Expansion vessels

[SLIPSHOD & SLAPDASH]

The one i have fitted can go to 3.5 bar

 

[Marvo]

If the tank is just there to absorb expansion on a closed system then I'd probably set the air pressure a bit lower than the normal water system pressure.

This doesn't apply to your heating system but with pumped water supply systems we always set the air charge at just below the cut-in setting on the pressure switch. New pressure tanks come with a precharge in them but as far as I know this isn't supposed to be preset as such, it's just there to prove it's not leaking. We check before installation and if we get one with no air charge in it we send it back to the supplier.