Hi how to increase the water pressure on ferroli optimax 25C?

Just do a Gooogle search for "ferroli optimax 25C F37"
That should give you results including the manual and several forum threads describing how to do the topup.

Things like:

www.ferroli.co.uk/brochures/Optimax-25-C-Manual.pdf

https://www.ukplumbersforums.co.uk/threads/ferroli-optimax-he-31c-error-37f-help.28956/

Plenty more out there

If the pressure continues to drop regularly, you may consider looking for a leak, either withing the boiler or on the system.

dsfds
October 2017