I recently remodeled two bathrooms in our house and installed two new Toto toilets. When I flush the toilet upstairs, the downstairs toilet bubbles, sometimes with enough force to overflow the rim. The bubbles are coming from the effluent side of the bowl (not the water jet inlet). The upstairs toilet is in the room directly above the downstairs one, and there is never any bubbling problem on the upstairs toilet. What would cause this? Is it more likely the vent or the drain? Why?
I don't know if the old toilets had this same problem because I just bought the house. I did look at the vent opening on the roof, and it is a 3-4 inch pipe, which is open at the top. Should this be covered somehow?
Thanks in advance.