Hot water cut out

Boilermate 111. The system runs ok then suddenly no hot water. I thought it might be a dead segment on the motor of the heat exchanger pump so I tested it by wiring it directly to the mains (and it worked). When I then re attached it to the PCB all worked again as normal!
I have now had to kick -start the sysem by this method about three times in the last month and I am totally at a loss as to what is causing it. I wish it would fail completely and then I might have a better chance of diagnosing it. Any ideas please?
Thanks

Hi radbor,

This sounds like a failing DHW sensor to me, but on a Boilermate III there is a flow switch too which can be known to stick. Next time you get cold from a hot tap, leave the tap running and give the flow switch a clout and see if the water starts flowing hot.

Or more elegantly, test the microswitch in the flow switch with a multimeter during the fault condition...

Cheers, Mike

Many thanks. As I have recently changed the store sensor ( I assume that's the one you refer to as the DHW sensor or are you refering to the pump speed sensor?), I'll go for the flow switch and let you know. Might have to wait a bit though until it fails again!

There is no 'pump speed sensor'! The DHW sensor to which I'm referring is on the HW output pipe from the heat exchanger, one of the two pipes connected to the top of the heat exchanger.

Cheers, Mike