new homeowner wanted furnace check over, husband, wife and infant not that it really matters.
original Coleman 2775-766 gas induced draft 80% unit.
the flue pipe was too big, 6" round, for the 5" oval flue collar on the metal collector box.
guess what they did,....
...they attached the pipe to the top horizontal plate of the furnace leaving a 1/2"-3/4" gap between the flue collar and the flue pipe. so much for the blocked vent switch!!
been this way since day one and it has been serviced by at least 2 other companies in the past.
maybe that is why there are 5 CO detectors in that immediate vicinity, 2 of them are hard wired types and dismantled.