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Why Do I Have a Gurgling Sound from My Sink?

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Do you hear a gulping noise coming from your plumbing? You may think that this is a clogged drain, but it may be something in the opposite direction.

If you read the legal description of a home inspection, you will find the phrase “visual inspection” or some variant incorporating the word visual. A home inspection may be more accurately described as involving all of the senses. Sure, our clients would love us to carry around fancy equipment which analyzes the home to give a detailed technical report; however, there is no piece of equipment that can go through the home to perform an analysis like you. The real problem comes in understanding what you are seeing or hearing. Certain plumbing problems can be heard, like the gurgling sound coming from a sink.
    Here is the path that I took to discover why I heard a gulping noise from a sink in a house that I was inspecting. We first start on the roof. I climbed up on the roof to examine the roof covering and anything else that I may find up there. This included the plumbing vents. These are the pipes that stick out of the roof. I did see some seals from the flashing breaking around these vents, but no other issue. I go up into the attic later, where I find stains on the air conditioning equipment. Those little black mold spots that occur with condensation were the stains. In the attic, this happens when conditioned air of one temperature comes into contact with the attic air at another temperature, so I looked for air leaks in the system, since I was running the air conditioning to check the cooling. I did not feel any leaks. As I moved out of the attic, I walked around the second floor, where I came upon a sink with a strong gulping sound. I had filled the tub near this sink earlier, so I checked the the other sink to hear if a sound came from that location. No gulping here. Then I had a revelation, so I opened the cabinet to find a tube connected to the drain line.

    Where did the gulping noise from the sink originate? All of the clues are in the last paragraph, but you need to understand how your plumbing system works. We turn on the water, and the water flows down the drain. If you have a two year old daughter like me, you may be taking apart your pipes because she decided to stuff toilet paper down the sink. (I had a lot of fun this weekend clearing that out). You will notice that the stuffed drain probably did not make a noise; you just had water that would not drain or drain slowly. For the water to flow smoothly down the drain, you need air in the line. My favorite example is the straw, and the discovery of a vacuum by children. They suck the drink into the straw, then they hold the top of the straw, keeping the liquid in place. Let the top go, and the liquid flows down. This is the purpose of the vents for the plumbing system. The gurgling sound is the water going down the pipe, and then stopping to let air into the pipe.
    In this inspected home, I mentioned that the vents on the roof had no problems. One cause for the sound in your sink can be a blockage of a vent by a bird. Birds will build nests in any convenient place, and I have found them on the top of plumbing vents. Maybe warm air rises during winter? Cool air during the summer? I am not sure why, but this does happen on larger vents. The vent being blocked was not a typical plumbing vent. There is another blockage that can occur that is not easily found. The drains from one part of the house usually share a vent. If the tub draining was causing the gurgling, because a shared vent with the sink was blocked, then I would have seen a problem with the water draining from the tub. I did not witness this situation.
    The oddball clue was in the attic. Remember the stains from condensation that I found? There was no air leak though. When I realized that the sound came from one particular sink, I checked underneath to see a pipe coming into the drain line above the P-trap. In older homes, this line may drain to the exterior of the home closer to the ground. The pipe is the primary drain line for your cooling system for your air conditioner. Your evaporator coil produces water when the air conditioning is cooling the home. This water drains through the primary, but it can drain through a secondary drain attached to a pan underneath the evaporator coil (this happens when there is a problem). In newer homes, the primary drain is often attached to an existing drain line. This is often the bathroom sink.  This is the scenario found under the sink. The primary drain from the air conditioning system was causing the gurgling sound.
   Going back into the attic, I located the vent pipe for the evaporator coil, discovering that it had been taped over. On older systems, there is enough air coming into the evaporator coil from the attic to allow the water to freely flow outdoors. On newer systems, a vent pipe is attached to the primary drain line. This is just an open pipe sticking up shortly after the primary drain exists the evaporator coil. When running the air conditioning for cooling, you can feel cold air coming out of this pipe. The cold air from this particular vent was lowing onto the air conditioning equipment, hence the condensation leading to the stains that I saw.  I have seen some of these vents installed at an angle to force the air away from the unit. Noticing the condensation, someone taped over the vent, forgetting that the vent was needed for the smooth flow of water in the primary drain. By poking a hole in the tape covering the vent opening, the gurgling sound stops.
    If you hear that gulp coming from your drain, then you can be sure that water is not flowing smoothly due to lack of air. Check your plumbing vents. This may require you to go up on the roof. A bird’s nest can be obvious from the ground, but there are other things that can block this vent. If water is draining slowly from another fixture, you may have a problem in the line. With the home mentioned above, we saw that the air conditioning could be the root cause. One last scenario to mention could be how the home’s drainage pipes were installed. A wet vent is where the drainage line from one fixture is the plumbing vent for another fixture. For example, you have a sink in a downstairs bathroom. In the same area directly above this sink, you have another bathroom sink on the second floor. The second floor sink drains down to the main drain on the same line used by the first floor sink. The pipe leading up from the drain is the vent. For the first floor sink, the vent portion is also the drain for the second floor, so we have a wet vent. If for some reason, there is a horizontal section of pipe for this drain/vent, we may have a situation where the pipe allows water to stay in one section like a trap. This would cause gurgling for the sink below this water stoppage, but allow water from the above sink to drain without issue. Once these situations are resolved, you will hear no gulps.

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© Frank Schulte-Ladbeck Professional Home Inspector Houston, Texas
Frank Theodor Schulte-Ladbeck
home inspector, TREC# 9073
Houston , Texas , 77063 United States

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