You lost me in the loop-t-loop but the primary and secoundary lines might be reversed,leading to all water going to emergency pan.

Your coil and associated condensate pan (inside the air handler) probably need to be cleaned to restore normal flow of condensate.

Is this a new installation? Is there a P-trap underneath the insulation? If not then that is probably the problem. There are You Tube videos explaining how this works.

If its a new install, get the installer back to look at it. If its not, check you furnace manual to make sure the primary and secondary drains aren't reversed. Some better quality furnaces have the P trap built inside the furnace - two P traps don't work well together.

The primary and secondary drain ports are identified by letters S and P.

This appears to be a coil. Unlikely to have a built-in P-trap. The product manual would indicate if it does.

Thanks everyone for your quick responses. I was calling it a "j loop" or whatever, it was late, ha. I knew it began with a letter. Yes, that is a P-trap under the insulation. I can pour water before the p-trap and it drains with no issues without backing up into the unit. Not a new install. recently purchased house.

I had already confirmed the primary and secondary, they are correct. Actually learned a bit there when taking apart to troubleshoot, noticing a little "gate/tab" on the secondary. Which explains how that works. I wondered how that worked.

I'm not sure if it's always or sometimes but the coil drain pipe for one of my AC coils is for sure under positive pressure and some air blows out. And it doesn't have a trap. I asked the HVAC contractor about it and he said code here doesn't require a trap (or at least didn't at the time, a few years ago) and they leave them off because they can be a source of blockage. I asked if the absence of a trap didn't cause some air loss and negative pressure in the house and his response was something like "Yes, a little, but not that much. The trap will dry out when the AC isn't in use, and especially during heating season when there's hot air blowing down the pipe. So, why bother?"

I don't know if he was describing common practice in this area or just his spin on how he likes to do it. I don't know if it matters but this unit is in a garage. There is a drain pipe that they poked through to the crawl space, it empties outside the house under a deck, a down-sloping gravity run.

I never mentioned code. But what do I know... Your "HVAC Contractor" doesn't seem to understand the operating conditions while cooling, however.

I'm gone.

How so?

The hit and run comment gets filed in the round file. I was watching a YouTube video done by an HVAC contractor in the South who, when fixing another's hack job, made the same comment, to the effect of "The coil has positive pressure. There's a trap in the condensate line, it's really not necessary". I don't really know or care if there is a consensus view but there seem to be opinions for yea and nay, notwithstanding stax's unenlightening comment.

Why is the riser there? Why is that airflow so strong?

The riser above the P trap is to allow you to clean the P trap. This is why the cap is not glued to the riser. You have a positive air flow set up. Air is being blown onto the coil. The air will escape out either drain.

Do you have a 6 inch level? If so check to see if the pipe from the coil to the riser is pitched down. You need at least a 1/4 inch drop per foot. If it is level or slightly pitched upward, what may be happening is when you remove the cap, the air flow is strong enough to overcome the slight backward pitch. This allows the water to flow down to the P trap. The fix for this is to adjust the pipe so it is pitched downward.

With the riser removed there is very strong air flow coming from the drain where the riser was removed, at the top of the T. I mean a lot of airflow. Why is the riser there? Why is that airflow so strong? With the riser in place, there is a very strong airflow coming from the secondary drain.

In HVAC everything has a cause and effect. The strong air flow is because you need air for AC. If that air doesn't have enough places to go you force it out thru every crack and crevice it can find.

Well that's cold air we are losing into an attic or where ever. So instead of fixing it, we will just cap it off.

Now that the air is capped off from exiting the drain tube it is forced further down stream. It can't go down the drain tube because it has a trap. SO now what you've done is created a strong suction pressure that causes the drain to back up and the next place for the water to go is? Thru the secondary into the emergency pan.

The duct system is likely undersized (an educated guess). The air for AC doesn't have enough room to flow and by capping off the primary port off the trap you create a new problem.

There could be additional reasons why. Each configuration can have challenges as well as different reasons why something is happening or not.

I service the Katy, Texas area.

Chas Ross if you let A Air get away with that you'll never again be able to look at yourself in the mirror.

No response needed for an explanation defying the laws of physics.

