Five hundred homes in York were left without water after a series of mains burst across the city on the hottest day of the year so far.
Engineers restored supplies to "most" affected homes by midday
A pressure surge at a pumping station caused 11 mains fractures in the early hours of Tuesday, Yorkshire Water said.
Several homes were flooded and one school, Westfield Primary, closed for the day because it had no water supply.
Engineers restored supplies to "most" affected homes by midday while others were reconnected later in the evening.
Repairs were prioritised, with those threatening damage and affecting large numbers of properties being looked at first.
Firefighters pumped water from flooded properties in Gale Lane
Meanwhile, the company distributed bottled water to residents in the affected areas.
Yorkshire Water spokesman Mark Penny said a power failure at a service reservoir had led to the system becoming over pressurised which caused the mains to burst.
"We've got to have a thorough investigation to find out what's gone on," he said.
"We've installed a temporary power supply at the service reservoir today but longer term we're looking at how we can remove that particular asset from the operation and put York on a completely pump-operated system to avoid this happening again."
Sally Hutchinson, from Age Concern, advised older people to get in touch with the organisation if they needed bottled water.
"We have an extreme weather team who can help people if they have a problem," she said.