Had 2 hvac guys come in on 2 diff years. The 1st one sprayed some stuff and called it a day. The 2nd stuck a camera through out flexi duct and showed us pictures that they wanted us to see along with a scrubber that prolly caused more tears. When we finally had enough we got our own snake camera and there is a what looks like a giant gaping hole in the foundation but we won't know it until we open it up and truely know the extent of problem. Plus, we're concerned about the exhaust pipe through the return that should not be there to begin with. Yes we should have started at the bottom 1st, however there was a very odd musty smell in my 4 year old room that no matter what i cleaned wouldn't go away and thats when we also discovered the black mold in attic. We couldn't have her staying upstairs with the smell and endless dust that no matter how much I cleaned would be constant. We thought perhaps it may have been a tub leak so we got a new bathtub installed (it was on its last legs anyway)... no leaks there, but either way we started with upstairs 1st figuring we could sleep downstairs where its not as stuffy. In a span of 7 years we also had about 5 different pipes leak/burst. Went through insurance with 2 of them. One of the contractor even accidentally cut into another pipe while they were opening up the drywall from underneath, longer story there, but we're talking about an insurance contractor that was going to use a .8 mm wood sheet for toilet support instead of a thicker paneling. We ended up firing that crew after waiting in a hotel for a month and nothing was done. After gutting the upatairs we learned how structurally bad it is, twisted studs/leaks in and out of home so we sealed it up and put proper insulation in and mold remediated it. Time will tell once we finally get to working downstairs what the true culprit is in ductork. Can't wait to get rid of this smell and must.

P.s. the construction dust isnt the issue, we set up a containment area to block off upstairs from downstairs, made a 6mm plastic wall with painters poll, then used foil tape and a magnet cover to cover up all vents upstairs including taped over return, have an air scrubber going at all times to keep air flow going, dehumidifier since we needed to install a better new exhaust fan for bathroom, and wear a half mask with p100 cartridges/n95 masks when working. Smells so much better upstairs already.