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Water Pressure Problems and Expansion Vessel

Postby NogoodatDIY » Sun Nov 26, 2017 5:12 pm

This is my first post, hope someone can help. I have a standard CH setup, expansion tank in loft, hot water cylinder in airing cupboard.
Last winter had big problems with CH, it took british gas a long time (and cost me a lot of money) to find out what was wrong. Had a powerflush done (advised by british gas) and also had it converted to a sealed system so we now have a balloon shaped expansion vessel. None of this made a difference, turned out the problem was the hot water cylinder!
I've noticed that the pressure gauge on the expansion vessel sits at zero when the boiler has been off for a short while. Is this normal or should I fill it up so that even when cold it's at 1B? This will mean that it will shoot up to 1.5-2B when hot, which is way past the recommended level.

Any advice appreciated.
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Postby ericmark » Thu Dec 07, 2017 10:07 pm

Three reasons for the pressure not to keep within the limits when hot and cold, one is the pressure vessel is not big enough, the second is the air or other gas has escaped from the vessel there is often a car type valve to pressurise it, and last is the diaphragm has a hole in it.

In theory the pressure vessel should not be too small, however in industrial installations I have found where who ever either modified designed the system got it wrong. I can understand the error, not only does the hydraulic fluid (water and additive mix) expand and contract, so does the radiators and pipes and in real terms one can't calculate exactly how much, however if it worked once, then it should continue to work.

One should not need to ever add gas, where I have added gas I have latter found the diaphragm has ruptured. The problem is many systems have an automatic air bleed, so you don't know the gas has gone into the coolant, all you see is the pressure drops.
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Postby NogoodatDIY » Fri Dec 08, 2017 9:37 pm

Thanks for the info ericmark.
I have topped it up so that it sits at 1B when cold and goes up to 1.5B when hot. I'll see how it goes.
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