Boiler flow temperature: should you turn it down?

The short answer

Yes, for many homes it’s worth trying. Your boiler’s flow temperature is the temperature of the water it sends round the radiators. On a combi, the central heating flow temperature is usually set separately from the hot water temperature, with its own dial or menu setting. Many are left high, and lowering it lets a condensing boiler run more efficiently, which means using less gas for the same warmth.

How low you can go depends on your radiators and your home. Around 60°C or lower is commonly suggested as a starting point. If the rooms still get warm enough, leave it there. If they don’t, turn it back up a little. We can’t give you a figure for the saving because it varies from house to house, but the change costs nothing to try.

Why a lower temperature saves gas

Almost every gas boiler fitted today is a condensing boiler. Burning gas produces water vapour in the flue gases, and that vapour carries heat. A condensing boiler cools the flue gases enough for the vapour to turn back into water, recovering heat that an older boiler would send out of the flue. That water is the condensate that runs down the small plastic pipe from the boiler to a drain.

The catch is that condensing only happens properly when the water returning from the radiators is cool enough, roughly below 55°C. If the flow temperature is set very high, the return water is often too hot for the boiler to condense much at all, and it runs less efficiently than it was designed to.

Turning the flow temperature down means the radiators run cooler and the water coming back is cooler, so the boiler spends more of its time condensing. The radiators feel warm rather than scorching, and the rooms may take a bit longer to heat up.

How to find the setting on your boiler

Every boiler is different, so check your manual or the manufacturer’s website for your model. As a rough guide:

• On many combis there are 2 controls on the front: one marked with a radiator symbol for heating, one with a tap symbol for hot water. The radiator one sets the flow temperature.
• Some boilers use a digital display and buttons or a menu instead of dials.
• Some smart thermostats and controls let you set the flow temperature from the app, or manage it for you.
• Some boilers show a number in °C, others a scale or a row of bars. The manual tells you what each step means.

Only use the controls on the front of the boiler or your thermostat. Never open the casing or adjust anything inside the boiler. That is work for a Gas Safe engineer.

How to try it

1. 1Note the current setting so you can go back to it.
2. 2Turn the central heating flow temperature down, towards 60°C or the nearest setting your boiler shows.
3. 3Live with it for a few days, ideally including a cold one. Leave the room thermostat where it was.
4. 4If the house warms up fine, you can try a little lower. If rooms stay chilly, turn it back up a step.

Expect the heating to take a bit longer to warm the house from cold, so you may want it to come on a little earlier. If someone at home is elderly, very young or unwell, make sure the house still gets warm enough. Comfort and health matter more than the saving.

Hot water and cylinders: keep it at 60°C

The advice above is about the central heating side. Hot water is different, especially if you have a cylinder.

If your boiler heats a hot water cylinder, as system and regular boilers do, the stored water must stay hot enough to control bacteria such as legionella. Keep a cylinder at 60°C. Don’t turn the cylinder thermostat below that, and don’t lower the boiler’s settings so far that the cylinder can no longer reach it. If you’re not sure how your system is set up, ask your engineer at the next boiler service.

A combi doesn’t store hot water. It heats water as you use it, so the hot water setting mostly changes how hot the taps run. Follow the manual if you change it, and keep in mind that very hot water at the tap is a scald risk for children and older people.

Controls that do it for you

Some controls adjust the flow temperature automatically. Weather compensation uses an outdoor sensor to lower the flow temperature on milder days. Load compensation adjusts it depending on how close the room is to the temperature you’ve set. Both help a boiler condense more of the time without you touching a dial.

Under the Boiler Plus rules in England, a new combi must be fitted with one of a short list of extra energy-saving measures, and weather compensation, load compensation and smart controls with automation and optimisation are among them. If you’re having a new combi boiler installed, ask which controls suit your home. We can also fit smart thermostats to existing boilers where they are compatible.

When to call an engineer

Turning a dial down is safe. Anything else needs an engineer. Call us if the boiler can’t keep the house warm at any setting, if it keeps cycling on and off, or if radiators stay cold in places. An annual service is a good time to ask the engineer to check your flow temperature and controls are set sensibly for your home.