Sealed Combi boiler system losing pressure

I have a three year old combi boiler system. Recently I have had to refill the boiler using the filling loop quite frequently (weekly..), because the pressure has dropped below 0.5bar (and the boiler stops)
If it is a fault with the boiler, I will not be able to do anything to it myself, and will have to call out a plumber.
But is there anything I can do to try and work out what the problem is before I call him? I cannot find any leaking radiators (I assume this could cause the symptoms). And it does not appear to be 'venting' excess water out of the 'overflow' pipe (difficult to tell because of recent wet weather
So where could the water I am adding be going?
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The expansion vessel may have failed/need refilling with air. Tie a plastic bag over the safety valve outlet and see if water is coming from there as the system heats up. if so, indicates problem with expansion vessel.
SV is probably inside boiler cabinet or nearby. The expansion vessel will be in there too.
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On 12/12/2012 14:53, Neal wrote:

Fix a small plastic bag over the vent pipe coming out the boiler (probably outside) and tie in place with a rubber band. If that fills up, then you know that is the source of the leak (and the cause is probably the expansion vessel)
--
Cheers,

John.

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On 12/12/2012 14:53, Neal wrote:

I haver a similar issue. New boiler and radiator valves throughout. Found one leak under the boards (at a threshold); fixed it but still loses a bit of pressure. Lifted a couple of other boards and can't see anything, but I think this is a very slow leak so I may have my work cut out. One radiator gradually loses heat from the top down - takes about 3 weeks before it's noticeable, and no movement on the pressure gauge.
A couple of pipes have a furry blue residue at/around joins. This doesn't appear to be wet - but is that indicative of something?
Rob
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Indicates a tiny leak. The corrosion may or may not make it up.
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On 13/12/2012 07:43, harry wrote:

Bear in mind that it can be greater than you'd otherwise think, as the heat of the pipes will evaporate off the leaking liquid as it emerges (and it might only leak when the system is on).
David
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On Wed, 12 Dec 2012 23:43:35 -0800 (PST), harry wrote:

Well there wouldn't be as the gas in the top of the rad is pressurised. Bet the guage falls when you bleed that radiator. If this has been going on for more than 6 months you need to find out why. Is this rad the highest/first one from the boiler? Has the system got suffcient inhibitor?

Compression or soldered joint? If soldered it might be flux residue reacting with the copper, though that tends to be greenish rather than blue. Clean it all off with a damp cloth and spot of detergent and see if it returns.
--
Cheers
Dave.




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On 13/12/2012 08:51, Dave Liquorice wrote:

Both, as it happens. Lifted a board to see a chunky T compression fitting. The other pipes are soldered. I say 'green', perhaps more turquoise. Anyway, no sign of damp in the surrounding area, but I cleaned it up and will have another look.
Rob
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Finding leaks like this can be hard, because pressure is highest (leak fastest) when system is hottest, and dries up small leaks before they are noticeable.
In the summer when you're not using the heating and the system is cold, top it up to reach what is normally the highest running pressure and leave it like this for a while. Then check for leaks, and you're more likely to see wet spots, as they won't dry so quickly.
--
Andrew Gabriel
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Thanks for the replies. The bag with an elastic band trick is a good idea, I shall try that
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