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  1. #1
    Join Date
    Jan 2010
    Posts
    1

    Default Vent or Cold Air Return ? Help

    I need help. The furnas/laundry room of my new wifes house has these sheet metal tubes coming from out of the ceiling. They are located between the furnas and water heater, both of which are gas units. I do not think they should be blocked, or maybe not both. Please look at the pictures and let me know why you think they are blocked off?



  2. #2
    Join Date
    Jan 2010
    Posts
    163

    Default Re: Vent or Cold Air Return ? Help

    Excellent pictures, which make it easy to answer your Q:

    What you are seeing are "combustion and ventilation air ducts", which are code-required for the purpose that the name implies:

    The upper pipe, or duct, is intended to ventilate the space containing the water heater and furnace. The lower pipe is intended to allow outside air to flow into the appliance enclosure, or closet, as a source air that is needed to adequately burn the gas. (It takes at least 15 cubic feet of air to burn 1 cubic foot of gas.)

    Both of these ducts run to the outside air. Somebody noticed the cold air flowing in, and thought it would be a good idea to plug the flow. This may or may not create a deficiency of combustion air--it depends on thing like, how leaky is the house? Is the appliance enclosure open to the rest of the house so as to allow air from the rest of the house to get to the enclosure? Are there exhaust fans that de-pressurize the house?

    For your safety, I recommend that you unplug the upper pipe, at least. Recent editions of the fuel-gas codes have allowed for there to be only one opening, or duct, sized at 1 sq. in. per 3000 BTU's of input for the total of all the appliances in the enclosure. For example, if the WH is 30,000 BTU's and the furnace is 60,000 BTU's, the total would be 90,000 BTU's and the required area of the combustion air pipe would be 30 sq. in.

    The code also allows you to consider other sources of outside air that could contribute to the required amount, such as air infiltration (air leaks from the outside), but the calculations necessary to determine the proportions are complex, since it's hard to quantify the amounts without testing methods that are not easily accessible to the average homeowner or inspector.

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