Unidentified pipe constantly leaking water outside

Can anyone tell me what a pipe coming out of the house might be? There is a half inch copper pipe with a ninety degree elbow facing down coming out of the house below the kitchen sink area of the kitchen. It is constantly and heavily dripping water. I first thought it might be an AC condensation drainage line. It is not. I have located those. I then thought it might be from the temperature/pressure relief valve for the water heater. It is not. I located that too. The basement is finished, so I cannot investigate further without cutting drywall. What else could this pipe be for?

It sounds like you have checked the most common culprits already.
Did you turn of the main water shut-off and then check the mystery pipe? If water stops flowing, then it could be an open valve or leak upstream of the pipe.

Did you check for a sump pump? This is a possibility.

Did you try to shut off the main electrical breaker? If water stops flowing then it is likely an appliance discharge, not a plumbing system leak.

Please let us know if you solve the puzzle.


I did not try those ideas. They are great ideas! Thank you! I will try today or tomorrow and get back to you.

Years ago I was working on a construction site and a backhoe operator cut a 4" sewer line that wasn’t on the plans. The contractor plugged the line and three days later the gas station across the street said his sewer line was backing up…Owner of unknown sewer line was solved. :thinking:


Half inch copper pipe sounds like some sort of condensation or overflow line.

Was there a water softener installed?
Was there only one HVAC zone?
Was there an air handler anywhere besides the basement?
Was there only one water heater?
Was there a first or second floor laundry?


If the pipe is located under the kitchen sink it might be from a instant water heater that is located under the sink. If there is no water heater located under the sink it may possibly be from a reverse osmosis system?

If none of those two items exist it most likely is condensation from an attic air conditioning unit.

Hi Martin, I thought the same thing, but there is no instant water heater, no filtration device, and no furnace in the attic. It is very confusing.

Ron follow the water. Trace it from the bottom up and find the source.

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No water softener.
Only one HVAC zone.
Only air handler is in the basement.
Only one water heater.
There is a first floor laundry not unreasonably far from the unknown leaking drain pipe. I thought of that but couldn’t think of why it could be involved. The leak is constant.

Is there a way for me to do that without opening up walls? I am going to try Bert Hull’s suggestion first. After that I don’t know what other choice I have. The customer is insisting that someone must have the blueprints for the house and I should be able to find them somewhere. The house is thirty years old. I don’t know how I’m supposed to do that.

Is this from an inspection you performed or are you doing contracting work for the customer?

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The reason I mentioned first or second floor laundry, there may be an overflow pan under the washer, and if there is a leak in the supply valves, hoses, or in the machine itself underneath, it may be leaking into the pan, and although these pan drain lines normally terminate in the basement, the possibility exists that they may have terminated it to the exterior.

Just working the process of elimination.

I’m doing siding repairs on the house. The customer a new about the pipe. I’m a handyman studying to be an inspector