Should I turn my boiler’s flow temperature down?

The boiler thermostat is a control on your boiler that adjusts the flow temperature, which is the temperature of the water it sends to the radiators.  

The water cools as it flows through the radiators 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. All boilers are more efficient when the return temperature is low, and this can save you energy and money. There’s more information about this on our advice page.

In this blog, we’ll look at what your options are when it comes to turning down your boiler’s flow temperature and some things you might want to consider.  

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 first before you try to adjust the boiler’s flow temperature with your boiler’s thermostat.  

Your boiler will have either a dial to control the flow temperature or some buttons and a digital display. You’ll probably need to open a flap on the boiler to access this – don’t worry, you’re allowed to open the flap.   

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

• If you have a combi boiler with no hot water cylinder, then there will be two dials or settings: one for the radiators and one for the hot water. You need to adjust the one with a picture of a radiator next to it, not the one with a picture of a tap, which is for your hot water. 

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

You don’t have to turn down your boiler’s flow temperature if you don’t want to. But if you decide to do it, these are your options depending on the type of boiler you have.  

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, then the flow temperature will be too high for most homes and heating systems, so you can probably turn it down. 

• Conventional boiler with hot water cylinder 

  If you have a conventional boiler with a hot water cylinder, then you could 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 60 degrees. The temperature must be no lower than 65 degrees to keep your hot water supply to your taps safe, otherwise there’s a risk of legionella bacteria growing in the cylinder. 

• Combi boiler with no hot water cylinder 

  If you have a combi boiler with no hot water 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.  

This means going a step further and working out different temperature settings depending on the weather.  

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. If 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 approach will save you more money than option one, but there’s more of a risk that you’ll have a cold house at times.  

If you live with other people, it’s a good idea to make sure they also know how to adjust the flow temperature.

This isn’t something you can do on your own. You might be able to get a heating engineer to fit a weather compensator 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 and should minimise the risk of your house not getting warm enough.  

If you already have weather or load compensation and think it needs adjusting, you can speak to your installer for guidance.

• The house, or some rooms in it, might not get warm enough if you turn your flow temperature down. Every home and heating system is different, so you won’t be able to work out if this will happen in your home based on general advice.
• If the house gets too cold it could be dangerous for elderly 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, it won’t lead to the expected outcome.
• You might adjust the wrong settings on your boiler by accident.

We’ve seen several claims for average gas bill savings by reducing the flow temperature on a combi boiler. These range from an 8% to a 13% saving. However, these claims need to be considered in context.

Research shows that turning down the flow temperature improves the boiler efficiency by around 4% to 5%. You may find that your gas use goes down more than this, but that will be because other things are going on that you might not expect or want. 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.
• Whether your home was too warm before.
• How much you’re prepared to experiment with your boiler’s flow temperature.

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