Expansion Vessel - right capacity?

15 Jul 2011
Reaction score
United Kingdom
Hi, I have a Potterton Prima B system boiler. The hot water cylinder is the kind where it has the combined F+E on the top section of the cylinder and the cylinder has a small (o.5 litre) expansion vessel on it by the 2 port valve. However, the expansion vessel started leaking a year ago, I got a direct replacement and that only lasted a few months before leaking. I am wondering if the capacity is too small and the pressure is causing it to leak? Should I go for a larger vessel next time? Or maybe I am just unlucky and the vessels are not much good....
Sponsored Links
0.5 lites :eek: ??
If the boiler system is sealed, you need something like several litres, half a dozen at least.
If the heating system has its own header tank you don't need a pressure/expansion vessel at all.
Sounds like your talking about a shock arrestor for water hammer.
I wonder if the op has a thermal store, any chance of a photo ?
Sponsored Links
Is the Prima designated for use on a sealed system?
a photo showing the complete cylinder would help these guys.
Maybe pressure reducing valve is faulty causing excessive water pressure to vessel , what is the max pressure rating for the vessel?
The pressure vessel reads 'maximum working pressure is 10 Bar' and the pressure reducing valve - assumng I am looking at the right component - rads 2.5 bar max. thanks
The expansion vessel should be fitted upright not sideways. (schrader valve at the top)
This is the most likely cause of it leaking.
What was the charge pressure when it was fitted it should be 1 bar below water mains standing pressure.
The units are charged to approx 2-3 bar you adjust it when fitting and check the pressure yearly.
That looks like a shock arrestor (damper) on the cold mains feed to the cylinder and the combined header tank ballcock. It also looks like it's a heatstore, the cold mains tees-off just below the shock arrestor to what looks like a mixing valve....do you adjust the hot water temperature here?

In any case it seems that the expansion vessel is being damaged, it has already been suggested that the pressure reducing valve may be faulty. There should be a pressure gauge port on it. so you can check the output pressure.


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

Sponsored Links