I have a Viessmann gas boiler, everything works fine except for the following:

I have flue damper with an automatic flue control that closes when the boiler fully reaches the call for heat and opens when the thermostat again calls for heat to conserve the boiler heat from being drawn up the chimney. As long as the Blocked Flue Switch (not pop out) is in the normal position the automatic flue opens, the boiler ignites and when the heating temperature is reached the boiler shuts down and the automatic flue closes and every thing works as normal. This can be successful for more than one heating cycle. I'm not actually sure when the Blocked Flue Switch actually actually activates (pop's out) and prevents the ignition system from re-igniting. Once the switch pops it needs to be manually reset. This status prevents the boiler the bolier from igniting for safety. If I set the automatic flue control to manual f(flue always open) the furnace operates without a problem, and there is no activation the blocked flue switch.

Here's the steps I've taken so far. I opened the flue and sight verified that the chimney is free of any obstructions and is clean all the way up the chimney top. The draft works fine I put lit match into the chimney and it immediately was snuffed out by the draw of the draft. I also have a squirel protector on the top of the chimney to prevent anything from getting into the chimney, so I feel confident that the flue and chimney are clear and not blocked, and has a good draft.

I suspect that maybe the blocked flue switch is defective but I've been told by a heating serviceman that's not a normal condition. I'm not convineced the switch is not the problem

If anyone has some tips or expererience with this type of problem I would certainly welcome your suggestions.

Thanks,

Papa_1