Ferroli Modena 102 Combi, DHW stops after a while

[stapo]

The system is using a concentric flue connection.

CH working fine, but while supplying DHW, (usually in middle of shower ), the flame goes out, waits a minute the comes back on.
This has been happening for some time now intermittantly, but has got worse with the cold weather.
I have checked on PCB, the Air Pressure Switch seems to be the thing causing the system to drop out.
If I pop the blanking plate on top of the boiler (the one meant for use in 2 pipe flue system), all is well - no dropping of the DHW, except I now do not have an enclosed system as it is drawing air from the room.
Is it common for the air-pressure switches to fail?
Would it be a dodgy flue arrangement?

I'd be grateful for any advice.

Thanks

John

 

[oilman]

It might well be a leaking flue, and you would need a gas analyser to check it, I think you will have to get someone in.

 

[oilman]

It might well be a leaking flue, and you would need a gas analyser to check it, I think you will have to get someone in.