I purchased and installed my 600 last year. I love the unit and easy
to install. The duct is tapped into the supply and the water/pad is
on the returned. Normally you would want this the other way around
but my heater/air conditioning coils would not allow for that. I also
check with the vendor and some mechanical engineering friends. This
setup is not a major issue.

Be sure to hook the water solenoid to the heaters main (24v) supply so
that the water comes on with the fan. They do make the inductor coils
but I prefer the direct power source.