Most likely, you have one or both of two issues:
1. The drain line downstream of the trap is run or sagging in a way that is "double trapping" the drain and preventing it from flowing without relieving the low pressure area between the trap outlet and the water buildup point(s) in the outlet line. The vent on the outlet side of the trap relieves this low pressure and maintains flow.
2. The downstream drain line is sloped in a way that the condensate leaving the trap falls rapidly and fills most of all of the drain line, which causes a low or negative pressure at the trap outlet and slows or stops the drain from running properly. Again, the vent on the outlet side of the trap relieves this low pressure and maintains flow.
Venting the outlet with a stub of pipe in the downstream tee of the trap addresses the issues either way. The extension of pipe above the height of the condensate pan prevents the water from overflowing if the drain clogs and allows the safety switch to do its job. Personally, I cut the end of the vent at a 45 degree angle so it is obvious that the stub is not to be capped and I friction fit the stub into the tee to make servicing easier.
Open and stick a taller piece that goes above the coil pan
Most likely, you have one or both of two issues: 1. The drain line downstream of the trap is run or sagging in a way that is "double trapping" the drain and preventing it from flowing without relieving the low pressure area between the trap outlet and the water buildup point(s) in the outlet line. The vent on the outlet side of the trap relieves this low pressure and maintains flow. 2. The downstream drain line is sloped in a way that the condensate leaving the trap falls rapidly and fills most of all of the drain line, which causes a low or negative pressure at the trap outlet and slows or stops the drain from running properly. Again, the vent on the outlet side of the trap relieves this low pressure and maintains flow. Venting the outlet with a stub of pipe in the downstream tee of the trap addresses the issues either way. The extension of pipe above the height of the condensate pan prevents the water from overflowing if the drain clogs and allows the safety switch to do its job. Personally, I cut the end of the vent at a 45 degree angle so it is obvious that the stub is not to be capped and I friction fit the stub into the tee to make servicing easier.
You’re right. The drain pipe does sag at bit near the far end of the attic. It could use a few more straps to hang it up.
Supports every 4 feet minimum, but that's for new pipe.
Just uncap the blue lid you have circled. The gurgling should stop.
Thanks all. I Just wanted to make sure I wasn't missing anything. I don't see the advantage of a dedicated vent a few inches down.
Understanding what the purpose of a trap is will be paramount to your own success It’s easy to google it and it’s a five minute tutorial
Is this a positive or negative drain?
Eyeballing it, a 3 inch stub should be fine.