Expansion Vessel slightly below spec - ok?

Well I had an engineer round today to look at my dodgy Potterton Puma 80 - as I feared the expansion vessel has died. Replacement from them is £100 + vat, on the net it's £63 +vat so that's more tempting.

However, the manual recommends an external expansion vessel be fitted 'in the unlikely event of failure', and these are much cheaper - £25 or so. I told the guy I'd get one of those and he'll come back to fit it and service my boiler..

Problem is the 12litre one is too wide to fit in the cupboard below the boiler, the 8litre one would fit fine - HOWEVER, the one it would replace is 10litre. Would this cause any problems? I'm thinking/hoping that it won't, as I'm running 7 rads in a fairly small flat, and that 8 litres ought to be enough (and if not, it'll just mean that the pressure when CH running is slighly higher than it would otherwise be and not too much of a problem?)

Thanks in advance!

PS It's amazing how much terminology I've picked up over the past few days from checking out this forum to help with my boiler problems - never heard of an 'expansion vessel', 'prv' etc before then

Get one the minimum correct size or bigger. DON'T use a smaller one. It can be put anywhere on the system, so if it wont fit in one place, put it in another.

An 8 litre will be fine. Check the pre-charge on the new vessel - they often come with way too much pressure - set it to 0.8 Bar.

Most boilers are now fiited with 8 litre vessels (and some only 6 litre). The hot pressure will be marginally higher but thats of no real consequence.

Just to confirm that 8 litres is fine.

Id check the hose leading to the expansion vessle is not blocked on the elbow connection..Ive never changed a puma expansion vessel but unblocked loads of them. id be suprised it it had bust and wasnt just a blockage.

HarrogateGas said:

Id check the hose leading to the expansion vessle is not blocked on the elbow connection..Ive never changed a puma expansion vessel but unblocked loads of them. id be suprised it it had bust and wasnt just a blockage.

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The engineer seemed to think it was knackered, he pressed it slightly and water gushed down the wall by the side of the boiler. Would that indicate a blocked hose?

chrishutt said:

Just to confirm that 8 litres is fine.

Click to expand...

Thanks! I will probably go with this option (it's just a stopgap anyway until I can afford a nice new condensing combi). The guy also mentioned I'd need 22mm hose and t-compression thing (not entirely sure what they are!) and that it'd be miles cheaper for me to source them than for his company to...

Any significant water expelled would indicate a holed diaphragm.

Replacement is your only option.

A blocked hose (normaly on the swaged elbow pipe) as Harrogate indicates is another common Puma problem.