An interesting little part of the condensate drain in a residential air conditioning system is the p-trap. Note that it is called a p-trap because of its shape and that it is not a “pee-trap”. That is something completely different. So, NO, that is not what that little vent pipe is for. You’re just going to have to climb down the ladder and use the restroom like a civilized person.
I don’t know why they don’t call it a “u-trap”. Yes, it would make more sense. I had no say in the matter.
The p-trap traps condensate (water) so that air cannot pass through. Because the coil is under positive pressure when the system is running, air would rush right out of the condensate lines. The p-trap helps prevent this. Condensate trickles in from the coil side causing an equal amount to trickle out the other side and down to a sewer drain or some other acceptable location.
Code requires a vent-T that allows air to get in behind the escaping water. Someone must have thought that relatively large amounts of water would be passing through, prompting the need for the t-vent. Normally a vent like this is required to prevent the drain from gurgling or trapping air, much like vents used in sewer lines. I’m quite sure that condensate drains would work just fine without the vent-T, but it is required by code.
The sad thing is that even though this is a fairly simple concept to understand, all too may times it is installed with the vent-T on the wrong side of the p-trap, making the p-trap completely irrelevant. While not a big deal (the leakage out of the t-vent is only a few cfm), it does say a lot about the installer. I can’t tell you how many times I’ve seen this. I would guess probably 30% of the time. If they don’t understand how a p-trap and vent works, how are we supposed to trust them around gas piping and refrigerant lines?
By the way. I did my BPI field exam in a friend’s house a while back. They had a brand new furnace in their attic. The p-traps were wrong. We also discovered that the gas line leaked where it was attached to the furnace. It was only finger-tight. They never used a wrench to tighten it down. Coincidence?
Here is a little quiz for you. If the pressure inside the coil is 90 Pascals, how much higher will the water level be on the right side of the p-trap compared to the left side in the diagram above? Answer below.
If 249 Pascals equals one inch of water column (iwc), then 90 / 249 = 0.36 iwc. So the water would be displaced by 0.36 inches. In the lower digram it would not displace much, if any, because the pressure is escaping out of the vent-T.
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Russell King, M.E. 
Sep 22, 2012 @ 15:14:01
I commonly see this problem and many times the installer will put tape over the Vent-T to stop the air leak. You have indicated that, aside from building code, the vent is not needed. Other than having air escaping through the vent-T, are there any other dangers or potential problems if it is backwards or taped off?
Sep 22, 2012 @ 18:48:56
Hi Roy, Thanks for commenting.
There are not any serious problems with it being backwards unless it is one of those one-piece, pre-made types where one side is higher than the other. Notice in my diagrams that the outgoing side is lower. If it is higher, the condensate could back up and overflow. A problem with taping the vent (or not having a vent) could be that algae grows quicker. I’m not sure. I’ve heard of algae getting so bad that it actually plugs the condensate line. That’s all that I can think of.
Sep 26, 2012 @ 09:45:55
Airflow out a drain with an improperly placed T can cause water backup in the coil, which may lead to leakage into the ahu or duct work. The trap prevents airflow, and thereby prevents air from “blocking” water flow out the drain. It’s also a fair “duct leak”
Sep 26, 2012 @ 09:49:03
The vent is as necessary as it is on your toilet or sink. Without it the flow could potentially suck the water out of the trap, defeating the purpose of the trap.
Oct 27, 2012 @ 15:51:28
The code (UPC) & (UMC) requires when the condensate line is more than 25 feet in length that a vent “t” be added. This breaks the vacuum that may be set up because of the “p” trap at the beginning of the drain system. Think of it like this: a straw with water / soda drawn to the top, now hold the top with your finger to prevent air from entering the top, and the water / soda will stay inside the straw until you release the top and let air in. When a condensate piping network has a vertical column, then a horizontal line, (particularly when the horizontal line is long,) the water tends to hold like a vacuum. You have to install a second tee somewhere in the horizontal to break the vacuum. Otherwise the water may not drain properly. This scenario has muffled many a service man… 🙂
Oct 27, 2012 @ 16:27:01
Thanks for the comments, everyone! I just can’t imagine that much water coming out of a residential coil for the vent to be needed. Especially in our dry climate. In order for a vacuum to be created, the outgoing water, downstream of the vent would have to completely fill the cross section of the pipe.
I have relatives in Louisiana where it is considerably more humid than it is here and there I’ve seen a constant stream of condensate coming out, but still nothing even close to completely filling the pipe.
Oct 30, 2012 @ 09:48:24
Especially when we get monsoon weather up from Mexico….. this year has been the wettest in years. Also note, that because of contractor failure, designs are more latent than sensible. (Insufficient airflow.) These failures are due to contractors failing to provide sufficient supply / return ducting. As a result, more condensate than expected. When a coil is placed in the attic fur space, and the drain runs horizontal, then vertical, then horizontal, the opportunity for water to build is available and create the blockage I’m speaking of. After 42 years in HVAC, I’ve seen a few. (And scratched my head trying to figure out the why – ) As a result, letting in air through a vent is a necessity.
