faulty Combi / Central Heating system

I have a five-year old Valliant combi boiler, and inherited a 'warm sure' boiler maintenance scheme with house, so do not have to pay for call outs or spare parts.
The boiler has been malfunctioning for more than a year now, with hot water going cold after a few minutes, then coming on again warm, then cold. The first engineer, who came three times, didn't seem to have much of a clue, but replaced a few parts and the warm water wd work well for a week or two then revert back to bad old ways.
Finally warm sure started sending out a Valliant guys, the first one said that the system needed a power flush to correct problem.
Second guy says there is a prob a leak in c heating system system which is causing boiler to overheat so it cuts out. He says leak needs to be found (he wouldn't try to find it...) and then system needs a flush. I'm not sure if he really knew if this was the fault as he was on phone to other engineer asking for advice...
Both seemed to think that boiler was not at fault. Thermo was replaced and found not to be at fault.
Obviously power flushes and so on cost $$$ and might not even remedy the prob. I'd really appreciate some advice about the best course of action, simple things I can check myself. Where do I begin checking for leaks? And problem solving?
thanks for any advice
Fiona
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On Fri, 18 Apr 2008 03:28:36 -0700, FionaS wrote:

The most likely causes of these problems in my experience are: 1) A faulty air pressure switch. About 5 years ago a number of Vaillant combis were shipped with troublesome air pressure switches.
2) A faulty diverter valve. I think this is less likely because you would have said sometimes it works fine and other times I get nothing.
3) A partly blocked up DHW heat exchanger. Also fit the symptoms exactly, further tests are needed to decided what the cause of the blockage is and if the cause(s) are still there.
Anyway even if you had no model specific experience you need to find out if the water going hot/cold is due to the burner coming on or off and if so is the burner coming on/off due to over heating or the air pressure switch (or even some other less common problem).
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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Having benefited as a "lurker" for some time on this forum (Ed Sirett, you're a gem) at last I might be able make a contribution...
When you say "hot water going cold... then warm... then cold" I assume you mean domestic HW flow. I had the same cycling prob with a Vaillant combi a few years ago and eventaully fixed it by replacing the system bypass valve (which is not the same as the diverter valve, and is a darned sight cheaper; also less fuss than a power flush, I reckon).
I think in my case the weak bypass valve was causing the flow to recirculate through the heat exchanger instead of through the system, causing an overheat and cutout. I got the clue from the fact that the bypass loop was heating up (just from feeling it) when there was no obvious reason for there to be a bypass flow. Perhaps that's where the idea about the power flush comes in; if the CH was blocked then the bypass flow would come in to action. (But I can't figure the logic behing the suggestion that it might be a leak in the CH.)
Incidently I only figured this out AFTER I had changed the diverter valve and the heat exchanger (); as Ed Sirett says, this and the air valve seem to be the obvious culprits.
(I now have a new problem with the CH cutting out if I set the flow temp below the half-way mark; still have to figure that bit.)
btw if your CH does need a flush -- and it might if it is old -- I think you might find that the rads have hot and cold areas on them, instead of uniform heat, due to sludge build-up. This would not apply if you happened to have solid lump stuck somewhere. But again, if I understand your post, the problem is with the DHW not the CH.
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When you say "hot water going cold" I assume you mean domestic HW flow. I had a similar prob with a Vaillant combi a few years ago and fixed it by replacing the system bypass valve (which is not the same as the diverter valve, and is a darned sight cheaper; also less fuss than a power flush, I reckon). I think in my case the failed bypass valve was causing the flow to recirculate through the heat exchanger instead of through the system, causing an overheat and cutout. I got the clue from the fact that the bypass loop was heating up (just from feeling it) when there was no obvious reason for there to be a bypass flow. Perhaps that's where the idea about the power flush comes in; if the CH was blocked then the bypass flow would come in to action. But I can't figure the logic behing the suggestion that it might be a leak in the CH .
Incidently I only figured this out AFTER I had changed the diverter valve and the heat exchanger ().
I now have a problem with the CH cutting out if I set the flow temp below the half-way mark; still have to figure that bit.
btw if your CH does need a flush I think you might find that the rads have hot and cold areas on them, instead of uniform heat.
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