Expansion vessel heavy

9 Apr 2012
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United Kingdom
Hi :)

I have a Grant Vortex 36e combi boiler.

It has two expansion vessels on the heating circuit.

The one inside the boiler is heavy and full of water.
The additional one is light as a feather.

When the heating is on the pressure rises from 1.5 bar to 3 bar.

Questions :

Why is the lighter expansion vessel not taking some of the pressure ?

The internal expansion vessel has an isolation valve and a shreader valve for re-pressurising. How can I tell if the diaphram has gone or if it simply needs re-pressurising ? If only re-pressurising, how can I do this.


Thank you
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Just pressed schrader valve on the expansion vessel attached to the boiler and water comes out so it is bust ! I need a new one.

Anyone know why the other one does not absorb the pressure ?
It's all relative - the actual amount of 'expanded water is fairly small. As the one EXV is full of water the other will always feel light by comparison as it's functioning correctly. Have you checked the precharge pressure on the second vessel?
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You have two pressure vessels for one of two reasons:

a. The system is a big one and requires more expansion capacity than the internal vessel provides.

b. The internal one has failed at some time, and an external one has been fitted as it is often less trouble and expense than replacing the internal one.

If the internal one has water coming out of the schraeder valve then its likely to be broken, so I'd concentrate on the external one. See if that has water coming out of the schraeder valve. If it has it will need to be replaced. If not then it may need to be re-pressurised. See FAQ (I think) but it will need the system to be de-pressurised.
I agree, its much more likely that the second one was fitted as a repair when the first was found to be failed.

But even so if the system is properly configured then the pressure should only rise slightly, perhaps 0.3 bar when the system gets warm.

Although there is nothing that a DIYer cannot deal with, a professional could assimilate and identify the problem.

The Grant Vortex 36 is a storage combi with a MASSIVE primary water store, the 14 litre vessel on the front of the boiler can only cope with a very small heating system in addition to the water contained within the boiler!! it is very common to see a 2nd vessel elsewhre in the system - the boiler M.I's make reference to the need to asses the total volume of water in the boiler and system both vessels will need to be correctly set, it is likely that the on-board vessel has lost it's charge therefore it is not doing it's job, the additional vessel may be over-charged with air again preventing it doing it's job. The correct way to set the air charge is to release the water pressure to zero, and then charge each vessel with air to 0.2bar less than the cold fill pressure -it is important to keep a drain valve open while pumpin with air to allow the displaced water to go to drain. once both vessels are correctly air charged refill the water pressure to 0.2bar grater than the air charge (cold) ;)

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