If your toilet bubbles while the tub drains, you may have noticed that the tub drains more slowly than it should, and there is a simple explanation. The vent lines, which are installed to allow air into the drain lines, probably are blocked or they may have been installed incorrectly. Without venting, the water flowing from the tub creates a vacuum that slows the water and sucks air through the toilet, which causes the bubbles.
The Purpose of Venting
Plumbers invented the P-shaped pipes attached to every drain, called P-traps, to prevent the incursion of sewer gases and vermin into sink and tub drains. They soon discovered that water rushing through the pipes created a vacuum that sucked out the water unless they added another pipe with the sole purpose of allowing air to fill the vacuum. This extra line is called the vent, and it is required by modern plumbing codes on all drain and waste lines. It usually extends horizontally from the drain, after the P-trap, and then changes direction to run vertically through the roof.
The Force of Suction
When water rushes through a drain line, it pushes air in front of it and creates a vacuum behind. If there is no air in the line to fill this vacuum, a suction force is created that can be strong enough to slow the water or stop it altogether. It also can be strong enough to suck water out of P-traps connected to other drains on the line, but before this happens, air usually rushes in through the traps, displacing the water and causing them to gurgle. When the traps eventually empty, sewer smells begin emanating from the drains.
Symptoms of Poor Venting
Slow draining accompanied by gurgling in nearby pipes are two symptoms of poor venting. Because a toilet releases a large amount of water into the pipes in a short time when you flush it, gurgling sounds from the sink, tub and shower drains usually are the first indication of a venting problem. The obverse, bubbling in the toilet when you open the tub drain, isn't as common but is just as possible. The bubbling represents pockets of air moving into the waste line, not out. You'll often hear gurgling sounds from the sink at the same time.
Correcting Venting Problems
You usually can clear blocked vents by climbing on the roof and clearing obstructions from the vent opening. In some cases, debris may have fallen into the line and collected at the point where the vent line curves upward. You often can clear this by spraying a jet of water down the vent with a garden hose or using a long plumbing snake. Prevent future obstructions by fitting the vent openings with caps or grates, but be careful not to obstruct the flow of air by fitting them too tightly. Check the vent openings periodically and clear them when necessary.