"This may sound odd but I have water coming from my furnace. There is a 3/4 inch drain pipe that comes out of the furnace and down to sump pump. The water softer is tied into this about half way down. It all runs down hill at a good slop and has been this way for 20 years. It seems to happen when the water softner back flushs. The water seems to be travrling back up the drain pipe and into the furnace.

Separate the two drains.