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Excess Air

Hey I'm not new to combustion theory but lately getting more involved in what it all really means and what I can do to benefit the client to the best of my ability....so I've been looking at excess air a lot lately on furnaces...oil, propane ,natural gas....and I've noticed that my excess air is roughly 30-50% when the indoor blower isn't on....but the second the indoor blower turns on my excess air sky rockets to sometimes up to 300%...now I've then checked for carbon monoxide in supply plenum and have found nothing by I've been taught that the indoor blower motor shouldn't directly affect my combustion chamber unless there's a hole in my heat exchanger can anybody please tell me if this is true and or what is exactly going on before I approach a client with this id like to be able to know and understand better thank you


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    Does this happen......

    Does this happen every time? If so, either way, stick 2 different CAs in the flue, just to check. But certainly this is a sign of a large crack in the exchanger. You wouldn't get CO in the supply because the blower is overpowering the exchanger. You may get it before the blower comes on.

    Also make sure after a cleaning, that all the clean outs are properly reattached and sealed--no loose gaskets. This would give you the same kind of readings as a cracked exchanger.

    More thoughts, what happens to the other numbers, before and after the blower comes on? Does CO2 nosedive (or O2 go up)? Does draft change?

    Some analyzers have a set up to do a heat exchanger test, which basically measures combustion before and after the blower comes on.

    Other things that could effect numbers pre/post blower:

    1. Open return (with oil, you usually see the draft regulator move when the blower comes on.

    2. Restriction in the return (clogged air filter) robbing combustion air.

    There was an error rendering this rich post.

  • Weezbo
    Weezbo Member Posts: 6,232
    would you say ,...

    this is something that you notice on all new equipment you have been testing ? or mostly older oil fired equipment?

    do you recall any of the burner s ?

    Do these systems have supply and return ducts?

    are these furnaces in flood disaster areas ?

    mostly up scale homes ?

    trailer courts ?

    where are all these furnaces located in closets , overhead in ceilings ?

    in crawl spaces ?

    in garages?

    how many of these have you found ?

    is forced air everyone's choice in the area ? are you located in a state where the furnaces are seldom run ?

    i like to know to what i need to focus my reply to help.

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