If you take an 2-liter bottle of soda and rotate it 180 degrees, you’ll see the soda flow out, then a sudden inrush of air as the pressure is equalized and then the process repeats. It results in a “gurgling” sound. This same process occurs in sewer and drain lines and results in the familiar sound of a gurgling toilet.
Early plumbing designers understood this phenomena and designed vents as a secondary method to allow air into the system air to equalize the pressure. Sometimes, however, one or both, become blocked or clogged resulting in a gurgling toilet. Overwhelmingly, the problem of a gurgling toilet results from one of two problems.
Blocked Main Line
The main sewer line in a residential home is set by local ordinance but is usually a 6” inch diameter pipe. Obviously, anything larger forced into the system will cause a problem. Tampons, diapers and any other material may be easily flushed down the toilet but they swell and will eventually block the system.
Then as other smaller debris such as toilet paper, grease and hair, add to the buildup until the water and air no longer flow freely. When this occurs, the sewage backs up until sufficient pressure builds and it passes the blockage. As the air rushes past, it produces the gurgling sound.
The solution to this problem is fairly straightforward, if a little messy. Every residential property has a mainline “clean out.” This is an exterior entry point into the main sewer line that allows the use of an industrial sized drain auger or “snake.” Clean out is usually quick and easy.
As mentioned, vents are the secondary method for allowing air into the plumbing system. They are intended to provide the same steady stream of air used to equalize the pressures within the system. Problems with vents come in two main varieties, blockages and vent pipe movement.
Blockages are usually from leaves or snow. On occasion, birds will nest in a vent or an unfortunate animal will become trapped and die inside. Obviously, the solution to obstructions is to remove them and, possibly, to increase the height of the vent pipe.
As for vent pipe movement, sometimes, as a house settles, it will crimp or unseat a plumbing vent pipe. Simply check, restore and reseal them and they should function normally.