Should I turn my boiler’s flow temperature down?

Your boiler flow temperature is the temperature your boiler heats your water to before it gets sent to your radiators.  

As it flows through the radiators, the water cools before it returns to the boiler. The temperature of the water at this point is the return temperature.   

Reducing the flow temperature also lowers the return temperature. Boilers are more efficient when the return temperature is low, and this can save you both energy and money. For more information, see our guide on boiler controls.

Yes, your boiler controls are separate from your central heating controls, such as your room thermostat, programmer and radiator valves. The central heating controls turn your heating on and off, while the boiler thermostat changes the temperature of the water in the system. 

It’s important to get your heating controls right before you try to change the flow temperature.  

If you have a conventional boiler with a hot water cylinder, there’ll be just one dial or setting to adjust.  

If you have a combi boiler with no hot water cylinder, there’ll be two dials or settings – one for the radiators and one for hot water. You need to adjust the radiator setting, which you may see as by an icon of a radiator rather than a tap.

For more information on the different types of boilers, see our boiler guide.

Before you make any adjustments, it’s a good idea to take a photo or make a note of how everything’s currently set. That way you’ll know what to turn it back to if you need to.

The easiest option is to turn the boiler flow temperature down to a level that’s just warm enough and leave it there.

This option assumes that the installer probably set the boiler thermostat to maximum when they fitted the boiler, and no one has adjusted it since. If that’s the case, the flow temperature will be too high for most homes and heating systems, so you can probably turn it down. 

Try turning the boiler’s flow temperature down to 65 degrees. Don’t set it any lower than this or the boiler won’t be able to heat your hot water cylinder to a safe temperature.

The temperature must be at least 65 degrees to keep your hot water supply to your taps safe. Otherwise there’s a risk of legionella bacteria growing in the cylinder. 

You can turn the radiator’s flow temperature down as much as you like without affecting the hot water. Some people suggest that turning the radiator flow down to 60 degrees is reasonable. But you’ll have to find what works for you by trial and error.

Bear in mind there’s a risk of the house getting too cold while you find the right temperature.  

If you don’t want to keep your boiler flow temperature down, you can adjust it periodically or as the weather changes.

If the weather is mild and you have a combi boiler, you’re more likely to be able to heat your home with a lower flow temperature, maybe 50 or 55 degrees. When it’s very cold, you’ll want to set it a bit higher.

If you have a conventional boiler, you can’t go below 65 degrees, so it’s unlikely you’ll be able to do this.  

This isn’t something you can do on your own. You might be able to get a heating engineer to fit a weather compensation kit or a load compensator, which will change the flow temperature for you. You can only do this if you have a combi boiler.

The engineer will still need to check if your boiler is compatible, and you’ll have to pay to have it fitted.  

Doing this will give you the maximum possible savings from adjusting the temperature of your boiler. It should also minimise the risk of your house not getting warm enough.  

If you already have weather compensation and think it needs adjusting, speak to your installer.

We’ve seen claims that average gas bills could go down by as much as 9% by turning your boiler flow temperature down. However, consider these claims in context.

Research from Nesta shows that turning down the flow temperature to 60 degrees improves your boiler’s efficiency by nearly 4%.

You may find that your gas use goes down more than this, but that will be because of other factors.

For example, your house may be heating up more slowly, or rooms may not be getting quite as warm anymore.

How much you’ll save will depend on several factors including:

• what your boiler was set to before you made the change
• whether your home was too warm before
• how much you’re prepared to experiment with your boiler’s flow temperature

• The house, or some rooms in it, might not get warm enough. Every home and heating system is different, so you’ll need to adjust your settings until you’re comfortable.
• If the house gets too cold it could be dangerous for older residents, people with health conditions and anyone living in a home that’s already underheated.
• Turning the boiler thermostat below 65 degrees on a conventional boiler (with a hot water cylinder) can increase the risk of legionella bacteria growing in the cylinder, which is dangerous.
• If the heating controls aren’t set correctly when you adjust the boiler thermostat, you might not get the results you expected.
• You might adjust the wrong settings on your boiler by accident.

Sign up to Energywire, our monthly newsletter full of news, advice and tips on energy saving and renewable energy options.

Sign up to our newsletter