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multiple furnaces going off on flu limet

i have never seen anything like this before we have 3 furnaces about 100,000 btu non condensing induced draft all going off on high limit that is on draft motor outlet. I took flu pipe off and there is a strong draft from chimney i do not have proper tools to check but it appeared ok. I am getting 4.7 02 and about 160ppm co sometimes fluctuating a bit +-20 ppm. flu temp is about 400F. There appears to be plenty of make up air. all 3 furnaces are doing the same thing.

other notes:

filters clean

ambient room temp 70

gas pressure 3.5" manifold side

natural gas

3 furnaces in same room common venting


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