It could be that the condesate drain line has been connected to the wastewater/blackwater stack. The trap (hopefully installed) preventing sewer gas from moving up the condensate line may have dried out. If dry, sewer gasses are no longer blocked by the hydraulic seal... resulting in a nasty smell when the unit is turned on and the fan kicks in. Just a thought.
Yes, my brother's house in CT. has the same issue and the cause is still undetermined. I initially thought that the house was so tight, that the fireplace "draw" was pulling the gas right thru the traps. That does not appear to be the case as the smell doesn't appear to be coming from the sink. He doesn't know of any vent inside the house, that could explain this. I'm going to suggest that he crack a door or window in the family room and see if that short circuits the draft and has any effect.
Various fitting degrees
Either you have a blockage present or your vents on your sewer lines have become blocked or you need to install more vents.
Install a one way check valve in the line from the house to the sewer.
ummmmm............. your not supposed to i suggest you get that checked!!
I have seen this where water has leaked into the ductwork and stagnated.
Sewer smell from toilets or some other problem
Code there doesn't always require a 'P' trap to hold water and thus block sewer gases from backing into the house.
Change the wax seal under the toilet
Awful.
Awful.
Lime
It could be that the condesate drain line has been connected to the wastewater/blackwater stack. The trap (hopefully installed) preventing sewer gas from moving up the condensate line may have dried out. If dry, sewer gasses are no longer blocked by the hydraulic seal... resulting in a nasty smell when the unit is turned on and the fan kicks in. Just a thought.
yes. water caught in the drain line may contain food particulate which can decompose and cause gasses to escape back into the house.
Bad electrical fixtures.
poop