Furnace: pipe is leaking when switched to air-conditioning

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

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