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I observed that when I enable air-conditioner, after awhile the valve (red one on the photo attached; correct me if it's called differently) starts leaking, the water drips from under the red "valve". The pipe which is next to "red valve" is fine, no leaks, it withdraws the water in to drain hole in the floor.

I wiggled it, but it looks it fits in quite tightly. I'm not sure if it is possible to replace it or put some rubber ring under it.

Is it common problem? What would you recommend to do? Thanks.

Furnace Valve

  • What JPhi said -- can you see air in the clear tube, or is it completely filled with water? – Hot Licks Dec 4 '19 at 23:28
  • @HotLicks, actually the clear tube doesn't look like it is completely filled with water at all. The other end of the tube has the water really slowly dripping. – Mark Dec 5 '19 at 2:27
  • Well then the problem is simply a bad seal with the plug. It may be as simple as removing it and coating the seal with Vaseline. – Hot Licks Dec 5 '19 at 2:34
  • @HotLicks there may well be a sludge dam in the pan that is partially restricting flow – Kris Dec 5 '19 at 23:45
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That plug is where I add biocides to prevent mold buildup in the pan, mold is the main cause of obstructions , if the pan is starting to drip you probably have some cleaning to do as the water should not build up in the pan and a small layer has probably raised the water level because it is acting like a dam.

I have had customers ask how can mold grow in there it’s only water and metal, well this is almost true but dust and fine organics get trapped in the coils and drip down to the pan, when the conditions are right a mold colony starts growing, For this reason pros recommend having your unit serviced, adding this biocide treatment is a requirement for large commercial systems to prevent legionnaires disease, I have not heard of it in home systems but it is possible. So you may need to clean and treat your pan or eventually the drain line will be totally blocked and the water may come out in other areas less noticed and cause problems.

  • Legionella is starting to be a bit of a problem at home too, thanks to new relatively low temperature heaters (power efficiency, scalding protection etc.). To cause the disease, you usually have to have a compromised immune system, so it's easy not to notice the growing colony. – Luaan Dec 6 '19 at 8:14
  • I believe it can be a problem but may not have shown up in home systems because of the very small amount of water and with things working correctly it is not really stagnant. I have use sanitizer tablets since the mid 80 on commercial systems and started the smaller pill size ones for home systems back around 2000 , kind of expensive per bottle but a bottle lasts years and eliminates the possibility of both mold and legionella so I would say that is a win win. – Ed Beal Dec 6 '19 at 17:12
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I think the other answers make it clear what's going on, but I wanted to add some more details. That red cap is probably not meant to be water tight, so when the water level in the pan is high, it will drip.

What is common in my area is that there is a "main" drain line, usually plumbed with PVC pipe (not that flext tubing you have) that will drain condensate into a drain pipe in the house somewhere. The second drain hole will be plumbed to a "backup" drain that will drip in a safe, but obvious place. It might drip out of the eaves of the house or onto the driveway, etc. The point is when you see that dripping, you know the main drain is clogged.

In cases where a secondary drain is not feasible or to make sure a leak never gets out of control, an A/C tech can install a float valve on the second drain hole that will cut power to the unit when the first drain gets clogged. The drain water backs up into the second hole and trips the float valve. If the A/C can't run, it can't generate condensate and leak. You're forced to fix the drain problem rather than ignoring a drip from the secondary drain.

Anyway, my recommendation would be to make sure that primary drain is going where it is supposed to. It really should be re-done properly with PVC. Also, installing a float valve is pretty important in your case because an overflow in the pan is going to drip water all over your furnace, possibly causing some expensive damage.

  • I would disagree with a float valve , a simple biocide added to the pan will prevent mold growth , and the line that is there is recommended by several MFG’s so the condition of the line can be viewed. – Ed Beal Dec 5 '19 at 15:39
  • @EdBeal Correct me if I'm wrong, but I thought the float valve was to prevent leaks, not mold growth. How is a biocide going to prevent leaks? – Glen Yates Dec 5 '19 at 17:44
  • @GlenYates, the only reason you typically get a leak is if the primary drain clogs and the pan fills up, and the only reason that normally clogs is mold growth.. I agree with Ed to a degree, but people forget the normal maintenance, so a float valve is like a failsafe to shut the system down if there is a problem. – JPhi1618 Dec 5 '19 at 18:30
  • @JPhi1618 - Agreed, the float valve is a failsafe, and can be very important depending on where your air-conditioner is. Where I am from, many AC units are (stupidly) placed in unconditioned attics above living spaces, and the drain lines run to the outside where they could easily be clogged by debris (not mold). In this case, I would consider a float valve a necessity - unless you like water spots on your ceiling. – Glen Yates Dec 5 '19 at 19:21
  • @GlenYates, I have ceiling damage for that reason exactly. Clogged primary, a tilted backup drip pan that didn't drain, first summer in a new-to-us house and I didn't think to check that. – JPhi1618 Dec 5 '19 at 19:27
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That looks like just a thin plastic filler plug for a second drain hole. If you pull it out or unscrew it there's probably threads under it. You can buy a pipe plug and some thread sealant at a local hardware store to plug it properly.

  • 1
    1/2" PVC NPT plug, and sealant, +1 – Mazura Dec 5 '19 at 1:36
  • Actually, I think those are 3/4". – Mazura Dec 6 '19 at 2:09
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Those two openings are for draining the pan that catches the condensate off the coil. One is usually a bit higher than the other.

Make sure drain line is attached to the lowest hole. If it is correct or or there is no difference I’d look for some blockage in the drain line.

A dirty filter can also cause draining problems by increasing the internal vacuum pressure which holds more water in the pan.

The drain line is not a standard looking arrangement. Usually a 3/4” minimum pvc pipe is used and must have a trap installed to prevent the unit from sucking air back into the plenum. enter image description here

When a trap is missing the suction drawn back into the unit causes inefficient draining that allows the water to stack up in the pan which also could cause your symptoms. For details on why a trap is so important for proper draining see this link

  • thanks for comment. In my case, both openings, one with the clear tube and the other with the plug, are actually at the same level. – Mark Dec 5 '19 at 2:29
  • Ok so look for a blockage. Good luck – Kris Dec 5 '19 at 2:43
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    @Mark - They'll either be off-set or its bottom half is shunt-off. You'll see what I mean when you take the plug out. Even a slight blockage can cause these to leak, where that would otherwise not be a problem if it was capped right. These will leak at some point if you don't take the manufacturer's cover plug out and plug it correctly, which I expect already done from a competent installer. – Mazura Dec 6 '19 at 2:18

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