While you wait for answers, we may have found some information which could help you.
Click here for Air Pressure ExpansionI have a Saunier Duval Isotwin condens F 35 E with hot water No heating?
saunier Duval is over pressurising when the heating is on. ?
'f19' error report - Saunier Duval 'Thermoclassic' boiler?
Saunier Duval F35E auto-vents down to code 21 in minutes?
How to repair F4 code?
Saunier Duval Thema F23 anyone have a link for manual?