No hot water unless radiators on

[chemistpm]

I have a large two year old condensing boiler. There is a Drayton controller (about 4 years old) and Honeywell tank thermostat and 2 way motorised valve on cylinder heating circuit (both older).
With CH off on room thermostat, there is no hot water and boiler does not fire.
When CH is turned on with room thermostat the boiler fires and hot water also circulates through storage cylinder heating coil – and gives hot water.
Switch off CH on thermostat with hot water “on” and boiler shuts down.
System has worked correctly for last two years and just gone wrong.
How do I set about working out which component is faulty?

 

[Millsy 82]

It sounds like a motorised valve might not be motoring across do you have 2 valves in the airing cupboard or 1 valve with 3 pipes. Photo would be good.

 

[jtsplumbing]

Same as Millsy, sounds like diverter valve is faulty, photo would be usefull !

 

[chemistpm]

It sounds like a motorised valve might not be motoring across do you have 2 valves in the airing cupboard or 1 valve with 3 pipes. Photo would be good.

Thanks guys
There is one 2 way motorised valve in the airing cupboard on the heating coil incoming side and there is another 2 way motorised valve in the boiler house presumably in the CH line. Two pipes run from pump with this valve in one of them. But remember that water will heat if CH is on. Two photos attached

 

[Millsy 82]

To me it sounds as though the motor in the DHW head is not travelling far enough to call for the boiler to fire up. It needs double checking with a multimeter but thats where I would start.

 

[jtsplumbing]

Faulty valve next to hot water tank, look's quite old, so maybe better of replaceing complete valve, could be motor or valve body that is stuck, on older valves I always replace whole unit, not alot of differance in price.

 

[chemistpm]

To me it sounds as though the motor in the DHW head is not travelling far enough to call for the boiler to fire up. It needs double checking with a multimeter but thats where I would start.

Thanks very much

 

[chemistpm]

Faulty valve next to hot water tank, look's quite old, so maybe better of replaceing complete valve, could be motor or valve body that is stuck, on older valves I always replace whole unit, not alot of differance in price.

Thanks very much

 

[tamz]

Try giving the HW valve a knock with a hammer. The microswitch is either sticking or the arm is not travelling far enough to hit it. If it is sticking a hit will get it going but it will do it again so valve will need changed.

 

[chemistpm]

Thanks i'll try that!