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  1. #1
    Barbara Miller's Avatar
    Barbara Miller Guest

    Default Water heater duct & attic insulating


    Our basement water heater vents out of what used to be a gas furnace flue through our attic (we now have a furnace that vents out of our basement.) I notice that even when the water heater is heating, the 8" or 10" duct hardly feels warm up in the attic.

    My plan was to make a jacket out of flashing to keep our attic insulation at least an inch away from this duct, but since it doesn't get hot, I am wondering if (non-combustible) Roxul mineral wool could be used right up against the duct.

    I will check with our local MA plumbing inspector, but since codes sometimes don't make such distinctions, I am curious about whether, for all practical purposes, my rationale makes any sense or whether there are other issues to consider. I did air seal the utility chase where this duct is located at the attic and basement levels, with flashing and intumescent caulk.

    Thank you for any comments.

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  2. #2
    Join Date
    Mar 2007
    Atlanta, Georgia

    Default Re: Water heater duct & attic insulating

    The problem is the exhaust flue is over sized. Typical water heaters use a 4" flue. With an over sized flue, the exhaust gases are likely cooling to fast and falling out the damper hood in the basement leading to potential carbon monoxide poisoning.

    There should be an owners manual with the water heater that describes the proper size flue. If the owner manual is missing, use the web to contact the manufacturer for the proper size. Hire a competent plumber to install a new correctly sized flue before your family dies of CO. At the same time, the plumber should correct the insulation around the flue in the attic.

    "The Code is not a peak to reach but a foundation to build from."

  3. #3
    Barbara Miller's Avatar
    Barbara Miller Guest

    Default Re: Water heater duct & attic insulating

    I have almost always seen 4" water heater ducts connecting to much larger metal furnace flues, so I'm interested to look into this further. Luckily our CO detector in the basement and on other floors isn't making noises.

    Thank you.

  4. #4

    Default Re: Water heater duct & attic insulating

    I have seen a great many water heaters "backdraft" flue gasses into the home, some over 200ppm and the CO alarm did not make a sound.

    This from our Kidde CO detector:
    This Kidde carbon monoxide alarm meets response time requirements as follows:
    At 70ppm, unit must alarm within 60-240 minutes.
    At 150ppm, unit must alarm within 10-50 minutes.
    At 400ppm, unit must alarm withing 4-15 minutes.

    When I perform home performance testing, BPI requires me to abort testing if the ambient CO reaches or exceeds 35ppm.

    Consider hiring a licensed plumber to fix your flue.

    Michael Carson
    Inspect It Right Home Inspections L.L.C.


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