Thread: vent on wrong side of p-trap

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Trap can be sucked dry allowing sewer gas to get past the water seal in the trap.

If the drain is on the negative pressure side of AHU, then you can draw sewer gas and/or attic air into the system.
If the drain is on the positive pressure side of OAHU, then you lose conditioned air into the attic while it is running and air can move either way during off cycle.
The trap comes first, then the vent downstream of the trap just like in a sink or other fixture. Keep the conditioned air in the air conditioner.
Of course there should not be sewer gas since the condensate drain should discharge into a trapped drain with an air gap.

Jeff,

Usually in new construction you will have the builder tell you that they install that vent for the homeowner to add bleach every Quarter to 6 months.

Tell them that that's ok, but they will need to cap it off and tell the homeowner to remove the cap when they add the bleach, then replace it when they are done.

Now, the issue still remains that they need an adequate vent AFTER the trap to allow for proper drainage.

The only thing that is accomplished if the vent is BEFORE the trap is that air escapes into the attic and as Jim stated, the vent can dry out. When the system is under negative pressure meaning is draws air thru the line, the primary condensate drain line siphons air thru the line, possibly bringing some pretty nasty smells from the sewage drainage into the unit and re-distributed into the home... Nasty stuff.

Richard

Sorry Jim, I was typing as you were posting...

RR

In most cities you can not dump condensate into the sewer system because it is clean water....do you guys see this setup very often?

Clean water not allowed in sewer systems?

A lot of municipalities around here don't allow sump pumps to discharge into the sewer, and probably AC condensate as well. The idea being - if everyone does it, the water treatment plants get overloaded with water that don't need no stinkin' treatment!

correct, no condensate, sump, or water treatment discharge allowed. City systems can't handle the overload.
Septic systems can't handle the mineral build up.
Now back to the real world.