While you wait for answers, we may have found some information which could help you.
Click here for Fires Normal HotSaunier Duval Thelia 23- no heating/hot water?
saunier duval thelia 23?
What's wrong with my Saunier Duval Xeon Central Heating Boiler?
High pressure on saunier duval boiler?
How can I fix the boiler?
Is the drop in pressure on saunier duval boilers a common fault?