Thread: P-trap on wrong side of vent

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Originally Posted by Brett Foster

Another inspector myth....it can be on either side.

Nope, you are incorrect. The main goal of the trap is to keep from sucking air into the evaporator unit. If it is before the trap, it will suck air into the unit. The trap keeps air from entering the unit, so you want any vent after the trap.

Dan, I don't believe this is in the codes. I have only seen it in the manufacturers requirements. This is not to say the some AHJ's might have a different take on it.

Dan,

That does not 'cause' a running trap, a "running trap" is a trap where the inlet and outlet are approximately the same elevation, i.e., you have a pipe and bend the pipe down and back up to the original level.

A "trap" for an a/c unit has the inlet higher than the outlet, typically at least 2" higher.

Otherwise, all you have is a 'bent piece of pipe'.

Originally Posted by Brett Foster

Another inspector myth....it can be on either side.

Brett,
Your running theme seems to be Inspector Myths. May I assume you are not an Inspector? None-the-less:

In the future, instead of just spouting Rhetoric, how about some meaningful backup? Otherwise, you sound like just another guy who got pissed off because some inspector cost you money when you had to fix what you were paid to do correctly the first time.

We have plenty of non inspectors on this board and always welcome more. Nobody here has a problem with being proved "wrong". The object is to be better, not right in our first opinions.

And please do fill in your profile.

In those pictures posted the trap is serving no useful purpose as it is being bypassed.

Best thing to do in that situation is recommend a cap be placed over the existing pipe & cut a tee in on the outlet side of the trap with a standpipe for a vent.

the trap has to be the first thing that water flows through before it enters the rest of the plumbing system. Vents are in the plumbing system, to help water flow once it enters, after the P trap.

Standpipes are a way to enable high flow drains like washing machines to put their water out first and not fight against resistance from a P trap. They are like temporary holding tanks. Not applicable here.

David

I think I will continue to put the standpipes in.

Last time I placed a straw in a full glass of water & pulled that straw out with my finger on top the water didn't flow out of the straw till I removed my finger.

I want as little resistance on that drain as possible while the fan is running if it is on the return side of the fan.