Covered before I know but this is from a different angle..
The internal vessel in my boiler is just about large enough to handle the cold to hot system expansion: remaining between 1 and 2 bar.
However, there can be water hammer from (I think) the by-pass valve opening during the change from DHW to underfloor heating.
To overcome this I installed a second (5L) vessel close to where the by-pass is fitted. The installation instructions advise pressurising the vessel to 1bar. Fine through the Winter but now we have some warmer weather the water hammer has returned.
The boiler spends more time fully shut down as the roomstats are not calling for heat. As a consequence, it is not uncommon for the system pressure to drop below 1 bar.
My thinking is that I need to more closely monitor the cold system pressure but also that the additional vessel pressure should be dropped to something like 0.75 bar. (with the water side at zero.)
Any thoughts? (conventional chalet bungalow so no huge static pressure concerns)