Condensate line issues.

[maxster]

1" minimum between the bottom of the 1st elbow out of the pan and the top of the last elbow out of the trap into the run out line. 2" is better on larger style Trane units where the unit is pulling the air thru the coils

[NWAIR]

I have always installed a riser vent prior to the trap to alleviate this problem.

[hands]

I have always installed a riser vent prior to the trap to alleviate this problem.

I think the vent needs to be after the trap, if it's before the trap, the unit will suck air through the vent.

[hvac69]

Vent prior to trap would negate the trap. I worked with a guy years ago that said he used swing check valves, never did it myself so I have no idea if it would work.

[NWAIR]

The vent needs to be before the trap. Yes, the unit will suck air though the trap during operation instead of sucking \ holding the water in the trap. The vent can be screened for sanitary concerns.