Expansion vessel problem - help!

This is pretty spooky, having posted a very similar query yesterday on behalf of a friend - however, this one relates to my own CH system which I've been working on today.

I flushed it all out and added fresh Sentinel, and set about checking the (separate) expansion vessel before repressurising the system, and I think it's failed. I've checked Ed's FAQ, but am I right?

This is a sealed system with unvented HW tank. Unfortunately having mucked about with the valve on top I'm not entirely sure what the status quo was; the boiler pressure gauge has been fluctuating, though. I'm finding it incredibly hard work to pressurise the expansion vessel by force air in via my car tyre pump, and only a very few strokes of the pump seems to pressurise the 19-litre vessel to about 2.5 bar, implying to me that the available volume within is tiny. Plus when I pump I can hear gurgling noises within the vessel, and some HW is forced out down the tundish. (During this time the water side is de-pressurised, and there's an end-plug removed from the bathroom towel rail). No water comes out of the Schrader valve on the vessel however.

So assuming I need to replace the vessel, it seems to be simply a case of unscrewing it (it's connected via a taper BSP screw fitting) and bunging a new one on? Correct? Are all expansion vessels the same - ie I just need to get one of matching volume?

I'm confused about pressures. Everywhere I read that expansion vessels should be charged to about 0.7 bar, with the wet side depressurised. Indeed, the manual for the boiler (Vaillant EcoMax VU226) says it should be between 0.7 and 0.9 bar; however that refers to an internal vessel - does that make a difference? Mine was fitted with an external vessel. However, there's a label on my HW tank (Vaillant) which states "Expansion vessel charge pressure: 4 bar" and "Expansion valve setting pressure: 6 bar" Furthermore there's a label on the side of the expansion vessel itself Varma) which seems to indicate 3.5 bar is appropriate - see picture here:

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seems to be a hell of a long way off the 0.7 bar mentioned

Any help welcomed. Thanks David

Reply to
Lobster
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Are you sure that you're looking at the right expansion vessel? The one for the heating system will be on the primary circuit - whereas I suspect that the one in the photo relates to the pressurised (secondary) hot water system - which is separate from the primary. Since pressurised HW systems are potentially dangerous - and have by law to be professionally installed and serviced - it sounds like it might be time to send for an expert!

Reply to
Roger Mills

That photo may be of your unvented cylinder's expansion vessel, which is unrelated to your central heating's pressure vessel, which will either be internal to the boiler, or a complete separate (usually red) device positioned nowhere near the hot water cylinder.

Christian.

Reply to
Christian McArdle

Hmm. That figures, yes it will be: it sits on top of the cylinder. There is indeed another expansion vessel inside the boiler. I hadn't appreciated that the cylinder required its own vessel, but that makes sense. (The installation instructions for the boiler make mention of fitting an additional, external expansion vessel where an old system is being modernised - which was what happened - so I assumed that was what my external vessel was for.)

Not sure where that leaves me now, other than following Roger's suggestion of calling in a pro... :-( ?

David

Reply to
Lobster

The vessel you have been trying to pump up is the one for the unvented HW cylinder. If you have instructions for the cylinder then they are the final authority in this matter.

Generally the unvented cylinders will have an inlet pressure reducer to about 3 bar (3.5 in this case) and the vessel will have a dry-side pressure around that value (4 in this case). If the vessel fails then the pressure rises will become large and at 6 bar the first of the two pressure relief valves will dribble.

The vessel for the HW cylinder will likely be coloured blue or white and be suitable for potable water and corrosion resistant.

You should find that if you turn off the inlet to the cylinder and open a HW tap you can pump the vessel up. However if the vessel needs pumping up then you have to ask your self why? You will probably need to replace its pressure regularly.

Reply to
Ed Sirett

Yes, thanks Ed: once I'd sussed I'd been trying to pressurise the whole HW cylinder, realisation dawned and I applied the above solution last night, with great relief.

Now I know this vessel has nowt do with the CH I'm sure it is fine, though obviously I'll keep a close eye on the pressure for a bit to confirm. Have to say that it was my fault it needed repressurising due to me fiddling with it; which with hindsight was pretty bloody stupid, not understanding what all the bits were for, I'll be the first to admit. A case of a little knowledge is a dangerous thing, and all that!

(And stone me - I certainly don't want to have to do that again - pumping an 18-litre vessel up to 4 bar using a handpump, with the valve

6 feet off the floor!)

David

Reply to
Lobster

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