I am trying to figure out where water is coming from that is dripping out of my cold air return. We have filled a couple 30 gallon totes already. There is no water source or drain anywhere close to the return. It is below zero outside, so the only thing I can think of would be condensation. But the drip is in the basement, which is also heated. We do have a humidifier on the system, but that all looks as if it is working fine, and the drip is 20 feet or so before the humidifier. Any help would be greatly appreciated.

Temporarily turn of humidifier to see if it is the source of the moisture.