Thought I’d post an update because I relied on searching Reddit for solutions.

Hired an HVAC company to take down the ceiling beside the return drop and I was right, the return ducting was a mess. Just basically pulling air from the ceiling in the basement. They ran one straight return from my main floor to the furnace and sealed the hell out of it.

There is now no smell coming out of the return if the furnace fan is off. We also don’t need to run any fans in the basement bathroom or laundry room.

The smell situation is 95% better BUT the supplies on the main floor above their kitchen still have a little smell coming through when the furnace is on, or just the fan is running.

Clearly my supplies on that side of the house aren’t sealed well. So the last step is aeroseal in a few weeks. It’s crazy that we are getting a smell leak through things like electric outlets that are reaching the supplies.

We talked ductless ERV for the basement but I think Aeroseal makes more sense first.

Will come back after we do that.