When Bill Tuck first started installing air source heat pumps six years ago, other heating engineers were skeptical about whether the green technology would take off. “Now, the conversation is completely different,” he said.

“I’m getting approached by them saying: ‘Mrs Jones, my customer for 15 years, she’s enquiring about a heat pump. What can you tell me?’ The whole narrative has changed.”

But despite a surge in interest in cheap and sustainable ways to warm homes, the government is understood to be slashing the amount of money available to subsidise heat pumps as it looks for ways to cut spending in Wednesday’s budget.

Businesses that install the renewable systems are fearing the worst. “It’s going to damage us as a company,” said Tuck, the operations director at Sustain Homes in Watford. About 75% of his customers pay for their pump using the government subsidies. “It’s the foundation of the business.…