residential hvac question?

Yeah you can cap it but the fact that it is blowing out cool air makes me quesiton if it has a P-Trap in it. The proper drain would have a sufficient dip between the drain outlet and the air handler. The 'secondary' drain that you see is more than likely a cleanout access in case the drain becomes plugged somewhere between there and the main drain of the house.

Back to the 'P-Trap' It would look almost like a goose neck and it serves the same function that the 'trap' or gooseneck of the toilet does, It creates a barrier for the air. Although the trap on a toilet prevents odors from coming back up the drain line the water barrier or P-Trap on a air handler would prevent air from blowing out or being sucked in to the airstream.

If you do have a P-Trap then it may need to be primed by removing an air handler access cover and pouring a descent amount of water into the condensate pan. If you are lacking a P-Trap on the drain line for the condensate then capping the cleanout/secondary will not stop air from blowing out of it, it just won't blow into the closet, it will be pressurizing the drain and blowing it further down the drain. To install a trap if it doesn't have one you could cut into the existing drain and add a P-Trap yourself by routing a U shape in the drain that drops roughly 2 to 3 inches center to center. Before doing so I would remove the access panel and trace out the drain to the condensate pan, it may be trapped internally to the air handler unit and just need to be primed.

Additionally if the cooling runs long enough then likely it will eventually produce enough condensate water to prime the trap automatically.

EDIT : Figured I'd check back, the purpose of the P Trap on the unit condensate drain is to isolate the air stream in the ducting from the closet it sits in, the main drain it goes to, etc. The trap works by creating a water barrier between the two. If you don't have a P Trap on the condensate drain then it was likely an oversight during install, it should be trapped. Since you sound pretty sure that it isn't trapped you can install one yourself and utilize the secondary or open part of the existing drain as a source to prime the trap. You would want to cut out the existing drain leaving about a 1" nub off of the existing drain coming from the coil. Install a 3" trap as mentioned before, and on the outlet side of the P Trap install a T that would leave one end vertically open for access and priming. This will make it easy to prime since it sounds like your coil and condensate pan have difficult access points.

Additionally whether your system is creating a positive or negative pressure on the coil side it may not drain properly and will allow condensate to back up in the coil pan due to air flow through the existing drain. This may eventually overflow from the pan into your ducting and/or the air handler compartments. This is why it needs to be trapped and primed.

sure you may merely replace the coil. by way of fact it incredibly is an older equipment. it a especially worry-free replace out.. You pump the unit down so which you deliver all the refrigerant to be saved interior the condensing unit. Then with a torch the might unbraze the previous coil. eliminate, reinstall, set up a sparkling drier (if warmth pump bi-bypass) Pull a vacuum turn provider vales back on and your achieved. examine rigidity and on condition that choose suitable off. additionally, if it replaced into that undesirable seem on the blower wheel. The blower wheel paddles could desire to be cupped. in the event that they're caked with grimy and are close to flat. you're able to desire to eliminate the blower assembly. Unplug 2 screws and the entire deliver jointly comes out. which will additionally make a extensive progression for air bypass. the clarification for the crappy coil is from the previous proprietor not changing the filter out. As I cant see the coil, Ive seen some especially undesirable ones. Ive merely wiped sparkling them with coil cleanser. sure, you may get the finished unit replaced. If the coil is in a seperate container (cased coil) then that makes it large worry-free to alter out. additionally, If I have been you i might sparkling the exterior unit. feels like the final guy or woman did 0 PM paintings on the equipment. Goodman make a especially solid unit. as with all equipment no pis* undesirable upkeep ends up in what your coping with now

Yes you can plug it.

You'll notice that this second tap is a tad higher than the primary drain. This is an auxiliary drain which won't start draining water unless the primary is clogged. It's often piped in as an "alert" drain, piped to terminate where the draining water will be noticed, but not cause any damage. As often as not they are plugged and not used unless the unit is in an attic or somewhere that condensate overflow can cause major damage.

Hope this helps