A "sanitary tee" is a specific kind of pipe fitting used where waste is involved (hence "sanitary") that allows one pipe to join in with a main run (a "T" shape). It is different than a "straight tee" in that the way it is shaped, flow down the main branch doesn't have as much of a tendency to flow into the branch.

Energy, the installing contractor has already demonstrated that they don't think it is their problem. I should have written my response differently. I think that the OP needs an inspector that is competent with moisture/mold issues. I think that they need an independent authority to confront the contractor, if is is an HVAC problem and to guide them to fixing other problems if it is not HVAC. I agree that It would be better if this person is not someone that does remediation and stands to profit further. The single picture that was posted, and the description indicate that there could be severe damage much like a home with a burst water heater or other standing water in the house for weeks. At the very least, there is a lot of clean-up that needs to be done and a lot of that is beyond the capability of an average home owner. I find it difficult to believe that, even with heavy rain, a properly-designed HVAC in a normal house would do that. The evidence points the new HVAC system as the culprit assuming that is only thing that changed in this house, and that mold has never been a problem. Now they have an unlivable home with "...mold on everything.... " The OP needs data (humidity) and an independent source of advice to confront the contractor and maybe for court. This is going to be an expensive mess to fix, with either sweat equity or for contractors. OP, start with general home inspectors like the one that did your home inspection when you purchased the house. Look for one that has personnel that has specialized training in mold and moisture issues within that group. Your best bet might be to look at larger firms first that have several inspectors. With some luck, you might find someone that can tap expertise in s a certified Home Efficiency Ratings System(HERS), Residential Energy Services Network (RESNET) Member and experience in the mold remediation industry all under one roof. It is likely to cost some hundreds, but with your problem, I think it will be money well spent. Put in a call to your insurance agent as well.

That's horrible! I have no idea where anyone would turn to in this situation. Maybe the county health department? I hope someone can help.

That’s what I figured. I know they’re trying to use the trim to cover up their mistake of not removing the carpet that was previously there earlier. I’m going to have them remove it and texture the wall approximately so there’s nothing running across the air vent. I did play around with that idea, but our intention is to keep it as open as possible and get new railing. I appreciate the feedback!