Given the fact that water enters this plumbing system with such a force, this revealed a piping issue/consideration in the basement of this home.
Since the drain cleaning of this particular fixture yielded no result, our next line of questioning was pointed at the installation of the piping regarding the shower. We are of the professional opinion that no vent for the shower exists, and that when this shower was installed in the bathroom directly below the bathroom above it created a 'siphoning' of the trap.
We experimented with different methods of flushing the toilet above and sure enough, a full and slower flush minimized the amount of water that ejected from the shower drain. Since this toilet on the first floor is the most used, this shower over the course of a day will eject its contents in the trap, causing sewer gases to eventually leak from the drain itself until someone pours water into the trap to prevent a sewer gas leak.
This whole event was a great learning experience as it provided some good insight as to "what if we installed that toilet and this happened" outlook.
We will now always tell our customers ahead of time of the conditions that exist with this powerful flushing american standard champion 4 toilet has, and what might create as a result of it. The customer decided to switch toilets from the downstairs which is an American Standard Cadet 3 toilet to the American Standard Champion Four which is upstairs. By switching these the less powerful flushing toilets will now be on the basement level instead of the first floor, directly over the incorrectly installed plumbing that was done for the shower.
State of Kentucky enforces venting for each and every plumbing fixture in the home. This is an important feature of the plumbing system as it allows for the free air movement when water is passing through.