morning and evening.

What temperature should a combi boiler be set to?

Start the heating flow at 60°C and live with it for a few cold days.

If every room reaches the temperature you want in a reasonable time, try 55°C and repeat. Some homes with generously sized radiators will happily sit at 50°C. If one or two rooms never quite get there, put it back up in small steps rather than jumping straight back to 75°C.

Hot water on a combi is a separate matter — that dial wants to stay where it is, high enough for a comfortable shower and a properly hot tap.

There are two situations where the general advice does not apply and you should leave things alone until someone has looked at the system. The first is a heat-only or system boiler with a stored cylinder, where the cylinder has to be able to reach a safe storage temperature. The second is an older, non-condensing boiler, which was never designed to run cool and can suffer from the acidic condensate forming inside a flue that was not built for it. If you are not certain which type you have, that is a good question for us on 01732 792 454.

Why the radiators feel cooler and nothing is wrong

Once the flow comes down, radiators stop being too hot to hold. People take that as a fault and turn everything back up, and we get called out to a system that is working exactly as intended.

A radiator at a lower flow temperature is still putting heat into the room — it is simply doing it over a longer period at a gentler rate, which is a far better match for how a house actually loses heat. The check that matters is not how the radiator feels but whether the room reaches the temperature on the thermostat and holds it.

What is not normal is one radiator staying cold while the rest are fine, or a radiator that is hot at the top and cold at the bottom. Those are faults — air, sludge or a valve problem — and no amount of adjusting the flow temperature will fix them. That is a job for our boiler and heating repairs team.

Getting it set properly

Turning a dial down is easy. Knowing how far you can take it without leaving a back bedroom cold takes knowing the system, and it is the kind of thing our engineers set up as a matter of course while they are already there.

If your boiler is due its annual check, we will look at the flow temperature, the controls and how the system is actually running as part of it — our boiler service is a flat £95+VAT, and you can see exactly what a boiler service includes before you book. Any work on a gas appliance must be carried out by a Gas Safe registered engineer, and we are registered under 951167.

If the boiler is old enough that running it cooler is the wrong answer — and some are — we would rather tell you that than fiddle with a dial. In that case we will quote properly for a new gas boiler installation, in writing, after we have seen the system.

Call us on 01732 792 454 and we will talk it through, whether you want a service booked in or just want to know what your boiler should be set to before the heating goes on daily this autumn.