The unit goes on the hot plenum, because that's where the heat is.
Only humidifier I recommend is Aprilaire, which is an excellent design
and trouble free. I also recommend getting one that is powered by a
small fan and not bypass type.
Since there are different designs for furnaces and humidifiers and
different situations, I suggest looking in the install instructions that
should have come with the humidifier.
Do you have the humidifier yet? How much access do you have to the
furnace and ducts? Access to water supply and drain?/
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