You are trying to fix the wrong problem. It would be a real stretch of my credulity to believe that the fitting could actually cause a stoppage in a vertical pipe. But, the real problem is that the condensate line should NEVER have been connected to the drain line in the first place. ANY TIME there is limited humidity in the air, meaning the entire winter at least, that trap will dry out and the furnace will "inhale" sewer gas from the drain line and blow it all through the house. The condensate is supposed to be connected to an "regularly used" fixture which already has a trap, such as a clothes washer or laundry sink. REmove the 3/4" fitting and glue a section of a coupling over it.