What temperature should I set my boiler to in winter?

If your home feels slow to warm up, or your gas bill has crept up now the heating is back on, the setting worth checking is your boiler's flow temperature. For most condensing combi boilers, a flow temperature of around 60°C keeps the house warm while letting the boiler run at its most efficient. If you have a hot water cylinder, the advice is different, and the stored water needs to stay at 60°C or above.

Below, we explain what each dial does, what to set for your type of system, and how to tell if you've gone too low. Everything here can be done with the controls on the front of the boiler. You never need to remove the boiler's casing to change these settings, and you shouldn't try.

Your thermostat and your boiler do different jobs

Most homes have three temperature settings, and it helps to keep them apart.

• The room thermostat on the wall decides when the heating comes on. It's set to the room temperature you want, usually somewhere between 18°C and 21°C.

• The flow temperature is how hot the boiler heats the water it sends round your radiators. It's usually a dial or button on the boiler marked with a radiator symbol.

• The hot water temperature is how hot the water at your taps and shower is. On a combi boiler it's a separate dial, usually marked with a tap symbol.

Turning the thermostat up won't warm the house faster. It just keeps the boiler going for longer. The flow temperature decides how hard it works while it's on.

Why a lower flow temperature can save gas

Most gas boilers fitted since 2005 are condensing boilers. A condensing boiler recovers extra heat from its exhaust gases, but it can only do that well when the water coming back from the radiators is cool enough, roughly below 55°C. Many boilers are left at a flow temperature of 70°C to 80°C, which keeps the returning water too hot for the boiler to condense properly.

Research for Nesta's Money Saving Boiler Challenge found that lowering a combi boiler's flow temperature from 80°C to 60°C could save around 9% of a household's gas bill. In controlled tests at the University of Salford, the same change cut the gas used for heating by 12%, with a trade-off: the house took longer to warm up in the morning, 37 minutes instead of 23.

What to set if you have a combi boiler

A combi boiler heats water as you use it, with no tank or cylinder. For most combis, these are good starting points:

• Heating (flow) temperature: 60°C. If your rooms still get warm enough, you can try going a little lower.

• Hot water temperature: as low as still feels comfortable in the shower. A combi doesn't store hot water, so a lower setting here is about comfort and cost, not safety.

With cooler radiators, you may want the heating to come on a little earlier on cold mornings. That's normal and still uses less gas overall.

If you have a hot water cylinder, keep it at 60°C

If your hot water is stored in a cylinder, usually in an airing cupboard, you have a system or regular (heat-only) boiler. Here the stored water must be kept at 60°C or above. Water stored at lukewarm temperatures can let legionella bacteria grow, and these can cause Legionnaires' disease, a serious lung infection, when breathed in as fine droplets from a shower.

The Health and Safety Executive's guidance is that hot water cylinders should store water at 60°C or higher. That's why Nesta's advice is aimed at combi boilers. On a cylinder system, the boiler's flow temperature also has to be high enough to heat the cylinder properly. So if you have a cylinder, don't turn anything down yourself. Ask a heating engineer to look at your settings, as there may still be savings to make without putting your hot water at risk.

How to change the setting safely

1. Check your boiler's user manual, or look up the model online, to find the heating dial or button.

2. Note the current setting, so you can put it back if you need to.

3. Turn the heating dial down to 60°C, or to the nearest mark if your dial has numbers or symbols rather than degrees.

4. Live with it for a few days, ideally including a cold one, before deciding whether it's right.

Never open the casing, adjust anything inside the boiler or touch the gas valve. If you smell gas at any point, turn off the gas at the meter, open windows and call the National Gas Emergency Service on 0800 111 999.

Signs you've gone too low

A lower flow temperature should leave rooms just as warm, only more slowly. It's set too low, or something else needs looking at, if:

• rooms never reach the temperature set on the thermostat

• the boiler runs for hours without the thermostat switching it off

• every radiator feels only lukewarm, even the one nearest the boiler

• some rooms are warm and others stay cold

If only some rooms are struggling, the radiators may need balancing, so the hot water reaches each one fairly. Our guide on how to balance radiators for even heat explains how. If the whole house struggles at 60°C, try 65°C, and if that's still not enough, a radiator may be too small for its room or the system may have a circulation problem.