Heat pumps are a key part of the government's promise to reach net zero in the UK by 2050, meaning no more greenhouse gas would be added to the atmosphere. Home heating currently accounts for 14% of the UK's carbon emissions.
The data published on Thursday on grants issued since the scheme started in May 2022, comes a day after the UK's climate watchdog said the government was "significantly off track". The scheme lasts until 2028.
The UK's Climate Change Committee (UKCCC) highlighted high heat pump cost, the low number of trained heat pump installers and the lack of energy efficiency measures like insulation, which help to improve heat pump efficiency, as the main reasons for low installation.
The £70 million left over due to grants not being issued last year cannot be used for grants in future years and will now be returned to the Treasury.
A spokesperson for the Department of Energy Security and Net Zero, which is responsible for the scheme, said: "Industry has reported an increased level of enquiries and we are confident that deployment will increase as the year progresses."
In February, the Lords Committee raised concerns that the scheme was underperforming. The government responded by saying it would launch a marketing campaign to promote the scheme.
It said that in the first 14 days of running the campaign there was a 62% increase in clicks to the gov.uk page compared to the previous 14 days. But the government's own figures show that the number of vouchers given out has dropped since March. The government intends to undertake wider promotion later this year, it said.
To meet the UK's climate change targets, the government wants to install 600,000 low-carbon heat pumps annually within five years - currently rates are around one-ninth of this.
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