Expansion vessel size unvented hot water

[Matt0029]

I think I looked into it a few years ago. That a modern 200litre unvented cylinder would need a higher litre expansion vessel then one that was saying 10-15 years old. As they are made of thinner steele now?

 

[oz-plumber]

They are probably made of thinner steel now, but that's not your issue.
You will need an expansion vessel suitable for potable water - different from expansion vessels used in heating systems.
There's nothing wrong with over sizing and expansion vessel, but plenty wrong with under sizing one.

 

[John.g]

The capacity of any EV should be what it says on the label, most are sized to ~10% of the UV cylinder capacity, for example a 200L UV with a 18L EV with a pre charge pressure of 3.0bar and with the PRV set to 3.0bar will have a final pressure of 3.88bar when cyl heated to 60C.