Hot Water Firing Boiler yet Timer is OFF

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I've got my excellent local plumber coming round asap but I was interested in any solutions you guys might suggest.

This is a very intermittent error as its been 2 weeks since it last did it then stopped. My guy changed the HW 2 port valve in October when the fault was fairly solid. It seemed to cure it but as I say its returned, every now and then.

On the timer the hot water is not on and even if I turn the timer for HW to OFF for the boiler still fires. Oddly if I turn the boiler thermostat down it stops and again if I turn the boiler thermostat up to max the boiler fires up again. Even though on the surface there is no demand for hot water. The valve is in the home/closed position and has resistance and returns to home if I try and open it so I guess not calling. But in any case the timer is "off". Radiator in the bathroom is "red" hot after a while of hot water heating when there is no demand from the timer.

I've just turned the whole system off at the mains to stop it overnight. Faulty micro switch in the Honeywell HW valve ?

Any thoughts appreciated
 
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Have you a heating zone valve as well ? If he calls when the fault is occurring it sh be easy enough to find,
Possibly reason’s frost stat ,zone valve end switch sticking
 
As per @cross thread, do you have a heating valve/3 port valve? No rads should be heating unless it’s been piped wrong, a failing 3 port or a faulty 2 port on heating.
 
Yes, sorry, it’s an S plan and I have a HW valve as well. It’s an unvented system with an old fashioned, but brilliant, Potterton Profile boiler.

Rads do not get hot just the hot water system.
 
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Chris

The rad in the bathroom is directly connected to the hot water. I should have said towel rail :D
 
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