16% of the UK’s carbon emissions come directly from heating homes with fossil fuels. The average UK gas boiler emits more CO2-equivalent emissions in a year than taking seven transatlantic flights.

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 from home heating. But we can’t install heat pumps across the country overnight.

That’s why we investigated 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 explored 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.

In 2005, condensing boilers were mandated in the UK. Condensing boilers are more efficient than traditional boilers because they capture and reuse energy that would otherwise be lost in gases that escape from the flue.

In order for these gases to condense, which allows the energy to be reused, the water returning to the boiler has to be below a certain temperature (around 54°C). The lower the flow temperature, the lower the return temperature will be, and the more efficiently the boiler will run.

Most condensing boilers in use across the UK are set with flow temperatures of 70-80°C, which is too high for them to operate at their maximum efficiency. A boiler with a high flow temperature will also switch on and off more often when in use. This is called cycling, and increases wear and tear on the system.

Our research found that if these boilers were optimised, households could reduce their gas use, costs and emissions by 8%.

This may not seem like a big reduction at first glance, but it’s comparatively large for a small and cost-free change. It will also help prepare households for the switch to heat pumps, which run at lower flow temperatures.

We looked at ways to design a scalable intervention to 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 was effective and could 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.

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 – our national consumer-facing campaign to raise awareness of the savings of reducing boiler flow, and direct consumers to the online tool.

We estimate approximately ~3.1 million households turned down their boiler flow temperatures over the course of the campaign.

The campaign reached more than 9.7 million (68%) of our target audience and 31 million people across the Great Britain adult population.

54% of those who recognised the adverts took or intend to take an action.

We also know that through our radio campaign, and ours and others’ influence on government and energy companies, the evidence and advice we created reached many more people through other channels (such as Martin Lewis’s weekly newsletters and the Government’s It All Adds Up campaign) – generating a much greater scale of impact.

Nesta commissioned pre, mid and post-campaign tracking through an external campaign tracking agency which surveyed a representative sample of more than 2,000 GB adults in each survey. In early October 2022, 21% of people said they had lowered their boiler flow temperature in the past. This rose to 32% when we re-ran our survey in March 2023. We estimate this equates to approximately ~3.1 million households turning down their boiler flow temperatures over the course of the campaign – saving an estimated 500,000 tonnes in CO2 and nearly £300 million in savings for households and £157 million for HM Treasury (by reducing spending on the Energy Price Guarantee).

These figures are based on carbon and cash savings for the past year alone; we believe that the continued impact of this work could, over the next 5-10 years, save several billion pounds depending on gas prices and how many consumers revert their boiler settings.

How many people used the online tool? 

Whilst millions of people were made aware of the advice and went on to take action during the course of the campaign, we are also able to quantify the numbers who specifically used our online tool.  

Between October 2022 and March 2023, more than 214,000 people used our online tool and told us that they had turned down their boiler flow temperature. We estimate this saved around £20 million off energy bills and 37,000 tonnes of CO2 per year (with an average saving of £96 and 173kg CO2 emissions per household).

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 also looked 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 identified a range of new ways these interventions might be achieved and how best to continue this work.

We have since developed the home heating advice generator – a tool to help gas engineers give bespoke energy efficiency and heating advice to their customers.