1st Floor Toiler Bubbles when 2nd or 3rd Flushes Toilet
Since I've moved into the first floor apartment of my 3-family, I've noticed that about twice a day, I'll get a large bubble coming up from my toilet bowl. It seems as though this only happens when the 2nd or 3rd floor flushes the toilet or turns on the shower, as I can hear water running through the pipes to go to the upper bathrooms.
The bathroom plumbing is on a "Philadelphia Single Stack" type of setup. The main stack (cast iron) goes through the roof. The main stack is 4 inches. At each cast iron toilet elbow, there is a cast iron vent line that travels up to the attic and ties into the main stack above the 3rd floor toilet.
What could this problem be? I'm think that if there's some sort of restriction in the main stack or the bend at the bottom of the stack where the main drain ties into it to send the waste horizontally through the foundation wall, then the falling waste in the main stack would cause air pressure to build up, which would find it's way to or toilet, where the pressure is released. Is this a proper guess?