A/C Discharge
The liquid formed from condensation of water in an air conditioner is collected and discharged to a wet drain line or to an outside gutter system or to the ground. Some air conditioners drain to the household drain or vent piping, although this is usually a code violation and not good practice. Condensates that discharge into drainage system equipment should go through an indirect waste pipe. Condensate drain lines can be routed to a sump pump or to a washing machine drain or other building drain using an air gap. Condensates should never be discharged downstream of the main plumbing vent stack.
Waste Vent
Running the condensate drain line into a plumbing waste vent is often done, but it’s not a good practice because it can cause the vent to become blocked. If you hear gurgling sounds from your bathtub, this could be your problem. Fortunately, it's not difficult to fix. Just run the drain line into PVC drain pipe and extend it a few inches beyond the edge of your roof. Add a 90-degree elbow, ensuring that it faces down so the condensate will drip on the ground.
Vent Pipe
On some air conditioners, the evaporator coil vent pipe is attached to the primary drain line. The cold air coming out of this pipe when the air conditioner runs can cause condensation on the air conditioning equipment. Some homeowners tape over the vent in an attempt to prevent condensation, but doing so will also keep air out of the line needed to keep the water flowing smoothly. If this is your problem, poking a hole in the tape may solve it.
Safety
Condensate drains should never be connected to house drains or to vent piping because bacteria can grow up the condensate line and enter the building’s air handling system. Dangerous sewer gases such as methane may also be drawn up the drain and enter the air conditioning system when the blower fan is running. The 2009 National Standard Plumbing Code includes standards, codes and good practices for air conditioner installations.