While you wait for answers, we may have found some information which could help you.
Click here for High Pressure Boiler Problemsaunier duval thelia 23?
How do i fix Saunier Duval boiler - Model Thelia 623?
boiler will not ignite, saunier duval xeon 50ff?
Saunier Duval Thelia 23- no heating/hot water?
Is the drop in pressure on saunier duval boilers a common fault?
saunier duval thermoclassic high pressure,,?