Originally Posted by chuckcrj
There is nothing wrong with either the venting or the drain. That furnace is approved for single pipe venting.
In fact, the burners will burn cleaner and be slightly more efficient with room temperature air instead of cold outdoor air in the winter.
I don't like drains run in front of furnaces from a service standpoint, but that setup will drain just fine.
That is a very restrictive return air drop and filter connection. I would want that redone if it was mine.