3 expansion vessels in 3 years, whats going wrong?!

[phatboy]

Hi,

Since I had a direct pressure fed water cylinder fitted 4 years ago, every year the expansion vessel fails and needs replacing. They fill completely with water.

Fortunately the company who installed the system have been very good in replacing it each time despite the warranty having long lapsed now.

I asked them what they thought about the fault and they said it must just be bad luck, but I am now on the 4th vessel and can't help wondering if something else has caused the repeated failures?

Thanks

 

[Dan Robinson]

Ask them to fit a bigger one and check the prepare pressure.

 

[St0rmer66]

How sure are you that they've "failed" and not just gone flat? Even if that is the case (very likely given how thick the rubber is for the diaphragm, it takes a lot for one to fail) it shouldn't go flat as often as that unless the pressure is incorrect or the vessel is undersized! They're definitely using a white 'potable water' vessel, right? How big is the cylinder in litres and what size is the vessel in litres?

 

[phatboy]

Thanks guys,

I suppose the failure is they have gone flat? They aren't leaking or anything like that.

The previous 2 vessels were a light cream colour - Aquasystem 18L, and said 'multifunctional tank' on it.
The new one fitted yesterday is blue, and also 18L.

The hot water cylinder is a 210 Litre CentreStore one.

Thanks!

 

[St0rmer66]

If they're just replacing it without even trying a foot pump on it then I'm glad they're not charging you! White and blue are both suitable. As long as it's not a red one.

18l on a cylinder that size is not unusual but oversizing might go some way towards stopping the problem.

 

[phatboy]

Thanks, I suppose I will need to wait and see if it goes again.

What could be the cause of this? My understanding of pressurised systems is rather limited.

I know there is a 3.5bar restricter connecting to the cylinder, so is the vessel there to take up any additional pressure?

 

[St0rmer66]

You fill it at 3.5bar cold, when you heat it the water expands and it needs the vessel to take up that expansion.

There is a cold water pressure relief valve just after the restrictor and then another on the side of the tank which also releases at a set temperature too (90 degrees usually) to stop it going above 100. If you store water under pressure the boiling point rises so if it was say 110 degrees it would remain liquid... Until you open a tap and suddenly it boils and explodes, taking out half the house with it! The number of things that would need to go wrong for this to happen make it very unlikely but that's why there are so many regulations and safety devices for unvented.

 

[phatboy]

That's a good explanation, I think I finally understand it I think!

The plumber wasn't able to explain it to me so well yesterday.