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I thought a holed plate heat exchanger would make the system pressure rise to approx 3 bar and the prv to open?

some boilers overheat when on maximum, the potterton profile is a common cause of this - I know it;s designed to but it just doesn't - like isolation valves under the boiler are designed to be used to change a part without draining down whole system etc - but we know these are not always...

sometimes instead of locking out some boilers just cycle on and off the boiler stat if air was in the system/circulation problem then that would also cause overheat and lock out if that was the case

try turning the boiler stat down a bit and see what happens - if set too high or to max they sometimes cause the boiler to overheat

Is there power going to the pump when hot water is selected? sounds like a pcb fault when ch works intermittently, what exactly happens - is there a pattern to it or is it random?

is the pump rotor free? you need to check the resistance of thw windings and also see if the impeller is ok.

so no hot water ? how does the room stat work then if there's no timer... what powers the room stat?

sorry there's only 1 open end connected to the primary system - so the vacuum holds then.... :)

does the hot water work? what's the pressure on the gauge? how fast does the blue light flash?

sorry APS- air pressure switch have you tested the pump? when you say it's running it doesn't mean it is working correctly Also the fan - have you tested that? Byt fan and aps are gas related and you need a gas safe enginerr to test them

PCBs are often faulty on these - also check the fan's running at speed and the APS

if the fuse keeps blowing / rcd trippig Id check pump and fan first, then possibly gas valve as use 240volts

know about the vacuum, but there'll be 2 open ends, not just 1 and we all know that sometimes vaccum's don't hold !

try bleeding the highest level rads - any water coming out of them? could be a blocked cold feed. or some air in the system somewhere - try turning the trv/manual valve not lockshield off on all hot rads and leave the cold one trv open

Thanks for your reply - yep good point about drainibg through the prv - but drain the whole system water?

Just a quickie - what would you recommened the best way/quickest and easiest way to replace a PRV in a combi boiler - as turning off the isolation valves often results in a leak. Is it best to drain the system pipework ?

I just wondered why this happens - what causes the pump to slow/fail when hot only?

Hi I've heard that an intermittent fault that can occur on a combi / boiler is that a pump can fail when hot - so when tested (cooled down) you can't find the fault! Is this true.... ? If so, how do you find this fault out :)

to the original poster: what CLICKS do you hear when the boiler starts up? Ignition sparking clicks? Sounds like a board fault if there's no power to the fan or pressure switch as mentioned. To: Slugbabydotcom/STevenJames125 or others: If the PCB RELAY was faulty - how would you...

how comes the air pressure switch was fine in hw mode but in heating ti kept tripping? very strange!....?