Gurgling Sound in the Shower Drain When the Toilet Is Flushed
When you flush your toilet, it releases one or two gallons of water into a 3- or 4-inch drain line. As the water rushes to the sewer, it pushes air ahead of it and creates a vacuum behind. The function of the toilet vent is to allow air into the line to fill this vacuum. If the vents are blocked, the air has to come from somewhere else, like the shower drain. Does this Spark an idea?
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The Purpose of Venting
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Plumbers first started using P-traps in the mid-19th century. A P-trap has a length of pipe shaped like an inverted "P" in which water collects and forms a barrier against the incursion of sewer gases and vermin into the residence. The vacuum created by rushing water in the pipes sucked the water out of the traps and caused them to fail until an unknown plumber devised the idea of venting drain lines to allow air to fill the vacuum. The first vents were made with 1/2-inch pipe, but when they proved inadequate, plumbers gradually increased the pipe size to its present 2 inches.
Symptoms of Poor Venting
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The sound of gurgling coming from the shower when you flush the toilet is one of the most common symptoms of poor venting. The sound is caused by water rushing through the P-trap. As the water in the bottom of the trap is displaced, it forms waves that pass air in cycles, creating the characteristic rhythmic sound. You can further diagnose a venting problem by noting how fast the toilet drains. If the venting is inadequate, the draining will be sluggish. You may also notice slow movement in the sink and shower drains.
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Causes of Poor Venting
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Obstruction in the vent lines or at the vent openings on the roof is the primary cause of poor venting. The main vent usually extends vertically from the main waste stack. There is an elbow at the point of connection, and debris falling in from the vent opening can collect there and eventually cut off air flow. It's also possible for debris to collect on the vent opening itself. This is especially likely if there are trees overhanging the vent. Small birds or animals can climb into vents and die there, and it's not unusual for a vent to be blocked by a baseball or tennis ball.
Clearing a Blocked Vent
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Climb on the roof to clear a blocked vent. It's a good idea to bring a garden hose or a plumbing snake with you. If the vent opening itself is blocked, clearing it is easy, but more often the problem is somewhere down the vent line. Spraying into a vent opening with a garden hose is sometimes enough to clear an obstruction, but you may need to use the snake for more stubborn ones. Feed the snake into the vent, in the same way you would a plumbing drain, unwind it and crank when you hit resistance. Reel it back in when the obstruction is gone.
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References
- Photo Credit red bathroom image by Jeffrey Sinnock from Fotolia.com
