Expansion Vessel question (s)

29 Oct 2008
Reaction score
United Kingdom
Can anyone explain to me why an expansion vessel loses its charge over time? Why does this happen? On top of that, why does an under-pressurized expansion vessel lead to the pressure relief/expansion valve being activated? I would have though that the expansion vessel with LESS air (charge) in it would be able to accommodate MORE water, and therefore the expansion valve/pressure relief valve, NOT needed to be activated?!

If anyone can explain I would be very appreciative!
Sponsored Links
I'll have a go Jack but I ain't a plumber.
I take it that you mean the gas pressure in the expansion vessel leaking?
If so, it can only be a) the Schrader valve in the pressurisation connection leaking or b) the diaphragm that separates the gas from the water leaking.
In the case of a) it can be proved by pressurising that vessel with gas and then putting soapy water on the inlet tube. If it bubbles, then it is leaking. Change the Schrader valve.
b) can be proved by holding the expansion vessel over a bowl and pressing the centre pin of the Schrader valve. If a lot of water pours out (as opposed to a short spurt) then the diaphragm is leaking. (usually this water is filthy black so don't squirt it on any carpets!) Change the expansion vessel.

Now to the why bit. Imagine (with the system cold and and the gas side of the diaphragm in the expansion vessel correctly pressurised.) The diaphragm will expand to fill or almost fill the expansion vessel. When the system heats up, the water will expand and move the diaphragm thus compressing the gas. As the expansion vessel should be sized to suite the amount of water in the system and thus the amount of expansion, all will be well. Now, imagine that the gas pressure is low and the gas side of the diaphragm only, say, fills one quarter of the expansion vessel. The system water is topped up to the correct cold pressure and now the expansion vessel is three quarters full of water. As the system heats up the water (which is now slightly more in volume than before) will expand by at least the amount it did in the first part of this example. Unfortunately the expansion vessel cannot now take all this extra volume so the system pressure builds up above the design pressure. If the pressure then gets as high as the PRV setting then the excess will blow over. The house owner will then top up with fresh water and the cycle continues. This is how I think it works and it'll do me but if not, I'm sure someone will say.
ah i see... thanks for that explanation.

what do you think would happen if the expansion vessel was over-pressurized with air???
Sponsored Links
If the gas pressure in the expansion vessel was too high then I think that for the water to move into the expansion vessel it must therefore be at a higher pressure than the design pressure and so there is a risk that the pressure may rise above the PRV setting and then dump overboard. So in other words it will show the same symptoms as if the pressure was too low. I have never thought about this before. strange isn't it?

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local