Sink, shower and tub drains often connect to a toilet waste pipe because the toilet pipe has a larger diameter and provides a direct path to the sewer. These connections are what produces strange noises or behavior in the toilet, such as bubbles forming of their own accord, or gurgling. It takes a lot of water in the pipes to make bubbles in the toilet; it usually comes from the washing machine or dishwasher.
Air In the Pipes
It's a good idea to have air in the plumbing pipes; in fact, plumbing code requires it. The air, which comes in through the vents, equalizes pressure when water is flowing and ensures it keeps flowing smoothly. An exit path is just as important as an entrance, or pressure builds in front of a head of flowing water. That exit usually is supplied by the vent as well, but in some cases, the venting may not be adequate. The vent pipes may be too small or in the wrong places relative to the drain traps.
Sink drains don't usually create enough pressure to push air through the P-trap of a toilet and make the water bubble, although they can. Dishwashers and washing machines, on the other hand, have pumps that actively eject water and force it through the pipes. These fixtures often connect to toilet pipes, either directly or by way of a nearby sink drain. It's common for them to be inadequately vented. Their vent pipes may be too small, or if they use an S-trap -- which isn't code compliant -- the vents may be lacking altogether.
Finding and Fixing the Problem Drain
Determining which appliance is draining when the bubbles happen is a good way to begin correcting the problem. It's probably a nearby one, but depending on your plumbing, it could be on another floor. Once you locate the culprit, you need to assess its drain and vent and, if necessary, repair it. If the fixture in question has an S-trap that empties into the floor, you need to replace it with a P-trap. The difference between them is that a P-trap has a length of horizontal pipe on its outflow side to which you can connect an upward-rising vent pipe.
If you can't find any issues with the appliance drain, it's possible the bubbling is caused by a blockage in the waste line past the point where the toilet connects. You may be able to clear the blockage, which is forcing air to come through the toilet, by plunging or snaking the toilet. Another possible cause is a blockage in the main vent stack. You have to climb on the roof to clear it, either by pulling debris away from the opening or spraying water down it. If it's deep and stubborn, you may need an auger to clear it.
- Barry Austin Photography/Stockbyte/Getty Images