[Zeke]

The vent stack is at atmospheric pressure at all times. There are vents that would not be appropriate but terminating into the and main vent should satisfy the need for the condensate to fall freely into air. There is a trap (or should be) at the evaporator unit preventing ambient sewer gasses from entering whatever space the thing occupies. Some are in a hall closet, others in the garage and some in the attic. All of the above have the potential to be in a closed place if not in a conditioned air space.

Getting the condensate out of any contained space doesn't have to be hard. But since there are many bozos installing HVAC, our code is what it is in order to prevent them from becoming creative and not understanding the requirement. All it has to do is run free of the pipe into normal atmospheric condition. By gravity, even if you have a pump at the evaporator.

The potential for methane build up is minimal, but it is there. Methane is lighter than air.