I just purchased a house that had some recent renos done to the bathrooms and I was experiencing some gurgling sounds in the bathtub overflow when flushing the toilet. I checked the trap and there was hardly any water in it so I pour some water in to the tub and the sound went away. I stared at the water in the tub trap and flushed the toilet about 5 times and it did not move. I also made some exploratory holes and did some research and here is what I found. There are 3 fixtures in the bathroom, the furthest upstream is the bathtub, next is the toilet and finally the vanity. All fixtures are draining into the same 3 inch main line under the floor. There is a 1 1/2" vent teeing off the 3 inch drain between the toilet and the sink. My research tells me that this is a circuit vent and that the vent should infact be a 2" vent and be located downstream from the last fixture which would put it between the tub and the toilet.
My question is how big an issue is this? Is there a concern for gases? would it suffice to tie an additional 1 1/2" vent to the existing 1 1/2" vent but put it in the proper location between the tub and toilet? I know this would not be code but I am trying to fix it with minimal damage to the finishes. To add the main stack/vent is about 6 ft away from the sink.
This might help and wont hurt but you could add a Pro Vent under the sink. Thats the easiest thing to get to at this time and it might help and youre right, there should be two 2" vents. One at each end that tie in together at some point at of sight.