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
