I've posted before about this - but I have new information.
I have not chosen a boiler yet, although Viessmann looks like a good contender.
Anyway, there are times in the winter where, if we say take a short holiday, my insurance says "turn the water off" - but I would like to leave the heating running.
Is this a problem (rhetorical):
Physically - if we've hydraulically locked both sets of our plumbing, a very small amount of expansion in the boiler's heat exchanger *could* result in a spike in pressure (copper pipes being not very elastic).
Well - Viessmann say no as they say the Vitodens combi has a small expansion vessel built in on the potable side in addition to the main one on the heating side.
But covering my bases in case I change my mind about the boiler, I looked at an old boiler manual of a boiler I had once:
and on page 9 it says, on the right of the diagram, half way down:
"Note: If the mains is fitted with water meter, check valves or loose jumper stop c*ck, then a DHW expansion device must be fitted."
That clearly says to me that combi boilers should not have a hydraulic lock on the potable water circuit while they are running (unless the manufacturer has built in a safeguard like Viessmann).
Anyway, I see no harm in adding a small external one - it might help with water hammer anyway. But which one?
I see an american campany, Sioux Chief make the ones Toolstation sell:
Does not say "WRAS" or "potable" but clearly its friend with the 3/4" threads on is designed to go on the cold mains near a washing machine. Checking further I think these things meet the equivalent USA version of WRAS. Neat, small and solid looking.
The other option is something like:
Bit lumpy, but more volume.