Carrier HVAC: CO Alarm and Smells
Where is the make up air for the boiler?
I think I see a vent to the top left but it appears blocked. If that is a fresh air inlet and it is blocked I hope your last will and testament is in order!
I lost a friend to carbon monoxide poisoning years back it is nothing to mess with. Your system needs fresh air for the CO to escape with no fresh air it escapes into the living space.
The fact that it is detected on another floor means it is an IMMEDIATE  HAZARD !
Make sure there is fresh air this is probably the issue. The small amount of cold air being drawn in by your boiler having to be heated is nothing compared to cost of a casket, no I am not kidding!