Jan 11, 2015 @ 19:00:41
If a condensate pump is used, is a p-trap still required? Is a typical pump air tight enough to serve the same function?
Jan 11, 2015 @ 19:51:17
Hello,
Most condensate pumps are the sump pump type and have their own little reservoir that is open to the air. Think of the pump as a small bucket with a drain that the condensate line from the AC coil goes to. The line between the AC coil and the pump is pretty much like any other, so yes, it needs a p-trap and vent as shown. But, the line after the pump is usually a rubber hose and it does not need a p-trap in the same way that a gravity fed line does. Sometimes the installation instructions will call for an inverted U or P trap, but no t-vent. I’m not sure exactly why, but be sure to follow all manufacturers instructions.
Jan 12, 2015 @ 17:57:23
Condensate pumps pump water under pressure and have lift. If the lift is high enough (say 13 feet or more) there may be an opportunity for the water to rescind back into the pump when it shuts off. If this happens, the pump has excess water in it’s sump and will pump out again. Sometimes it may cycle quickly trying to pump out the excess water that keeps “rolling” back in. The “t” portion on a 3/4″ drain pipe to the pump is necessary to prevent siphon-age. When a “p” trap is used to prevent airflow from interfering with water flow, and the pump at the other end has some residue water storage in the sump, a “vapor lock” of sorts is set up. A “t” to let air in after the trap is needed to prevent this “lock” from occurring.
Jan 12, 2015 @ 21:09:59
Great points.
Thanks, John.
Russ
May 26, 2015 @ 05:46:34
I am in Ahemdabad India – we dont have very strict building code and one major issue I face is bad ordor comming in from the Indoor unit of split ac. This I was made to understand comes in via the condensate drain pipe. I was wondering if a P Trap be able to prevent this bad odor from entering into my room? Right now the there are no valves or traps attached to the drain system…!
May 26, 2015 @ 07:52:18
Hello! Yes, that could definitely be a cause. It depends on where the condensate drain goes to. If it ties into the sewer system, you could be getting sewer gasses. I assume that AC coil is on the supply side of the air handler. This is most common. When the fan runs, the coil is under positive pressure and pushing air out. Smells should not get in while it is running. But when it stops, gasses can rise back into it. So, the smell would only be there when the unit first turns on and then stop coming out of the vents after a few minutes. It may still linger in the house, though. If the coil is on the return side, it will actually suck gasses into the house when it is running. A properly installed p-trap will fix both of these problems.
Another cause of odors could be stagnant water in the drain pan. I imagine that it is very humid there, so your drain pan never gets a chance to dry out. If the coil is not level the water will puddle in one corner and smelly things will grow in it. If the smell is more like a frog pond, then that is probably the problem. If it’s especially bad the mossy stuff can actually clog the condensate line and make the pan overflow. Making sure it drains properly and regular cleaning will fix that. There are special products for that, but I imagine that a mild bleach solution would work OK too.
Good luck and thanks for checking in!
Russ
Feb 17, 2017 @ 18:23:53
I think your picture diagram should say negative pressure not positive. It says coil under positive pressure. It wouldn’t need a vent t at all because the air flow is pushing not sucking. Some people do use traps on a positive pressure system. If it was a long run outside I would understand because of the possible cfm loss but that’s not much at all. Some people do use traps on a positive side coil application. You don’t need a vent t though. Let me hear your input though or your reasoning. Also I don’t think that is code like I said a negative pressure Air handler usually.
Feb 21, 2017 @ 15:16:59
Hi Daniel,
This is a supply-side (blow through) coil so the coil is definitely under positive pressure, but the pressure stops at the P-trap. That is the purpose of the P-trap. Without it the condensate line would be an air leak. Everything down stream of the P-trap is neutral pressure but could be subject to a negative pressure if condensate were to completely fill the pipe and flow downhill, similar to how a siphon works, even with some air bubbles in the tube. The vent T breaks this siphon. I personally find it hard to believe that there would be that much condensate, but we are a pretty dry climate here in CA.
Thanks for your comment.
Mar 04, 2017 @ 17:01:04
My condensate drain had no trap, but did have a vent pipe blowing cold air into the attic. I removed the vent pipe and installed a trap as per the diagram. Thanks to Russ, I no longer hear gurgling noises coming from the bathroom sink drain when the AC is running. So there’s the benefit! I didn’t use a vent pipe, but may add one after reading some of the comments here.
Mar 07, 2017 @ 07:22:16
I’ve heard of that happening, but not first hand. Thanks for sharing! Unless you live in a super humid climate and/or the condensate line has any flat sections in it, you could probably live without a vent pipe. They are only needed if so much water fills the pipe that the air can’t get past it. Most systems only have a small trickle of water.
