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Thread: Combustion Air intake?

  1. #1
    DIY Junior Member
    Join Date
    May 2005

    Default Combustion Air intake?

    The contractor on my rehab project has installed a foreced air furnace (AireFlow AF92MPE050U3 92%) unit in the 20 x 25 basement. I noticed that there is only an exhaust PVC line, and not an outside air intake line. There seams to be a 3" outlet on top of the unit where I am assuming the intake line goes. The contractor says that there doesnt need to be an outside fresh air intake with this unit. I find that hard to believe, but I have been wrong in the past. How do I check if this is correct? Is this common to have no fresh air intake on these units?

  2. #2


    Depends on who is paying for the fuel!!!!

    Just had the same situation on a rental property, it now gets it's air from the outside.

    Why would one want to remove conditioned air when it is not necessary? I think some installers just get lazy, or just want to save a few bucks.

  3. #3
    In the Trades MaintenanceMan's Avatar
    Join Date
    Dec 2008


    Some manufacturers allow it, some don't. Some local codes allow it some don't. Our local inspector had nothing to say about outside air intake on residential furnaces as long as there was sufficient combustion air for the appliance. IMO there, at the very least, should be a stand pipe off the top of the furnace to keep something from blocking the intake.

    We almost always ran the intake pipe on sealed combustion furnaces. A furnace is not going to malfunction without it. But with the proper intake pipe the combustion air is fresh outside air. With no intake pipe it's conditioned inside air therefore technically making the furnace less efficient. They are designed to be sealed combustion furnaces with fresh intake air from the outside. That's the way we installed them, just as the manufacturer specified. YMMV

  4. #4
    Like an engineer alternety's Avatar
    Join Date
    Apr 2006


    If your rehab also has sealed the house, there is another issue with not using the external intake air. Running a exhaust fan or a fireplace using interior air (or just running the furnace) can cause negative pressure in the house. If you have a regularly vented unsealed gas water heater the furnace could pull in the exhaust gasses from the water heater. If the combustion chamber of the furnace is not sealed that exhaust could return to the house. This of course puts CO in your house and that is a bad thing.

    At the very least, if the house is too tight, the furnace will not work properly which will probably lower efficiency and cause more carbon deposits in the heat exchanger.

  5. #5
    Master Plumber nhmaster's Avatar
    Join Date
    Jul 2008
    S. Maine


    Read the installation manual.


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