On 13/03/2018 10:01, Tim Lamb wrote:
Confused....

Either my known to be suspect mathematics is wrong or there is something
amiss with sites recommending pressure vessel sizes.
snip
Any thoughts?

Yes, Apart from physical space required, get a guestimate then go for
the next size up.
From my own experience the estimated size of expansion vessel based on
"at the time" UFH water volume was something like a litre more (or less)
than the closest size so I went to the next size up. Fortunately it
meant that when I re-fitted radiators in the bedrooms (not previously in
the system) I could fit bigger rads to work at UFH flow temperatures and
still have spare capacity.
Followed by previously un-planned front extension with another 300m or
so (3 zones on 4 circuits) of UFH pipework and still within capacity of
expansion vessel.
I still have another UFH zone to add in the bathroom so again this
wasn't originally factored into the volume calcs but will hopefully be
met with the existing vessel.

So bigger is better than being accurately calculated to meet "current"
system requirement.