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Community and Q&A

Outside Combustion Air

Fletcher Hall | Posted in Energy Efficiency and Durability on

My daughter has an 1860’s vintage house in New Bedford, MA with a hot air heating system. She has a damp and moldy basement which supplies the intake air for furnace. Can we easily install outside intake air to supply return duct and does it need any special equip. besides a vent hood and duct? Would it replace the damper attached to return duct? She has a new baby and a 4 year old and we are concerned about moldy air being sent to living space.

Thank you


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  1. GBA Editor
    Martin Holladay | | #1

    Combustion air is the air that is used by the burner in your furnace. The components in that air that are not used in combustion leave your house through the flue (that is, the chimney). Combustion air is not delivered to the rooms in your house.

    Even if the combustion air used by your furnace is stinky and foul-smelling, it will not affect the air quality in your home unless you have something seriously wrong with your furnace (like a cracked heat exchanger).

    The air delivered through the registers and diffusers of your forced-air system has no contact with combustion air. The air is pulled from your home (upstairs) through the return air grilles; is delivered to the furnace; and is heated by the furnace and then sent back upstairs to be delivered through registers and diffusers in the rooms of your home.

    However, if the basement is damp and moldy, that air can affect the air quality of your home, but the way that your basement air gets upstairs is not directly through the furnace ducts. It arrives upstairs through cracks in your floor, pulled by the stack effect. (It's also possible for basement air to enter your duct system through cracks in leaky ductwork; if you suspect this is happening, you should seal your duct seams with mastic.)

    Damp, moldy basements should be fixed, of course. If the floor is dirt, you need to install polyethylene over the dirt, followed by a concrete slab. If the soil around the house is moist, you need to change the grade at the home's perimeter so that the soil slopes away from the home. Gutters should be connected to conductor pipes that carry the roof water far from the foundation. The footing drains, if they even exist, may need to be checked -- an expensive repair. Some old basements need interior drainage connected to a sump equipped with a sump pump.

    In some cases, a dehumidifier can help, although dehumidifiers are expensive to operate.

    Finally, it may be possible to provide outdoor combustion air to your furnace if you want to, but it's not clear from your question that such work is necessary.

  2. Fletcher Hall | | #2

    Thanks, that was helpful!

  3. Expert Member
    Dana Dorsett | | #3

    It would take quite an air-sealing effort on the house to get an 1860s antique in New Bedford so tight that it needed a dedicated combustion air supply. In fact, warm-humid air leakage into the cool basement in summer will probably remain primary CAUSE of the mold conditions in that location, even after bulk-moisture issues are dealt with.

    Another strong vote for sealing the ducts & air handler, but also for air-sealing the foundation and rim-joists, as well as at the attic:

    In winter the stack effect of lighter-warmer conditioned space air leaking out the top of the house pulls air from the basement into the living space, while pulling denser outdoor air into the basement. By blocking/stopping the escape of air out of both the top and bottom that parasitic air movement slows to a crawl, even if the intervening levels are still pretty air-leaky to the outdoors.

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