My basement toilet has started to erupt as a bubble of air blows water up and out. As far as we can tell it happens on a random basis. We'll lift the lid and the underside will be all wet. My daughter was walking through the basement and heard - well, she heard whatever sound a large under water bubble would make.
Let me describe the drain and vent set-up:
First: This toilet has been installed for as long as I have lived in the house (25+ years) and has never bubbled before. Nothing has changed as far as the venting or drains. The toilet flushes fine and all other fixtures drain normally.
Second: This bathroom is in the corner of the house where all the drains meet under the slab just before leaving the house. The toilet, shower and sink in this bathroom are the last set of fixtures before the drains leave the house. The toilet is before the shower and sink which wye into the main sewer pipe about 3 feet from the exterior wall. There is a clean-out just inside the exterior wall under the sink.
Third: My house has only one vent - the soil stack, which is in the opposite corner from the bathroom. No fixture in the house has it's own vent, and never has. Right or wrong, this has never been an issue. All fixtures have always drained properly.
So, based on the information above, can someone explain why my toilet has suddenly started erupting like Mount St. Helens?
Thanks!