Sizing expansion vessels

[bright spark]

Lads which calculation do you use to size expansion vessels on jobs. I have to confess if I'm in a hurry I'll go with 1 litre vessel per kw of boiler.
It's just that I go to so may domestic system boilers where only it's internal vessel is installed, at correct air pressure but there's a wide variety in system pressure hot and cold so obviously vessel in too small.

There must be a short calculation you can do.
Any thoughts appreciated.

 

[Last Plumber]

Use 1/23 of system vol and then double that amount because it;s a vessel not a tank.

Is that what you mean ? or am I missing the point ?

It will be oversized (slightly) due to temp range causing less expansion than the 23rd in reality.

Others will no doubt use other methods but I find if you use that kw method you can be miles out especially on commercial systems

 

[Chalked]

10% of system volume.

Mr combi has an app where you put in ammount of pipework then how many litres are in rads etc. Think it's £1.50 is

 

[ShaunCorbs]

1 litre per rad so you have a 10 rad system your vessel will need to be 10l

 

[Dotty]

The thing is you can't oversize an expansion vessel. You can only undersize it.

Although, having said that, a 100 litre vessel in a 2 bed semi is a touch on the side of overkill.