How to optimise your combi boiler flow temperature and reduce gas bill

It's winter and that means your boiler will be working harder than normal. Follow some of our tips on optimising your combi boiler to keep it working efficiently. By making a little change to your combi boiler, you can reduce your gas usage by up to 9% to save money while keeping your home just as warm.⁽¹⁾

1. What is boiler flow temperature and how can I reduce my energy usage?

Your heating flow temperature handles the temperature to which your boiler heats water before sending it to your radiators.

When your boiler is installed, the flow temperature is usually set to around 75°C-80°C. This is too high for most combi boilers to run at maximum efficiency.⁽²⁾

At lower flow temperatures, the combi boiler is in condensing mode more often. This means it can capture more heat and recycle it back into the system.

Lower flow temperatures result in a more efficient combi boiler. One that uses less gas to heat your home to the same temperature, saving you money and reducing your carbon footprint.

2. How much can I save by reducing the heating flow temperature on my combi boiler?

Research from the Salford Energy House found that lowering your heating flow temperature resulted in gas usage savings of:

up to 9% by lowering the temperature from 80°C to 60°C
up to 12% by lowering the temperature from 80°C to 55°C

People who may be more vulnerable to the cold should be careful when reducing the flow temperature. For example, if you're elderly or have underlying health conditions. Innovation charity Nesta recommends not turning it below 60°C to ensure homes are properly heated.

The average household could save around £65 per year on their heating bill by lowering their boiler flow temperature.* Heating homes with fossil fuels accounts for 15% of the UK’s carbon emissions. So, by using energy more efficiently, we can cut carbon emissions and save money on our heating bills at the same time *Based on energy prices as of September 2023.⁽³⁾

All without turning down the thermostat.

3. Do I have the right type of boiler to reduce my flow temperature?

To change your heating flow temperature safely yourself, you must have a condensing combi boiler.

A combi (or combination) boiler sends hot water on demand to your radiators and hot water taps. The water isn't stored in a cylinder or storage tank.

Our boiler guide explains more about the different types of boilers. You'll need to ensure you have the right type.

If you have a system boiler or a hot water cylinder, please don't change your flow temperature on your own. It's important to have a qualified professional check to see if your system is appropriate and safe.

4. I have a hot water tank. Why should I not change the flow temperature?

f you have a water cylinder/tank, changing the flow temperature may not be right for your system.

The water stored in the cylinder needs to be kept at 60°C or higher. This is to reduce the risk of harmful legionella bacteria growing⁽⁴⁾. Bacteria can grow in the cylinder if the flow temperature is set too low.

The temperature around your heating system can vary and drop. Don't change the flow temperature on your own without a qualified professional confirming it's right for your system.

This isn't a risk within heating systems that don't have water cylinders, as hot water is not stored in a separate tank.

5. What temperature should a combi boiler be set at?

The lower you set your flow temperature, the longer it will take to heat your home.

It may take some trial and error to find the right flow temperature for your home. Innovation charity Nesta suggests a flow temperature of 60°C or slightly lower if your home is particularly well insulated.

What temperature should my boiler be set at in winter?

If you or anyone in your household is more vulnerable to the cold (for example the elderly or those with underlying health conditions) please don't set your flow temperature below 60°C. This should ensure your home heats up within a suitable time when your radiators initially come on.

The best flow temperature for your home depends on a few things: how well-insulated your home is, its size, and how quickly you want to heat it. It's about finding the best balance between saving energy and heating your home quickly.

6. How do I set the flow temperature on my combi boiler?

You control the flow temperature directly on your boiler. It doesn't take long at all and can be changed back at any time.

The Money Saving Boiler Challenge is a campaign launched by the innovation charity Nesta. To help people set their flow temperature, Nesta has an interactive online tool that offers help depending on your combi boiler display type.

If you're unsure, your combi boiler's user manual, which can be found on FreeBoilerManuals, will have extra guidance. If you're still unsure, seek help from a qualified and competent professional.

