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  1. #1
    Join Date
    Jan 2009
    Posts
    10

    Smelly Return Air Ducts

    I have an musty odor that comes out my forced air system under two scenarios:

    1. The furnace blower is running and the smell will come out of the hot air vents
    2. The furnace is not running and conditions are just right and the smell will come out of a couple of the cold air returns.

    I believe that the smell is originating from the cold air returns because I performed the following experiment:

    - Run the furnace blower...smell the hot air vents...confirmed smell is there.
    - Put tin foil over the furnace filter and open up the door to the blower on the furnace...run the furnace blower (only draws air from the basement now)...smell the hot air vents...confirmed there is no smell.

    The smell seems to be worst when it is rainy and humid, although I am not sure which one of these factors is causing the problem.

    The smell also seems to be worse in two ducts that run on the same wall from the basement. The ducts are basically just run between studs in my wall with no sheet metal liner. I stuck a flash light and a video camera down the ducts and didn't notice any mold or water...but I'm not exactly an expert.

    Any ideas?

  2. #2
    Join Date
    Aug 2003
    Posts
    906
    Could be anything, from a dead rodent to mold. Who knows. What does it smell like? If you can't tell, ask friends (good ones lol). What's the humidity like? Maybe a dehumidifier for your basement would be a good investment.

  3. #3
    Join Date
    Jan 2009
    Posts
    10
    The smell is very difficult to describe. I guess I would say it is musty, but it is hard to really pin it down.

    I doubt it is a dead rodent because we have smelled this on and off for a couple of years now.

    I have a Santa Fe dehu in the basement. The basement is consistently 37-41% RH during the warm months.

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,338
    Do you smell this musty odor only when you're heating the house? Only in cooling? Or both?

    The enclosed joists used as ducts...are these return or supply ducts?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Join Date
    Jan 2009
    Posts
    10
    I get a "mild" dose of the odor through the hot air vents whenever the furnace blower is turned on. It does not matter if the AC or heat is running...I get the odor either way.

    When conditions are just right (i think it is some combination of heat, humidity and recent rain) there is a stronger odor that tends to waft up through the cold air returns.

    The enclosed joints that are used as ducts go to the cold air side of the furnace. So, when the furnace blower is switched on it pulls air from various rooms in the house through the enclosed joints.

    All of the hot air ducts are lined.

  6. #6
    Join Date
    Aug 2003
    Posts
    906
    Quote Originally Posted by doylebry View Post
    I get a "mild" dose of the odor through the hot air vents whenever the furnace blower is turned on. It does not matter if the AC or heat is running...I get the odor either way.

    When conditions are just right (i think it is some combination of heat, humidity and recent rain) there is a stronger odor that tends to waft up through the cold air returns.

    The enclosed joints that are used as ducts go to the cold air side of the furnace. So, when the furnace blower is switched on it pulls air from various rooms in the house through the enclosed joints.

    All of the hot air ducts are lined.
    All I can think of is that it is pulling air from somewhere it isn't supposed to. That somewhere would get very humid in those conditions and develop mold. Is it possible that the end of the joists isn't sealed well, pulling some air from outside when the furnace runs, perhaps through the walls even. That infiltration then becomes very humid or even condenses when it cools at home temps. After a while the smell (mold) develops. Just an hypothesis.

  7. #7
    Join Date
    Mar 2005
    Posts
    997
    Id check to see if the liner has mold on it.

  8. #8

    same problem

    doylebry - read your description of your musty odor (describes my problems almost exactly) and what you have already tried and was wondering you have tried since your post? what is working for you?

  9. #9
    Join Date
    Jan 2009
    Posts
    10
    I could write an entire book on this, but I'll try to keep it short. The source of the odor at my house seemed to be mouse urine. Don't laugh...you can't even imagine this smell until you experience it. I almost fell off the ladder I was standing on the first time I smelled it up close. Then, afer I caught my balance I dry heaved.

    Anyway, the mice were in an air pocket above a bay window in my kitchen. Between the ceiling of the bay window (inside the house) and the roof of the bay window (outside the house). Now, imagine a box that consists of the kitchen ceiling on the bottom, the 2nd story floor boards on the top and the 2nd story floor joists on the sides. This created a tunnel from the bay window to a wall on the far side of my kitchen. This wall contained the two return air ducts. So the odor would waft through this tunnel and flow straight up the kitchen wall and out the return air ducts on the second floor.

    So, I removed and replaced all of the contaminated material in the bay window along with the entire kitchen ceiling (sounds like overkill, but the odor had soaked into the unfinished side of the drywall). Then I did my best to seal off the return air ducts and the bay window from each other (lots of rigid syrofoam insulation and great stuff foam sealant to fit in any open spaces).

    This seams to have put things under control, but I still am not sure if it is totally fixed....I'll post again if I have any new info.

    The bottom line is that there is no magic solution. You have to find the source and do your best to eliminate it. Once you have done that you can go the route of trying to control the residual odor with tricky stuff like charcoal filters and UV lights. But you first have to find the source and this will take a lot of time (in my case almost a year of all of my free time) and money (well over $1000 in diagnostic and investigative work from building engineers, inspectors, etc).

    My advice would be to talk to people and see if they have suggestions on how to design some experiments to help you narrow this down. It could be a million different things causing your problem so you need to start figuring out some ways to reduce the number of variables (i.e. does it happen when it's humid or when the sun hits one part of your house or when you run a bathroom fan, etc.)

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