Erratic hot water supply from Combi

[MisterBoy]

We have had issues with muck before - when the exchanger was totally gunged up a couple of years back our magnaclean was pretty much full and the water was black. We've had the system cleansed and filled with inhibitor since and been keeping an eye on it, it was looking OK when checked a little while back.
I think other than checking the magnaclean again, this is about as far as I want to go myself but it seems I can give some pretty clear diagnostics to a plumber. Thanks

 

[ianmcd]

yep sounds like the diverter valve again (could be the plate heat exchanger) the diverter, there is a simple test, there is an isolation valve on pipe 5 CH return (red handle is closed by turning 1/4 turn) close this and run the hot tap, if HW is now fine, the diverter cartridge has failed, if the boiler locks out on Overheat, then the plate heat exchanger is blocked

 

[MisterBoy]

yep sounds like the diverter valve again (could be the plate heat exchanger) the diverter, there is a simple test, there is an isolation valve on pipe 5 CH return (red handle is closed by turning 1/4 turn) close this and run the hot tap, if HW is now fine, the diverter cartridge has failed, if the boiler locks out on Overheat, then the plate heat exchanger is blocked

That's a nice tip. I actually had that valve fitted recently (and another one) to isolate the magnaclean more easily as we were periodically using it to introduce inhibitor.

 

[CBW]

It could be a blocked plate heat exchanger, but unlikely. Isolate the flow pipe underneath, run the hot water and if you get hot water then it’s the Diverter, if it’s still the same you have a blocked plate. If the diverter has failed, it could just be a mucky system.

yep sounds like the diverter valve again (could be the plate heat exchanger) the diverter, there is a simple test, there is an isolation valve on pipe 5 CH return (red handle is closed by turning 1/4 turn) close this and run the hot tap, if HW is now fine, the diverter cartridge has failed, if the boiler locks out on Overheat, then the plate heat exchanger is blocked

That's a nice tip. I actually had that valve fitted recently (and another one) to isolate the magnaclean more easily as we were periodically using it to introduce inhibitor.

 

[MisterBoy]

yep sounds like the diverter valve again (could be the plate heat exchanger) the diverter, there is a simple test, there is an isolation valve on pipe 5 CH return (red handle is closed by turning 1/4 turn) close this and run the hot tap, if HW is now fine, the diverter cartridge has failed, if the boiler locks out on Overheat, then the plate heat exchanger is blocked

@ianmcd so again on a cold system (CH turned off) I started the bath running. Temp readout rose to 64 and held steady, the hot water out got warm and the CH output very hot.
I then closed the shut-off valve on the CH return. Immediately the temp started to rise, it got up to 80 and then seemed to hold steady - I waited about a minute. At this point, the hot water pipe was now too hot to touch, as hot as the CH out had got.

Just to confirm, this would strongly indicate it's the diverter not the exchange... as soon as I stop the hot water escaping the boiler into the CH pipe, I get much hotter water out of the tap?

 

[ianmcd]

yes the cartridge has gone on the diverter valve , they are not very reliable, but easy to swap and not too expensive