7. Will this change how warm my home is?

Changing the flow temperature to the Nesta recommended temperature of 60°C shouldn't change the temperature of your home.⁽⁵⁾

However, by reducing the temperature of the water being delivered to your radiators:

Your radiators will feel slightly cooler to the touch.
Heating your home when the radiators initially come on will take slightly longer.

Only when you set the flow temperature too low will your heating system struggle to get your home temperature up to the level on your thermostat.

In the Boiler Flow Temperature Report heat-up times went from 23 minutes to 37 minutes after reducing the flow temperature from 80℃ to 60℃. Therefore, it took an extra 14 minutes to get the home up to the thermostat temperature of 21℃ when the radiators initially came on.

The heat-up time took an extra 33 minutes after reducing the flow temperature from 80℃ to 55℃. The exact heat-up times will depend on your home and thermostat temperature.

8. Should everyone reduce their flow temperature?

We know how important keeping your home warm is.

Lowering the flow temperature on your combi boiler can be a great way to reduce your energy usage without turning down the thermostat.

However, reducing the flow temperature may not be appropriate for everyone. If you're worried about heating your home quickly, it may not be right for you. If you're more vulnerable to the cold, having a higher flow temperature can help get your home up to the correct temperature more quickly.

And don't worry, there are plenty of other ways you can make your home energy efficient this winter.

9. What can I do if my boiler is old or hasn't been serviced?

A reliable boiler is essential for keeping your home warm and your energy bills under control. That’s why annual boiler servicing is so essential, it helps prevent unexpected breakdowns, keeps things running efficiently, and ensures your home stays safe.

If your boiler is showing its age - think rising bills, unusual noises, or frequent callouts - it might be time to upgrade. Newer models are more energy-efficient, helping to cut costs and reduce your carbon footprint.

Or, if you’re replacing your boiler altogether, why not think about an air source heat pump? Acting like an outdoor boiler, it captures heat from the outside and brings it into your home using electricity. These systems are incredibly efficient - over 300% more than standard gas boilers and can help you save on heating bills while lowering your carbon emissions. Ready to make the change? You can also enjoy the £7,500 government grant.

Find out more

10. What else can I do to save energy and lower my bills?

To help, we can give you a smart meter and access to our free Energy Hub. This will help you understand how much energy you use.

Cavity wall and loft insulation can save you up to £790 a year on your energy bills ⁽⁶⁾.

You can also read our tips on energy saving at home. Some of these are easy to do and completely free.

Notes

The Energy Savings Trust found that overall gas consumption savings could go down by as much as 9%. But this depends on various factors. (Those turning down to 60°C should expect to see 4%). This aligns with other estimations.
Which? found that many boilers are set to 75-80°C. Lowering the temperature to 60-65°C would mean that your boiler is using less gas and operating more efficiently.
Estimated gas bill savings for the UK typical medium household are based on a flow temperature reduction of 80°C to 60°C in the Salford Energy House Boiler Flow Temperature Report and with Ofgem's typical Uk household usage figures of 11,500 kWh of annual gas consumption.
The Health and Safety Executive Guidance is that hot water should be stored at least at 60°C.
The Salford Energy House Boiler Flow Temperature Report's model house found reducing the flow temperature on their combi boiler down to 60℃, 55℃ and 50℃ down from 80℃ did not change the room temperature from the desired 21℃ on the thermostat once the room heated up to that temperature.
Estimates are based on insulating a gas-heated detached home with a totally uninsulated loft (0mm) with 270mm of loft insulation. The recommended depth of mineral wool insulation is 270mm, but other materials need different depths. The average professional installation cost is unsubsidised, but prices will vary. Figures are based on fuel prices as of July 2025. Source: Energy Saving Trust for roof and loft insulation and cavity wall insulation.

How to optimise your combi boiler and reduce energy usage