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Black dust streaks are coming from under supply duct covers. What is it?

eiuckgaMLR | Posted in Energy Efficiency and Durability on

The dust streaks seem to come out the edges mostly, and it doesn’t happen with all of them. Some streaks are even coming out from under outlet covers.

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  1. GBA Editor
    Martin Holladay | | #1

    Forced-air systems pressurize and depressurize different areas of your house. Inevitably, these systems move dust and concentrate it in certain locations.

    Visible problems like yours can be minimized by:
    1. Keeping your house clean.
    2. Enforcing a "no indoor smoking" rule.
    3. Not burning candles.
    4. Regularly changing your system's air filter.
    5. Sealing all duct seams -- including the seam between branch ducts and register boots -- with mastic.
    6. Sealing air leaks in your building envelope (if necessary, with the assistance of a blower door).

  2. eiuckgaMLR | | #2


    Thanks, for your post. The first 4 possibilities are easily ruled out by the clients "squeaky clean" life style. Perhaps, I can use your opinion to bolster my recommendation to do a blower door test, and then air sealing accordingly.
    All ducts are in the attic, (earth contact home,) with the metal main trunks feeding a multitude of flex ducts leading to the register boots. "Sealing all duct seams," from the top will be a do-able job, all be it, very time consuming and body busting.
    We, also, are recommending "over insulating" all ducts as we blow a cellulose R50 topper over the meager R28. It's an all electric heat pump house which is scheduled to be replaced. (Even considering geo-thermal.) All the more reason to get things tight?

  3. GBA Editor
    Martin Holladay | | #3

    Regardless of whether it helps with the dust-distribution problem, the duct seams must be sealed with mastic -- since you now tell me that the ducts are in the unconditioned attic. This duct system is undoubtedly wasting tremendous amounts of energy. If the return ducts are routed through the attic, they may be pulling in attic dust through seam leaks.

    If the home is air conditioned, you can't use deep cellulose to insulate the ducts. Cellulose is vapor permeable; moisture in the hot attic air will diffuse through the cellulose and condense on the cold attic ducts, leading to a soggy mess.

    Before installing the cellulose, insulate the ducts with duct insulation (including an exterior vapor barrier), or encapsulate the ductwork in closed-cell spray polyurethane foam.

  4. davidmeiland | | #4

    You sound like a great candidate for blower door and duct blaster testing. If you can't get a duct blaster you can still determine dominant duct leakage with the BD. I'll hazard a guess that you have one or maybe two returns and a bunch of leaky supply runs. Ultimately as Martin says you need to seal and insulate the ductwork and make sure that the system is not pressurizing or depressurizing the house.

  5. eiuckgaMLR | | #5

    Thanks, for your help, gentlemen, but evidently, I blew this one.
    The client is going to have the ducts "professionally" cleaned; I wonder, will that cause more leaks? Then, there will be a new HVAC installed - all electric heat pump, or, perhaps, a geo-thermal.
    Finally, the walls will be repainted in order to give the new system a clean start.
    And, by the way, It was felt that the blower door test, which is where we as energy contractors always start, was too expensive.

  6. David Meiland | | #6

    It's not for everyone, but I decided to get rid of the customer's cost objection by eating the cost myself and getting my own blower door. It takes maybe 15 minutes to set it up, make the house ready, and get a single point number. You can run around feeling for air leakage from registers with your hand, or get yourself a pressure pan. Running the air handler with the manometer set up tells you something about which side has the greater leaks. The BD is as useful a tool as any of the nail guns or chopsaws I own.

  7. HDendy | | #7

    It seems so obvious that it's not just dirty ducts, since you mentioned the outlets. You'll probably be out to perform the blower door when the problem resurfaces. From your description I would be willing to bet there are supply leaks in the attic (outside the thermal boundary) depressurizing the house and brining in air and dust in the cracks you mentioned. Hard to imagine that a blower door test can't be justified before all the other make sure the actual problem is being solved.

  8. GBA Editor
    Martin Holladay | | #8

    Cleaning the ducts without sealing the leaks is an obvious waste of money. But you know that.

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