What is the fourth pipe in the plumbing under my sink?

Question

I think most sinks have three pipes into the wall under the bathroom sink. The hot water, cold water and drain pipe. I have a fourth pipe that connects above the trap. What is this pipe? You can see a picture of the pipes under my sink here:

http://thenadlers.com/bathroomsink.jpg

I have turned the water shutoffs off and disconnected the trap and there is water leaking from this fourth pipe which would normally go down the drain. I found this problem because I think the drain got clogged and the water backed up into my sink. So first, I'm not sure what this pipe is for. And second, I'm not sure why water would be coming in through it.

Thanks very much!
Eric

The air conditioner condensation line sounds likely. This picture is from my upstairs bathroom, the kitchen is downstairs. One other sink downstairs has a fourth pipe. I have two AC units, both above each of the "fourth pipe" sinks.

Answer 1

Unless your ac is on the roof, it is very unlikely to be a drain line for condensation. Is your water heater in the attic? That isn't done in California where I am, but I know it is in some areas with colder climates. If so, it may be a drain line for that. It is not plumbing for a double sink, double sinks are routed through one waste line. How old is your home? It may be a retrofitted vent stack, although I can't imagine why it would have water in it, unless it has been raining in your area.
How much water is coming out of that line? With a flashlight, see if you can determine which direction it goes inside the wall. This should help you determine what the line is for. It it goes up, it may be a vent stack. If it goes any other direction, trace it back to the source.

Answer 2

Usually the life of plumbing is dependent on the water condition. Galvanized piping rots out faster when the zinc plating is washed out. But this is not the usual case. Most failures are due to calcified deposits closing off the passage of the pipes. Traditional re-piping involves removing the lath or gypsum board on the wall so the pipe can be properly mounted. It is a small matter to repair the wall. usually harder to repaint. ( color matching and all..) You will certainly want to get the best of 3 bids on this kind of job, and be sure if the wall repair is included in the bid. If the pipe failure was due to rotting out then I understand why the plumber would suggest it. But if it is copper pipe or such, rotting does not occur unless you have acidic water. (unthinkable). These joints are simply cleaned and re-soldiered. Look for discolorations near the line of the pipe to indicate wetness from leakage. Listen to the wall and especially to the pipe. You can hear the leak. Look at the base boards and floor to find signs of water staining. These are a few clues to leaks. Leaks don't go away so you are looking for 'wet' signs. The only outer signs of failure in a pipe are bulging and visible corrosion. that is when the damage is immanent. But the real signs of pipe health are inside the pipe. You would need to run a bore scope into the pipe to see this. (yup, endoscopy for plumbers.)

Answer 3

Go up on the roof and pour a gallon of hot water down the PVC Vent pipe and have someone feel the pipe under the sink to see if it is a vent. If so, then pour some Liquid Plumber snake foamer down it and let it clean its self out. do it in the day time so you can open a window in case sewer gas backs up. Soap Suds going down the drain build up in the drain and chokes off the vents, so rinse your suds with cold water every time your use soap.

Answer 4

That is a connection from a condensate drain. Central AC is nearby above this sink?

Answer 5

It is hard to say for sure. There is often a vent pipe that vent gases outside the house. the way that is set up in your house if it is a vent pipe water could easily get into it. Also if the vent outside is open water could get in it that way. If you look out side on the wall or on the roof in the vicinity of that pipe you should see a vent. If you do that is probably what it is.

Answer 6

there is something upstairs or maybe an air conditioning unit in the attic if you have no upstairs.but it has been added to that drain in the photo link it looks like someone took the easy way out to add a drain line.

Answer 7

it looks like something else drains into this, have a sink outside? washing machine? whats on the other side of this wall or upstairs? I've not seen this done before.

Answer 8

This is a set up for a double sink. Where does the upper end go it doesn't show in the pic.

Answer 9

I have seen this arrangement when there is a dishwasher or washing machine in close proximity, it is the discharge pipe.

Answer 10

that is the pan drain pipe from your a,c. its called the condensation line.