15% of the UK’s carbon emissions come from heating homes with fossil fuels. The average UK gas boiler emits more CO2-equivalent emissions in a year than taking seven transatlantic flights. Unlike the emissions from the electricity that we use to power our lights and appliances, emissions from home heating haven't lowered much over the last 20 years.

It’s clear we need to decarbonise the way we heat our homes if we want to meet the UK’s net-zero target. Heat pumps are the most effective way of reducing emissions. But we can’t install heat pumps across the country overnight.

That’s why we’re investigating how to make it easier for households to increase the efficiency of their existing boilers, enabling us to start lowering household-heating emissions before boilers are replaced with greener technology.

We’re exploring the barriers to, and impacts of, households lowering the flow temperature of their condensing combi boilers – the temperature to which boilers heat the water that gets sent to our radiators.

Too many condensing combi boilers in the UK aren't operating to their maximum efficiency. They’re burning more gas, generating more emissions and costing households more than they need to.

These boilers should be set to a flow temperature of 60°C or lower to increase the chances of them running at their optimum 95% efficiency. However, many are currently installed to have a flow temperature of between 70-80°C.

If these boilers were optimised, households could reduce their gas use, costs and emissions by 6-8%.

This may not seem like a big reduction at first glance, but it’s comparatively large for a small and low-cost change. It will also prepare households for the switch to heat pumps, which closely match the heating style associated with lower flow temperatures.

Although the solution seems simple, there are many barriers in the way. Even though only a small number of very poorly insulated homes may not be as suitable for lower flow temperatures, engineers may feel safer setting up all boilers in a uniform, but inefficient, way to guarantee enough warmth and comfort regardless of property type. For this same reason boiler manuals often suggest using higher flow temperatures, and many installers and households understandably use this as guidance when configuring the boiler. Finally, many people are also cautious about changing boiler settings themselves and fearful of something going wrong.

We looked at ways to design a scalable intervention to tackle the barriers, raise awareness and support households to reduce the flow temperature of their boilers.

We worked with key stakeholders such as boiler companies, consumer advice organisations, energy companies and the UK government to ensure that our intervention is designed so that it stands up to scrutiny and can be easily scaled.

We started the project by designing a prototype intervention, informed by interviews with industry stakeholders and a small number of households. This took the form of a web tool that gives tailored instructions on how to change boiler settings.

We tested different versions of this web tool, culminating in a larger-scale experiment where it was emailed to 3,500 users of an energy advice app. We looked to answer a range of questions through this research, including:

• Does receiving this web tool result in more households turning down their boiler flow temperature ?
• Do households that turn down their flow temperature use less energy?
• Are homes still warm and comfortable enough for the household when they maintain their boiler with a reduced flow?

The initial results from this trial were positive. We also conducted experiments and modelling to better understand the interaction between lower flow temperatures, energy use, and household comfort. The next phase was to use these insights to develop an intervention to lower household emissions that could be scaled across the UK.

As well as testing the online tool with customers of the energy-saving app we undertook a wide-ranging research phase to give us a solid evidence base to recommend lowering flow temperatures. This included measuring the impact of lowering flow temperatures in the Energy House at Salford University; using modelling work by Cambridge Architectural Research to estimate the number of UK homes that could take this action; and working with Energy Systems Catapult to measure savings and impact on room temperatures on different types of homes when changing flow temperature.

Read our evidence summary to find out more.

The next phase was to use these insights to develop an intervention to lower household emissions that could be scaled across the UK. This resulted in the launch of the Money Saving Boiler Challenge.

There are additional boiler optimisation measures people could adopt, such as lowering a combi boiler’s hot water temperature or turning off the preheat function. As well as investigating these measures we are also looking at some other ideas.

– Supporting gas engineers, advisers and volunteers to optimise existing systems on behalf of consumers. This could include households that are digitally excluded and have not been able to access online advice or those that might need additional in-person or phone support from a gas engineer, adviser or volunteer.

– Ensuring that new heating systems are designed, installed and set up to maximise efficiency from the moment they are installed, rather than having to do it retrospectively.

We are aiming to identify and test a range of new ways these interventions might be achieved and make recommendations on the highest impact routes so we can decide how best to continue this work.