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replacement air ducting X 2

I live in a large newly constructed house with a basement. Entering the basement at floor joist level are two 4" ducts bringing in outside air. The one duct attaches to an elbow and more ducting and drops its cold air at the base of the forced air furnace. The second 4" duct stays at floor joist level and ends with an elbow pointed down, venting outside air at the joist level. The gas water heater uses a power draft so I can see that some of this air would feed the power draft blower. The furnace brings in its own outside combustion air and exhausts flue gases through 3" pvc pipe. My question: why two ducts?? and since I have two ducts why is one venting high and the other at floor level??


  • Zman
    Zman Member Posts: 7,572
    They sound like combustion air ducts. The code requires them to be high and low. There is a fancy word for why this is which a cannot remember at the moment. Essentially it is so the air will be mixed well in the room.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    EBEBRATT-Ed Member Posts: 15,613
    That's pretty much standard procedure for make up air for gas equipment, especially with a new home with tight construction.

    Fireplaces, clothes dryers, bathroom exhaust fans and kitchen exhaust fan also need make up air.

    I would look into a small Heat Recovery Ventilator. The ventilator exhausts (heated-summer) (cooler-winter) air and heats or cools the incoming air so they are about 80% efficient in heat recovery. Better than bringing in cold winter air