Have an older home that does not have any vents connected to any sink, shower etc... There is a 4" stack that all the drains connect to. Next to the 4" stack looks like a 2" line that connects to the 4" stack higher up. Is this the vent? Most of the drains are fairly close 4 to 6 feet. Others are maybe 8 feet or so. Some drains do not seem to drain as well. Is this an old way of venting? Anything to help the water drain? Thanks.