Combi boiler lockout on overrun

[DrNibor]

I've got a Potterton Combi 80 boiler. The central heating works fine. But whenever I run the hot water for an extended period (filling kitchen sink for example) When I have finished, the boiler over heats, makes some horrid gugling noises, steam comes out the top and goes into lockout.

I've ckecked the DHW thermistor and it reads 7.3 kOhms at room temp (which is within spec according to the installation manual)

I'm assuming that the pump is ok, cause if it wasn't, wouldn't the central heating not work?

Any other suggestions?

 

[ChrisR]

Standard fault on this boiler - look at the red light on the PCB. It should go out as soon as the tap water flow stops. Yours doesn't, because the DHW flow switch is sticking on.

 

[DrNibor]

Many thanks for the quick reply. I've just run the sink, and typically, the problem didn't occur!

Is it possible to clean the switch or is a new one needed?

Any preferences on suppliers of boiler spare parts?

 

[DrNibor]

So I replaced the hot water flow switch, and the problem appears to have been solved. However, when I run the cold tap on full, and then shut it, the red LED on the PCB flashes and I can see a relay firing momentarily. Is this normal? It didn't used to happen before I changed the flow switch.

thanks