Hello,

I have a really old combi boiler and I think the expansion vessel
needs replacing. What happens to them? Does the membrane eventually
perish?

The vessel is at the back of the boiler in a really awkward place, so
I was thinking of buying an external one. The manual says the fitted
vessel is 8L but says if you add radiators and increase the volume of
the system, a larger vessel will be required, which makes sense.

Now I do plan to add a few radiators and I also plan to download one
of those spreadsheets to calculate what size radiators I need, so I
may upgrade a few of the existing ones. Since a 12L vessel is only one
pound more than the 8L one at tool station, does it make sense to buy
the 12L vessel to give me some spare capacity?

I note that Screw fix sell a EV isolating valve with drain. Are these
useful? Is the idea that you isolate the vessel and depressurise the
CH once a year and measure the pressure in the vessel and top it up if
necessary?

I know I need to fit the vessel as close to the boiler as possible on
the return pipe. Does it have to be plumbed in 22mm or could I use
15mm?

I know there are two spreadsheets recommended here for CH sizing: one
in Myson, what's the other and where can I find them?

Thanks,
Stephen.