Ctal writes:

Thanks in advance for any suggestions.

Others have explained the primary and backup functions of the two
condensate pipes.

Most homeowners have no idea this is a critical maintenance item. Here
in SE Florida I hear all the time of severe damage to houses from
condensate leakage due to ignorance. Sometimes when visiting a neighbor
or friend, I'll point out the backup-pan pipe exiting their soffits, and
no one seems to know what it means when it drips.

Forget the advice about using chlorine. This is often repeated but is
based on more ignorance. The fact is it won't work for several reasons,
(1) the chlorine will only last perhaps a few weeks before decomposing
and/or flushing, (2) chlorine is inactive in the resulting high pH
condensate, (3) the chlorine demand to kill and dissolve a typical mold
clog is far more than you'll deliver this way.

A strong dose of hypochlorite may help the cleaning process because it
is so strongly alkaline, not from the chlorine. Lye drain cleaner would
work better, but I wouldn't recommend that either.

Polyquat tabs or pouches are also sold (or pushed by the repair trade)
as clog preventers, but I doubt these work for the same reasons, and
that the dose is tiny and short-lived.

In my experience, vacuuming won't usually work on a clogged condensate
pipe. On an easy clog, it will work a little, just enough to give the
illusion that it worked because the drain will flow again, just enough
for the condensate drip, but most of the clog is still there and will
quickly grow back.

The way that works is to install a disconnect at the air handler,
consisting of a PVC repair coupling or Fernco rubber coupler, and use a
rubber-tipped blow gun with 90 psi shop air to blow the clog(s) out.
This is a much more involved and expensive method, but it has always
worked for me. You get a hideos gooey clog splattered on the ground
where the pipe ends.

Make sure you have U-trap installed in the line. Otherwise air flows
through, collecting dust and fostering more mold growth that leads to
clogging.