Check - when the water runs cold, is the main burner off ?
Especially with the age of the boiler there could be a problem/blockage in the heat exchanger, if the heat is not getting from the boiler to the water fast enough the water will keep running cold when the burner cuts out - as the water in the boiler will be at temperature.
Hi htg engineer
Yes, main burner is off when water runs cold, I can turn on tap and watch as burner comes on, hot water, then burner cuts off, cold water!
we live in a soft water area, no problems with limescale in kettle etc
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