Water Dripping From HVAC Furnace PVC

While looking at the cap, it's probably worthwhile taking a look at the caulking between the crown and your flue, and or flue-cap, which may have eroded. This needs to be looked at about every year or two (more or less frequently depending on your local habitat, but both extremes in hot and cold weather will help age caulk, rot brick, etc.)

It is indeed condensate water, high efficiency furnaces wring water all year long (more so during heating, but A/C in summer too).

What has happened is the condensate collection point (on the inside of where the external PVC drain runs) has cracked. It is a very brittle and cheap plastic, just saw this on another customers furnace. It acts like a little trap, holding water until it overflows into the PVC to drain.

Possibly the PVC got kicked and the force made its way back to the box (good luck getting a show of hands for this one)..

In any case, your furnace vendor will likely have one in stock.