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danger of inlet vent too close to exhaust vent

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john123
john123 Member Posts: 74
in Chimneys & Flues
What is the danger in having your inlet vent too close to the exhaust vent. I know the code requires it, but what (bad) happens if it is installed incorrectly and too close?
Mad Dog_2

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  • hot_rod
    hot_rod Member Posts: 22,139
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    Exhaust gasses get recirculated 

     or pulled back into the boiler
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Mad Dog_2
  • SuperTech
    SuperTech Member Posts: 2,160
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    It will completely wreck any furnace or boiler. This is a problem I encounter frequently. For some reason a lot of installers seem to think this doesn't matter when it's a critical part of the installation. Recirculation of flue games destroys circuit boards, igniter and flame sensors, gas valves and and all other component exposed to it. 
    kcoppMad Dog_2
  • kcopp
    kcopp Member Posts: 4,432
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    Its for this reason I dont install concentric venting.
    I have seen way too many ruined boiler guts w/ them.
    Mad Dog_2
  • john123
    john123 Member Posts: 74
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    Thank you all. I thought perhaps it would be that the CO and other gases would displace the O2 and the boiler would misfire. I didn't want to start with this because it was only a guess. But would this be an additional reason for the code requirement?
    Mad Dog_2
  • Big Ed_4
    Big Ed_4 Member Posts: 2,785
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    Yes it can , cross contamination dilutes the normal 20% O2 intake used for combustion .
    I have enough experience to know , that I dont know it all
    hot_rod
  • Mad Dog_2
    Mad Dog_2 Member Posts: 6,924
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    A Real no no.  You wouldn't want to stand too close to a car's muffler either... Could cause "drain bamage!"  Mad Dog 🐕 
    Consulting by Mad Dog & Matt Jr.
  • Dave Carpentier
    Dave Carpentier Member Posts: 587
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    Do condensing boiler designs tend to dump the intake into the cabinet ( as opposed to piping the intake to the fan directly ) ?
    If so, running the unit with the cover off could potentially suck a good load of CO into the room, ?

    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 22,139
    edited February 2023
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    Some pipe into the cabinet, some direct to the fan.
    If the room meets the criteria in the manual, you can pull intake air from the space also.

    The exhaust is in a sealed pipe, hopefully, so no exhaust is leaking into the room, no CO either

    If you put your hand over the intake you feel suction, not unlike a carburetor, so air can only enter the combustion chamber when it is running.

    Most all have a blocked vent cutoff safety also.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream

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