No hot water unless heating is on(Alpha 28C)

[several]

Since the weather's dropped we can't get hot water unless I turn the heating on. Boiler is Alpha 28c, I have a rad off in the living room while decorating, I don't know if that matters, it's capped off both ends.

We have an expansion vessel connected to the boiler because in previous winter's, the pressure kept dropping and constantly needed to be topped up. Previous gauge on the boiler remains stable now but as said, can only get hot water when I switch the thermostat on.

 

[jackthom]

You say this happened since the weather dropped, have you just switched over to the DHW & Heating switch position?

I've no idea why this might be faulty but it would be useful to check it out by going back to DHW only to see if there is still no hot water.
(I'm assuming you have tried moving the DHW thermostat temp dial up and down to see if it's causing the problem.)

Otherwise I would be checking the DHW flow switch to atart with.

 

[45yearsagasman]

Very likely to be the diaphragm in the diverter valve holed, or a defective water flow switch. Very common to these boilers.