Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

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


  • unclejohn
    unclejohn Member Posts: 1,751
    Is this a home

    Or a business? Sounds like you have a sever negative pressure in the space.
  • drhvac
    drhvac Member Posts: 189
    air flow

    How about your temp rise? Is it right according to the nameplate?