DIY Doctor

New Vaillant Combi Boiler losing pressure

Postby DIYmonks » Fri Oct 23, 2020 3:41 pm

I have had a new Vaillant boiler fitted a year ago. I had no problems with it until I fitted a new radiator. I switched off the boiler, drained the system, fitted the new rad, filled up the system adding the correct inhibitor and re pressurised it up to 1.5 bar and then closed off the filling loop valves. All the the rads have been bled and it runs well with no leaks, but I am losing pressure after a few days. I have had to open up the filling loop several times. The PRV is dry and I have checked and rechecked for leaks too. Could it be the expansion vessel leaking? As I said the boiler is 1 year old and I am thinking of getting it checked under warranty but I need to make sure I haven't forgetten to do something before I go down that path. Thanks in advance.
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Postby plumbbob » Sun Oct 25, 2020 11:29 pm

DIYmonks wrote: Could it be the expansion vessel leaking?


In short, No. Any leak from the pressure vessel would be visible as a puddle under the boiler and anyway vessels rarely leak in this way.

Sorry, but given that the boiler is so new, I would say you have a pipework leak somewhere. You just haven't found it yet.
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Postby DIYmonks » Sun Oct 25, 2020 11:52 pm

Thanks for the reply. I understand what you're saying but it's beyond me because any leak would be marking the ceiling by now so I am at a loss.
Thanks again.
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Postby plumbbob » Mon Oct 26, 2020 9:45 am

DIYmonks wrote:........... any leak would be marking the ceiling by now so I am at a loss.


Not necessarily actually. This is only a small leak and any damage might only show after quite some time. A lot of the leak could simply be evaporating if the heating is on and the house is warm.

Assuming you don't have a hot water cylinder, water can escape in only two ways: out the prv or a pipework leak. There is a slim chance there may be trapped air in the system which is being purged and resulting in the required top-ups, but I would have thought that would have settled by now.
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Postby DIYmonks » Mon Oct 26, 2020 9:54 am

Funny you should mention trapped air. I have repressurised the system about five times now and this morning, after a few days it seems to have remained near on 1.5 bar. I am surprised that the automatic air valves wouldn't have done their job sooner but I won't count my chickens just yet. Cheers.
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