I can't tell you how many times I have seen air conditioning condensate being emptied directly into the homes waste plumbing line. Outside of this being against building codes (both common sense and written) it is also unhealthy.
What most folks do not realize is that the condensation drain line is connected directly to the evaporator coil cabinet and that it is under negative pressure. That means that it can actually suck air into the system and then into the living area. This is one of the reasons that the manufacturers require an air gap of trap on the line shortly after it leaves the cabinet. The air gap or trap works until it goes dry, this happens during the winter months when the A/C is not used. The water dries up and the systems starts sucking in air. If it is connected to the sewer line it will suck all types of little nasty's into the air that you are breathing. Legionnaire disease can start this way!
So can you see the direction we are heading and why it is not a good idea to connect a condensation drain line to a homes waste plumbing lines! If the condensation line is connected to the homes sewer system it can draw sewer gas and bacteria into the homes HVAC system and into the living are of the home.
It can drain indirectly to the homes waste plumbing system or it can drain to the exterior of the home, it just can not be connected directly to the homes waste plumbing and this would include the homes vent stacks or pipes.
I found this condensation line connected directly to a 4" waste line, a toilet was sitting directly above and on the top of the pipe! The HVAC contractor did this and really caused a problem. In order for this to be properly repaired the 4" section of pipe will need to be replaced, you can't patch or plug the hole that was made for the condensation line.