Recently, the drain in my second floor tub makes a gurgling sound when I flush the toilet, which is located right next to the tub. They are obviously on the same drain line. I know that there is a vent line going up from the bathroom through the attic, but I am not sure of the exact configuration.
In all other respects, both toilet and tub seem to drain normally.
What could be causing the gurgling?
Two options. Air is going in, or air is coming out.
"Gurgle" is caused by air being forced through the water in a fixture's trap. It's similar to the "glug" generated when you pour milk quickly from the milk jug. It's air being forced through liquid, in an attempt to equalize pressures.
Vents placed strategically throughout the plumbing system, typically provide an adequate amount of air. If the vent(s) get clogged/blocked/restricted, pressure differences inside and outside the system will develop. If these differences in pressure are large enough, air will be forced through fixture traps in an attempt to equalize the pressure.
In this case, clearing vents; or adding additional vents in the case of improper venting, will usually resolve the issue.
If a large enough object (liquid and solid waste) is moved through the system fast enough; in such a way that the object blocks the entire internal pipe diameter, a high pressure zone can be generated on the front side of the object. This high pressure zone may be able to force air out through fixture traps, as the object travels through the system. Think of it like the plunger of a pop gun.
In this case the solution is to increase the diameter of the pipe, eat less fiber, use less toilet tissue, or decrease the flush power of the toilet.
My tub loudly gurgled when the toilet was flushed. Found out it was a vent problem, but it was a bad vent cap under my bathroom sink that screws on to the top of the PVC pipe. I unscrewed it and put a new vent cap on and problem was solved. a vent cap can be bought at a local hardware store for $6.00 to $8.00
First check and easiest solution--if you haven't used the tub in awhile, the trap may be empty, simply run some water in the tub down the drain and then flush the toilet and see if the gurgling is gone!
I had the same problem and on another site it said to try plunging the toilet. I did that a few times and it worked! No more tub gurgling!
In my case, it was tree roots blocking the sewer line. (See the preceding post about air out). Over time, the glug got worse and eventually morphed into bad drainage for the sink/tub/toilet.
Cleared the sewer line and the glugs were gone.
