my Mum's two year old combi stops heating the water while she's running a
> bath, she says it's been like that since she moved into the house a few
> months ago. It heats water ok most of the time but when she's running a
> bath it stops heating the water after it is 1/3 to 1/2 full. I wondered if
> anyone had any ideas they could maybe suggest.
Sunds like a high limit cut out is being activated. Get Ariston, in High Wycome to do a full service.
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They will give you the name of their local service contractor.