Vaillant Ecotec 831 problem

22 Dec 2010
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United Kingdom
I am having a problem with a five year old Vaillant 831 combi boiler. Four days ago it failed to start up on the timer in the morning - F.75 on display (no pressure rise detected on pump start) I eventually managed to clear this fault by pressing the reset, and forcing a pump start through the diagnostics, I'm assuming that the pump had "locked" - boiler has been operating OK since then until this morning (I have been running boiler 24 hrs (with lowered stat. during night)) until last night. This morning the boiler failed to start again, but this time the display was blank (I.e. as if power was off) - mains supply from fused spur checked out OK. Turning isolator switch on front panel off, then back on after 5-10 seconds brings display back up, boiler attempts to go through start sequence but display blanks again and boiler shuts down, I have managed to finally get boiler firing by repeating above sequence a few times, but it seems to me that I may have a problem with panel I/O main switch or its associated PCB (which is an extension of main PCB)
The 831 is a little over-rated for this installation (1st floor, 1 bed maisonette) with 6 rads, two of which are large doubles, 1 radiator (downstairs - furthermost from boiler) does not have a trv fitted, all others do. The 831 was specified for this installation for its increased H.W. output,- boiler rating or system balance has not been a issue so far. Does anyone out there have thoughts on my suspicion regarding the power switch? I am unable to get our GS engineer out until the New Year.
Regards, warrior193. P.S. I realised that I put in the wrong fault code for the original failure three days ago - was F.75 (not F.65) have now corrected. Problem is now dead display - therefore no status information available - PCB fuse OK; mains power supply OK, Regards.
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