Every household must be engaged by the government in the shift to clean heating as uptake of heat pumps to replace boilers is running at less than half of expected levels, the public spending watchdog has warned.

A report by the National Audit Office (NAO) described assumptions on consumer demand for heat pumps, which use electricity to draw heat from the ground, air or water for heating buildings, as “optimistic”.

It also called into question public awareness of the availability of boiler upgrade grants to help smooth the transition from oil and gas-fired boilers amid the battle against climate change.

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The home energy sector accounts for around 18% of the country’s overall greenhouse gas emissions.

The NAO said that just 18,900 of the clean heating units were installed under the grant scheme from May 2022 to December 2023 – less than half the up to 50,000 expected by that point.

Since the grant was increased from £5,000 to £7,500 in September, the number of heat pumps being installed had risen, it added, but the study also reported doubts on whether the increase would be sustained.

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The report identified two barriers to increased installations, including poor awareness.

It said cost also remained a factor, despite aid from the grant, as heat pumps are four times more expensive than gas boilers despite far greater efficiency.

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The government has ambitions for the installation of 600,000 heat pumps a year by 2028.

Sky News has reported apparent divisions among ministers in the Department for Energy Security and Net Zero (DESNZ) on efforts to meet the deadline.

A mechanism, designed to punish boiler manufacturers who fail to meet electric heat pump sales, has resulted in an ugly clash that saw the companies raise prices to offset potential fines for missed installation targets.

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It was confirmed last week that Energy Secretary Claire Coutinho had asked the Competition and Markets Authority (CMA) to investigate the home heating market.

The NAO suggested that a decision on the role that hydrogen could play in future in reducing emissions should come before the government’s planned deadline of 2026.

Gareth Davies, head of the NAO, said: “Government needs to engage every household to achieve its objective to decarbonise home heating as part of the transition to net zero.

“DESNZ’s progress in making households aware and encouraging them to switch to low-carbon alternatives has been slower than expected.”

A DESNZ spokesperson said: “By helping rather than forcing families to install heat pumps, with a 50% bigger heat pump grant, we have boosted applications by nearly 40%.

“Our Welcome Home to Energy Efficiency campaign is running on TV, radio and newspapers, reaching 16.6 million households with advice and information about how heat pumps, insulation and solar panels can cut their emissions and energy bills.”