Handymen! AC Condensate Line...?

Factual Questions
1

My AC’s condensate line is apparently clogged. The line comes off an AHU for a heat pump (split system) design. The line is PVC, and I can cut into it easy enough to snake, but…

I cannot see how the PVC interfaces with the drip pan below the coil. (I opened a side panel, but the drain line comes off about 6-in lower. The view is blocked by the “V” shape of the coil. The twin PVC connections (which tee together downstream) must be at the vertex of the “V”. But, what dows the PVC/drip pan interface look like? Is it a nipple connection? Someone sugested it is more like a “pin drin” (smaller than a nipple) which may (possibly) be clogged with rust.

Any advice as to what I can expect to be snaking into would be helpful; links to pictures or diagrams on the web would be awesome!

Bonus Q: Does the condensate line need a “P” trap? I wouldn’t think so, but I was advised to put one in while I am fooling with the PVC line. Is it really necessary? (I know it cuts down on a backdraft which can pull condensate the other way and/or hinder drainage, but we’ve been in this house 7 years now without incident. So, do I really need a P trap?)

Please help!

  • Jinx
2

A couple of questions. Is the AHU/evaporator located in an attic or otherwise above habitable space, or is it in a basement/crawl space?

Whether or not you need a P trap is determined by where the drain line terminates. For example, a basement unit which drains condensate into a sump hole would not need a trap. An attic unit draining to the exterior of the dwelling would need a trap, to prevent critters from using the line as a means of entry.

I typically connect the drain line to the evaporator with a full union, so the trap can be cleaned out using a shop vac in either blow or suck mode.

3

Typically, the main drain pan has two female threaded fittings. The lower one is the primary drain, and the higher one is the secondary drain, and they should not be connected together. The secondary drain should terminate at a conspicuous location, such that when you see condensate draining from it, that indicates that the primary drain is clogged.

4

I have nothing particularly constructive to add, other than to say that I spent most of today cleaning my AC drain for the upteenth time. I find out that it’s clogged when my carpet goes squish. WHile you’re in there, you probably should check / clean the evaporator coils - mine were really dirty (of course, this system is possibly 20 years old). I find that my drain gets plugged with dirt/dust/glop from the evaporator - I used and air gun and a shop vac with a small straw taped to the nozzle to clean everywhere I could reach.

Good luck.

5

Thanks all, for the replies. I’ll have to try to mate the wet vac’s blower to the condensate line and test the results. The weird thing is that one second I was standing next to the AHU on a dry floor. And, in a blink, water was all over the floor around the AHU! Was it building and building just waiting to overflow the drip pan?

Now, some clarification: My AHU is in center of the basement. The condensate line runs across the floor to a (French?) drain. It is open to atmosphere, not flooded (such as by submersion in a sump.) As for the twin drain connections off the pan, they are at the same height, and they are teed together into the common condensate drain line.

As for my question if the drip pan has a “pin drain”, it is possible that on the inside of the drip pan, the drain resembling an ant trap (a raised section with a hole at each corner) that leads into a nipple connection. I noticed this when examining my dehumidifier’s drain pan. Maybe that’s what someone was trying to describe to me regarding the drip pan’s drain.

Last, I like the idea of adding a union for easy access, but is there an easy way to remove the existing PVC piping where it is glued to the nipple connection without damaging the nipple? If so, this would allow me to re-route the piping as I like from start to finish. Any ideas?

Thanks,
Drain-o Jinx

6

The PVC isn’t going to be glued to the evaporator. As mentioned, the evaporator has a pair of female NPT fittings, so once you saw the line apart, so long as you have enough of a stub to solvent weld on either side of the cut line, you should be good to go.

Since your unit is in the basement, you don’t need a P-trap. You should identify and correct the cause of the blockage. In some cases, I’ve seen poorly fitting filters/wrong size filters/what filter? which allow the evaporator to gunk up with crap. Ensure that your filter fits properly, and change it regularly to reduce evaporator fouling.

Dirty evaporators can be cleaned with compressed air, a shop vac, and if you really want to get thorough, use a product called Alka-Nu. This concentrated cleaner (sold by HVAC supply houses) is sprayed onto the evaporator with a pump tank, and allowed to foam off the accumulated dirt, before rinsing with clear water.

If your evaporator is old, and has been draining poorly (level and plumb is important for proper drainage), you may find that the bottom of it is deteriorating and blocking the line with rust flakes. If that’s the case, you’ll need someone to measure, draw, and have a new base/pan fabricated. There’s usually enough wiggle room to pull out the old and slide in a new pan assembly without having to D/C the evaporator.