There is some of that going around with another poster.

"Now that the air is capped off from exiting the drain tube it is forced further down stream. It can't go down the drain tube because it has a trap. SO now what you've done is created a strong suction pressure that causes the drain to back up and the next place for the water to go is? Thru the secondary into the emergency pan."

Whaaaaa what? Ray, have you fried your brain in the Houston summer heat?

The removable cap on the vertical riser above the trap is to provide easy access for cleaning. Condensate flows down the drain driven by gravity. If you've got condensate flowing from the emergency drain you've either got a problem with the pan under the coil leaking or not pitched toward the primary drain outlet or a blockage somewhere between the pan and the exit of the condensate drain outside the home (I believe all of the above have been mentioned by previous posters.) I've found the exit of mine blocked by spiders making a home inside it during the "off season" and by landscapers that covered it with mulch. The OP will be well served to make sure that whatever liquid they pour down the drain actually exits where it's supposed to.

Ray's explanation reminded me of Dr. Doolittle's pushmi-pullyu. Both are entertaining works of fiction, albeit the pushmi-pullyu is somewhat more believable than Ray's explanation.

For your intertainment on tonight's So You Think You Have Talent we have Ray competing against pushmi-pullyu to decide this season's champion. Now here's your host Chas Ross.

No response needed for an explanation defying the laws of physics.

You put a straw in a soft drink from your local c-store / restaurant.

You suck the straw and the contents in the container you are sucking from magically defy the laws of physics to enter your mouth.

So simple an engineer can understand it.

Whoa wait a minute the liquid in the cup you put the straw in those contents are held there by gravity. Congratulations open door #3 to find out what you've won. LOL.

The op said: "I can remove the riser at the T just before the "J" loop or whatever is is called, and pour water in, and it drains with no issues. It drains as fast as I can pour."

The suction has been broken while pouring.... well us engineers would rather create more problems in our own minds first. Why bother ourselves with what someone is telling us in written form.

Ray,

You might want to re-read the original post in its entirety. Here's some helpful content:

"With the riser removed there is very strong air flow coming from the drain where the riser was removed, at the top of the T. I mean a lot of airflow."

Air flow exiting the riser with the cap removed doesn't suggest suction--just the opposite. It's positive pressure and a normal operating condition.

Air flow exiting the riser with the cap removed doesn't suggest suction--just the opposite. It's positive pressure and a normal operating condition.

Well there's plenty we don't know from a forum board.

My job isn't to teach "former" engineers how to do it.

If it was "normal operating conditions" we wouldn't be discussing ways in which to fix it. Would we?

I've about stopped trying to help folks that drop in to ask a question in their first and last post but I'll try and help one more time.

SOoo if you haven't already disapppeared into the wild blue Cooper,I promise I can help you fix your problem if you promise to come back to let us know how it worked out.

Ok,I have popcorn and a drink,back to the regular program.

There are no blockages, I checked and double checked.

So we put everything back and the over flow starts dripping again.

It's like deJa vu all over again. ~ Mike, pure gold there.

Enjoy your popcorn klem1. The engineers, not so much. We must be at the nexus of the universe because this thing is defying the laws of physics!

Ha, ha, ha, ha. (Even I enjoy a good laugh)

~If I've seen it once, I've seen it a thousand times... just not on a forum board. ;-)

A skilled, experienced practitioner knows that when a homeowner says they've checked and checked, you still need to verify their assessment (no offense intended, Cooper.)

Condensate can drain from a partially blocked drain line. The problem occurs when condensate is generated at a rate greater than it can drain. In order to ensure there is indeed no blockage, someone would need to see free flow at the drain's exit point, not infer the lack of blockage because they can pour some quantify of water down the drain without it backing up. That's a two person job. Most pros would simply blow out the drain line to clear any potential blockage.

Former Engineer/ Home Builder: Condensate can drain from a partially blocked drain line. The problem occurs when condensate is generated at a rate greater than it can drain.........

From the OP: I can pour water before the p-trap and it drains with no issues without backing up into the unit.

~It's like déjà vu all over again.~

How is it going? "No response needed for an explanation defying the laws of physics."

ha, ha, ha, ha.