I know what a fan powered humidifier is, but a bypass?
Is that supposed to be called passive?
Passive humidifiers install in the duct and only work when the heating unit's fan is on. I like diffrerent ones depending on where the ductwork is or how accessible it is. Under the floor, in a basement, passive. Under the floor in a crawl space, powered.
My thought is that a duct system that is in a crawl space could condense more moisture inside it than a duct system in a temperature controlled environment like a basement. A fan powered humidifier would push the moisture on out of the ductwork where a passive system may condense and puddle in the ducts.
We all know that puddle ducks are best left outside.
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