I smell a dark heavy musky smell that comes and goes. It will travel around in my apartment, but it is also always in one spot...right in front of my ac closet when it does come about.

Although, whenever I stick my head underneath the compartment or in the AC closet I don't smell it. Just right in front of it a few feet away.

I took some pictures of the AC air in take.

Is the grey stuff on the wood mold?

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  • surely looks like it Oct 14, 2017 at 22:40
  • 1
    Do I see that right, there is no duct work connecting the filter? It just pulls air from the crawlspace???
    – Jack
    Oct 15, 2017 at 0:34
  • Yes, that is correct...the AC sits above the board platform and sucks air through that square cut out. Is that normal for air intakes?
    – dman
    Oct 15, 2017 at 1:44

1 Answer 1


The gray stuff on the side of the joists should simply be dust that also lines the interior of duct work when the ducts are present.The picture does not show everything but I presume the concentration of dust is there because the foundation is close to the area and the air is drawn through the space concentrating the dust that will accumulate. Again, this is a presumption, I have the same thing in my duct work, but coating all the sides because of the liner.

Onto the "non-question" question. I do not know what part of the country you are in, or continent, but in the USA in all cases the returns are enclosed. Some jurisdictions allow the joist runs to be "panned" - a piece of sheet metal that covers the entire run of the cold air return so the return air is supplied by the interior of the home, not the crawlspace, as in your case. Other jurisdictions require that the entire run be set in duct work, not just the bottom pan, but all 4 sides. I think the last is for fire safety. Some states or counties are more stringent than others.

  • Thank you. Would this be causing the musty old wood smell that comes and goes in areas of my apartment?
    – dman
    Oct 15, 2017 at 17:19
  • Not the dust itself but the actual air being drawn from the entire crawl space could be causing the musty smell
    – Jack
    Oct 15, 2017 at 17:26

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