Jul 21, 2018 @ 21:44:29
We are in Florida and think we have this problem after a new HVAC was installed according to the “wrong” diagram above. However, the leakage IS pretty major. We are using a turkey baster to suction water out of the t-pipe (placed before the p-pipe) and we have collected 2 liters in 4 hours. Installers are coming in the morning to fix it and explain themselves but if this was the problem and not an actual block somewhere else, well, I’ll be linking back here in my yelp review. Thanks for the explanation!
Jun 12, 2020 @ 12:43:41
Hi Russ, I’m a civil engineer specializing in septic systems (also have developed an IoT system for septic systems called SepticSitter. Anyway, a lady contacted me through our web site who is having a sewer gas/septic odor problem inside her home that was most pronounced when the shared septic tank was recently pumped.
I have been trying to make suggestions for potential plumbing problems which could be allowing septic gas inside her home. An Inspectapedia article mentioned: “An improper connection of the condensate drain from an A/C or heat pump system directly to a sewer line or drain / waste / vent piping system can permit back-drafting of sewer gases through the drain system and into the HVAC system air handler where in turn it may pass through the HVAC ducts into other building areas.”
I was just wondering if putting the Vent T in the wrong place is the kind of “improper connection” they are talking about or if its likely something else.
Thanks,
Kelly
Jun 12, 2020 @ 14:11:30
Hi Kelly,
It absolutely could be a cause of the problem. It’s all about pressures. Air moves from high pressure to low pressure. When the AC system is running the evaporator coil (the box that the condensate lines are coming out of) is under high pressure, so air will be coming out of it. If there is no p-trap, that air blows down the drain pipe, if that drain pipe goes down a waste vent, it will pressurize the waste vent pipes with air. From there, the air/gasses will go toward the path of least resistance, which could be into the house, or into the attic where it is somehow getting into the house through air leaks (recessed can lights are notoriously leaky). I know that in commercial buildings a common cause of sewer smells is when the p-trap in a floor drain in a bathroom dries out. The janitor has to remember to dump a bucket of water down the floor drain once in a while to refill the p-trap. Is it possible that the homeowner has a dried out p-trap under a seldom used sink or tub somewhere in the house? Also, if the AC ducts leak and the leakage is greater on the supply ducts than the return ducts, the house could be negatively pressurized. This would certainly pull smells into the house too.
Let me know what you find out.
Russ
May 25, 2022 @ 14:11:57
Hi, Russel, you talk about only the case that “cooling coil placed after the fan-blower section”. You forgot to talk about the more popular case that “cooling coil placed before the fan-blower section”.
May 25, 2022 @ 14:56:35
Hi Sam. I’m used to seeing those referred to as blow-through or draw-though coil setups. In my experience blow-through (coil after the fan) is far more common. The main difference will be that in a draw through coil (coil before the fan) the pressure inside the coil case will be negative, so the blue arrow in the diagram would be going into the coil rather than out of it. Everything else still holds true. The vent needs to be on the other side of the trap. I never thought about it before, but if the air was blowing in through the trap really hard, it could affect the ability of the condensate (water) to flow out or it could make it splash over the edge of the drain pan. I hope this helps.
Aug 10, 2022 @ 17:11:57
Just a quick note I don’t think I sent others mention this but there IS situations where a vent prior a trap or vent no trap is necessary. This is on negative side evaporator coil installations. The coil is not always on the positive side of the fan. In high velocity Unico fancoil units which we install regularly, they require vent prior trap and come with a manufactured vent trap that comes with the units for this reason. The evaporator coil is before the fan and obviously under negative pressure and with out the vent it pulls the condensate from the drain pan into the blower compartment, destroys motors, electronics and more!. I believe this setup may be relevant to downflow furnaces aswell. Hope this helps someone !
Aug 10, 2022 @ 20:19:04
Thanks for your comment. That’s an interesting perspective that I had not heard before. I think the goals of what you mention could be still accomplished with the vent on the downstream side. As a background, the whole purpose of why I wrote that article was because so many installers were failing the mandatory duct tests in CA. When we told them that the vent was a source of leakage because it was on the wrong side, they would argue about it. Hence the article. The installation you describe would still be a source of leakage, it’s just in the opposite direction. If the concern is water blowing back into the coil, this could be addressed by making the upstream side of the trap much higher than the downstream side. Water will only go backwards if the level of the water in the trap is sucked up that high. If the pressure in the coil is negative 1 inch water column, the maximum height differential in the two sides of the water in the p-trap will be 1″ higher on the negative side. As long as the p-trap has, say, 2″ of differential above the low pressure side, no water should ever leave the trap. Also, the whole reason for the vent is to keep water running out of the drain further downstream from siphoning water out of the p-trap. If it’s not on the downstream side, why even have one? I’d be interested in Unico’s reasoning for this.