7

Sometimes, when the blower shuts off, a lot of water held on the surface of the evaporator coil will fall into the drain pan. If the drain is clogged and the pan is already full, you can get a fair amount of water spillage quickly.

Your primary and secondary drains are teed together because 1.) there’s nowhere to direct the emergency drain to alert you to a clogged main drain and 2.) there’s no reason to route an emergency (also called ‘sight’) drain anywhere because the unit isn’t above a finished ceiling and a clogged drain isn’t going to cause damage to the structure.

Also, if you look closely at the two drain connections, I’m pretty certain you will notice that one is slightly higher than the other - the difference may be 1/2" or less. In normal operation, water will never run down that connection. But when the main drain gets even slightly clogged, the water will rise to the emergency drain’s level. In your situation, you’ll never know it has happened until that drain is also clogged and you have wet socks.

I’ve never seen anything like this in a condenstae drain pan, and I’ve seen a lot of them.

The nipple is almost certainly screwed onto or into the drain connection. You will need to cut the pvc pipe some distance away from the conection and then unthread it. You can certainly remove, replace or reroute the entire system to your liking if you feel that it is necessary. I have never taken the step of installing a bona fide union connection, but I may next time. What I have done instead is to cut the pipe to allow me to clear the drain, and then coupled it back together with clear pvc tubing and hose clamps for ease of future removal. It will clog again.

My pleasure.

8

Your air conditioning coil (the evaporator) is either under negative pressure, or positive pressure, depending on it’s relationship to the blower. It is therefore, either a “draw through” (negative) or “push through” arrangement.

An easy way to tell is to disconnect your drain and turn on your blower. As danceswithcats noted, it will normally be a 3/4 female threaded connection. Is air blowing out at you through that hole? If so, your coil is in a positive environment. Is air being sucked into the hole? If so, your coil is in a negative environment.

As why/how this relates to your drain…Years ago we put traps in part to keep critters etc out. There is another, different, reason for adding a trap nowadays. (and that reason applied to some better constructed units back in the day too)

That air you felt blowing at you (positive) or sucking in (negative) is often strong enough to keep water from draining from the evaporator.

Try to visualize this: You [re]construct your drain. The A/C is on, and humidity (water vapor) is is being condensed to water, dehumidifying your home. Those droplets make their way down your coil into the drain pan, where there is a big 3/4 hole for the water to drain out.

However…air is either rushing out, or sucking in through that drain you constructed, enough that the water isn’t allowed to drain. This applies to either a positive coil arrangement (where the air is pushing out) or a negative coil arrangement (where the air is sucking in). The velocity, flow rate and pressure is strong enough to essentially “fill the hole with air” and not allow, or give room, for the water to drain out.

It is common to see drain pans filled with water, despite the fact that there’s a big 3/4 hole in the side of the pan for the water to drain. Many times I’ve seen drain pans simply overflow the sides (and all over the floor/ceiling) and people concluded that the drain was clogged, and it wasn’t. The hole was rendered useless by air flow/velocity/pressure from the blower. The air flow essentially “closed off” the drain hole, filling it with air.

A common evidence is what Rhurbarb describes when he said, “Sometimes, when the blower shuts off, a lot of water held on the surface of the evaporator coil will fall into the drain pan. If the drain is clogged and the pan is already full, you can get a fair amount of water spillage quickly.”

That is a classic sign of a unit that needs a trap. The water is being held in the pan by air pressure—positive or negative—and the moment the blower stops (eliminating the pressure differential) and the coil is at atmospheric pressure (read: gravity drain) water come poring out. It was the lack of a trap that causes that condition.

Now I would guess that many (most?) homes that don’t have traps have this problem. Why aren’t more homes experiencing water all over the floor? Because in most instances the thermostat satisfies before the pan overflows it’s sides. And…when the thermostat satisfies and shuts the blower off the water "held’ in the pan comes tumbling out. In short, the A/C and blower didn’t run long enough to overflow the sides, and once the blower shuts off the pan is able to “relieve itself.”

The long and short is this: Most homes could use a trap, IMO. For energy efficiency reasons, many AHUs and furnaces are being made with tighter cabinets, and so there is less air “leakage.” That exacerbates the problem.

There is a product that is a wonderful trap that I highly recommend. It’s clear (so you can see if it’s clogged) and even comes with a brush. Here’s just one place that sells them. You can also find them on ebay.

Lastly, the condensate comes from the A/C system, and if your AHU/furnace is near carpeting, or in an attic above a finished space, for example, you can put in a “wet switch” that disables your A/C system before water ends up all over the floor or coming through the ceiling. It is cheap insurance and lets you know you have a problem before the water lets you know.