Sealed system boiler losing pressure

[Lobster]

I have an unvented Vaillant condensing boiler which is regularly losing pressure - I guestimate te loss to be of the order of about 3 pints per week judging by the length of time I have to open the valve on the filling loop. (The gauge on the boiler falls to zero and said boiler starts to make strangulated noises...)

This has been happening all summer, ie with the CH switched off. I can't see any leaks anywhere, and the tundish by the pressure relief valve is dry; but logic tells me the water must be going somewhere, and surely at this rate ought to be detectable. Where's it going?!

Thanks David

[Ed Sirett]

Have a read of the SealedCH FAQ and then see what else you need to know.

[Lobster]

Already have actually! Decided it couldn't be the expansion vessel because the pressure only drops, it doesn't cycle (have I understood that correctly?); and the pressure-relief valve seems OK as I said. And I'd have thought I'd notice the effects of the volume of water which is apparently vanishing... so I'm stumped!

David

[Ed Sirett]

Is there any place where it could go unnoticed? You could probably get the leak to stop with leak sealer. It could be in the boiler itself and evaporating with the heat.

[dave]

my boiler did this and it turned out to be the thermostatic valves. 3 out of

4 leaked around the top under the thermostat control. Dave

[Lobster]

Thanks Ed. Well, I thought I had had a good look... the pipework twixt boiler and HW tank does pass under the upstairs floor for several metres, and I haven't pulled up carpets to inspect that; but you'd think that level of leak would have manifested itself, wouldn't you?

Mm, maybe that's where I'm headed; seems a bit of a bodge solution though?

Because my problem is not tied to the central heating being on? Presumably I just need to be looking at the boiler/HW tank/intermediate pipes, is that right?

David

[Ed Sirett]

No. All the primary pipework ( that includes the rads) are under pressure, although some types of leakage happen only when the pipes are warmed.

IME you can loose a litre or two a week and not be flooded out, look carefully for damp patches. Assuming its not the relief valve itself then the most likely places are:

1) The radiator valves. 2) The boiler

HTH

[Lobster]

Thanks again Ed. Armed with that knowledge I'll have (yet) another good look round.

Cheers David