Engineers at British Gas are starting the first of five strikes in protest at the closure of the company's final salary pension scheme to new entrants.
Contingency plans are in place to provide coverage during strikes
The engineers who repair household boilers are holding a 24-hour strike, with further ones planned for 19 and 21 December, and 6 and 9 January.
A total of 6,000 workers, members of the GMB union, are taking part.
At the weekend British Gas management said they were "disappointed and frustrated" after a breakdown in talks.
The company said it had contingency plans to prioritise the needs of vulnerable people who might need urgent help.
GMB national secretary Brian Strutton said members had "fully co-operated" to ensure the contingency plans would work.
The union said it had asked the company, which trades as Scottish Gas in Scotland, to suspend closing the pension scheme to new entrants for a year while the scheme was re-evaluated.
However, British Gas's parent company Centrica has said its new pension package is one of the best on the market.
It said the long-term viability of the final salary scheme, to be closed to new entrants from January, depended on the changes.