Yesterday afternoon my mother noticed a problem with her hot water - it
wasn't hot, it was stone cold. Soon after that the boiler came on and
the heating and hot water were fine (note: whenever her boiler comes on
the heating and hot water heat simultaneously - it's one of those
boilers where you can't ONLY turn on the heating (manually or via the
timer) instead you need to turn on the hot water first THEN the heating
a split second later)).

All was well this morning, but she's been out during the day and came
back in to find the hot water is stone cold again.

This is not normal - there is usually some hot water left in the tank
and, as mentioned, this only started yesterday.

I can't check it myself as I live a few hours drive away.

No taps are on or dripping, there's apparently no sign of any damp
patches on the ceiling.

My gut instinct says it's perhaps a leak underneath the floor somewhere
(she lives in a house with a ground and upper floor, plus an attic).
But she hasn't had any really really hard frosts in her area (unlike
some) and even if she had I can't imagine pipes with warm water in them
freezing that easily.

But what else could it be? It's rather worrying.

Thanks
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