Some additional info, the instructions that came with our house stated to pour water down the drain line (there is a cap for doing this) every 6 months. If the line is backed up you will know it when you do this. I do not understand how the unused filters had mold on them. Do you have your system checked for both air and water leaks? I added a piece of screening at the end of my J trap, so that critters cannot climb into the drain pipe.
I have a small piece of old garden hose that can go from the hot water heater drain to the area of the HVAC drain, and when I drain the water heater (also every 6 months), I do this to the HVAC drain. I usually run the hot water heater drain for 2 minutes, and I check that there is a puddle when the HVAC drain empties outside.
Hope this helps.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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