A little history....
I have a 25 yr old 4 bedroom colonial built on a slab. I have a clean out in the rear outside of the house and a vent in the front of the house (2 ft off the ground) that goes into the wall of the garage and then elbows down into the garage slab. On this pipe that goes down, a drain for the water softener goes into the side of it and also a drain for the boiler condensation goes into the side of it. A few years ago I had a problem with the boiler drain freezing there and it backed up into the boiler and causing it not to work right. So I put a tee in the pipe about halfway between the boiler and the pipe junction. I figured if the joint froze again, the water would simply leak out that tee and not cause the boiler to fail starting. No big deal as it's a gallon a day tops in the winter.
Once during the summer, water started pouring out that tee and flooding into the garage. It certainly didn't come from the gas boiler. Happened again today. I have 1 toilet on the bottom slab level and it bubbles when this is happening and will overflow if flushed. Usually happens when someone is taking a shower upstairs I believe
Am I right in assuming that there is a blockage somewhere under ground or below my slab to cause this The line runs to a street sewer line 75 ft out.
What to do???