Air Conditioning - Air Conditioning smells like the sewer

My heating/air conditioning unit is 17 years old. For the past 3 years
periodically throughout the day when the air conditioner is turned on all
vents blow out air that smells like the sewer. It also happens immediately
when only one faucet of one of the upstairs bathroom is turned on. When
a small amount of bleach is placed down the sink, the smell of bleach is
immediately detected all throughout the house. I do not understand the
relationship between the plumbing system and the air conditioning system.
I put some bleach in the pan under the condenser but that did not help. To
my knowledge, all systems are closed without any leaks. Under the sink in
the upstairs bathroom as mentioned above, I could not see any other drain
pipes that lead into the main drain where the trap is located. I want a professional opinion on this as no one has been able to give me a difinitive
answer or logical explanation to say nothing of a positive diagnosis. Can
someone out there please help me? Thank you so much.

Sounds like someone tied the evap pan line (a-coil in furnace) to the sewer line in the house. Perhaps you are getting sewer gas backing up through the line into the coil case housing and getting sucked into the duct. Not sure but perhaps you should check to see if there is a water "trap" int the line that way the drainage water acts like a trap (like a toilet) and sewer gas cannot back up.

well it sounds like the drain from your ac unit was piped into your drain line from the bathroom and now its getting backed up and going into your ac unit throw drain line . i would fine a new place to run the ac drain too since the drain its tied into is not draning like it once was

You counld possiblr sucking the vapors from the sewer vent. You may have a vent that became plugged. As Lennie said, move the drain. If you are draining to a floor drain try to find a laundry tub if possible.

Dick