When Hot water tap is running the central heating flow pipe gets hot ... right here we go ...checked plate to plate and all is clear ...changed two diverter valves ..body and head ...replaced thermistor and pcb ...and still no joy ...it fires up on full rate as expected but dumps the heat into the heating ... pump is pumping the right way ... this has me baffled ...even manually with the head off it ....and remotely moving it ...no joy ...yet .. when tap off ...and what appears to be ...preheat ....the plate gets warm ..... any help would be more than welcome ....thank you very much guys .... :-(

SEBO, October 2011

Just observe it's rotation when you take it out, it only goes back in one way, if you get it wrong, you won't be able to refit the flow connection to the manifold.

Carefree, October 2011

Thanks carefree ... altho I did take the water pressure switch out ...and notice the filter was clean ...but maybe it was blocked on the outlet to the plate to plate not sure .....but gonna strip it down to look ... will keep you posted mate ...other chap that came before me put all the new parts on ...

SEBO, October 2011

Had this very problem last year, it was a blocked filter due to an unclean system, the same symptons, pre-heat would run but H/W demand would push heat down the C/H flow.

Test by turning off the C/H flow isolation valve, run the H/W (not pre-heat), if the filter is blocked, the temperature will rise rapidly until it reaches about 85c then shut the flame off, it will then stay off until the temperature cools sufficiently and will run through the process again.

The filter is located in the bottom brass L/H section of the boiler, if you take out the C/H pressure switch (older models), you will see the filter in behind through the hole. It is extracted to the left after removing the flow connection to the brass block.

Carefree, October 2011