Mystery Combi Boiler leak

[Andrew Mawson]

I have a combi boiler feeding two small holiday cottages. Total of two large radiators, two small radiators, two showers and kitchen sinks. There had been a history of loss of pressure, so when the cottages were renovated and the floors taken up (suspended floor) every joint was checked and there was no evidence of leakage or staining so the floor was re-laid and tiled. Everything pointed to the Worcester 20-25 boiler which had other problems (it had already had it's expansion vessel, safety pressure valve and auto air vent changed).

Symptoms were that if the boiler stayed warm it was ok but if allowed to go cold (power cut for example) the pressure would go to zero, no sign of any pressure on the gauge what so ever.

So we had the boiler replaced (Grant Vortex 21 combi). Then we had a power cut, and ARGH !!!!!! the pressure fell to zero. Re-pressurised and it was ok for a few days - then the oil control solenoid failed (infant mortality?), the boiler went cold, and again the pressure went to zero indicated on the gauge. Last night again a repeat - system not working as no pressure this morning - (it was fine when I checked it at 20:00 last evening).

So logically the leak is most unlikely to be the new boiler as the symptoms are pretty well identical to the old one, which begs the question, how can a leak suddenly develop, allow total loss of pressure, yet seal fine when the system is topped up and show no evidence?

Andrew

[Davey]

I know nothing of pressurised boilers, but to me, it sounds as though there is a powered normally-open solenoid valve somewhere, which fails open, and lets the pressure out. Or a valve which needs a certain minimum pressure to stay closed, and when the system goes cold, the pressure drops below this. Just thinking.

[Dave Baker]

Perhaps a system drain point which leads outside so there's no internal evidence of leaks?

Other than that if the boiler itself has been changed and the rads don't leak the only other things that could affect this would seem to be the pressure relief valve or the non return valve on the filling loop.

[newshound]

I agree it sounds pretty odd. Is there perhaps an external expansion vessel (not replaced) with a failed diaphragm so that the system gets overpressurised on warmup? In my experience if the "pressure relief valve" has ever lifted it sometimes does not re-seal well (erosion of seat from cavitation? deposits on seat?). So you could have had a failed PRV on the old one, and now have a failed one on the new combi. The "hot/cold" effect could be because sufficient pressure keeps it sealed while it leaks at slightly lower pressure.

Or of course could there be a leak somewhere uninspectable, e.g. where pipes pass through walls? Same hot/cold process (although this sounds less likely than for the PRV which I think has a rubber seal).

[Andrew Mawson]

Expansion vessel is incorporated in the boiler. as is the pressure relief valve, all electrically operated valves (Honeywell zone valves) are in the boiler room and visible. Leaks on the filling loop would increase boiler pressure to mains water pressure not reduce it.

My only conclusion is that there MUST be an under floor leak, but we searched every pipe and joint when the floor was replaced - it's a bit of a b*****r

Andrew

[Capitol]

Is there any chance that there is air in the system? You state that the system doesn't lose water when running hot, but loses pressure if it cools down. With air in the system, wouldn't the pressure relief valve come into operation when the system is hot, thereby losing water? I don't know anything about these systems, just a thought.

[Andrew Mawson]

"....just a thought. " and indeed a very good one. The whole system is so small, confined to one floor, and we >>think

[Davey]

You need a ferret-mounted camera........

[Capitol]

Wireless I assume?

[Davey]

There was a guy on TV recently who swallowed a pill with a camera inside, so that he could see, live, pictures of the inside of his stomach and intestines. He had deliberately swallowed tapeworm cysts, and the resulting tapeworms were clearly visible.

[Fredxxx]

Snake cams are pretty affordable.

Are these copper or plastic pipes?

When filling from 0 to say 1 bar, how long does it take to fill? It sounds a modest leak.

Have you checked the pressure relief valve vent pipe, to see if any water comes out when running?

Air is very compressible, so wouldn't be much different to the air in the expansion vessel.

[Andrew Mawson]

Unfortunately it's plastic pipe. Re-pressurising takes perhaps 10 seconds, no sign of leaks from the pressure relief vent pipe

Andrew

[Capitol]

Are there push fit joints under the floor?

[Andrew Mawson]

Sadly yes :( Andrew

[Capitol]

Oh dear! Looks like a camera job is called for. What's the floor! Does it come up in sections? Or, can you procrastinate and say, we'll pretend it isn't happening? Just make sure the water is kept hot!

[Andrew Mawson]

Floor is 18 mm plywood, tiled all over :(

Plumber has suggested an automatic top up system that fills from a reservoir of Fernox treated water. OK I suppose if I can monitor water usage to see if it escalates. The very odd thing is it can be fine for weeks, or at lease has been previously.

Andrew

[Davey]

Ah! So you need to use a leak-plugging additive! My sympathies for this one.

[Bob Minchin]

If you can re-pressurise in seconds then I think it is unlikely that you have a lot of air in the system. You said somewhere that there is no sign of water coming from the PRV drain - which would be a common culprit

I can only suggest adding a dye to the water and look for under floor weeps.

The other 'fix' which I could not possibly condone is the fit a pressure regulator, set to the preferred cold pressure setting, upstream from the filling loop and its NRV and leave the fill valve open a crack.

Incidentally I find having such a pressure regulator on my fill loop very useful indeed - even though I DONT leave the fill valve open. It makes bleeding and refilling a breeze with no concerns of over pressurising.

Bob

[Onetap]

Did you pressure test it?

I used to witness pressure tests, before the insulation went on, and contractors would have countless cunning stunts to try to get a leak through; pump up the pressure when (they think) you're not looking, valve off the leaking bit that they'd already insulated, etc., etc.

The plastic pipe stretches and a different test procedure is used. The 10 bar test is part of the makers' warranty and must be done to set the grip rings on installation.

It sounds like it might be a plastic pipe pulling on a fitting when it's cold & shortest. They expand a lot when hot.

[Onetap]

10 bar HYDRAULIC test.