TYPES OF HUMIDIFIERS
Evaporative Humidifiers work on the principle of providing moisture on a surface called a media and exposing it to the dry air.
This is normally done by forcing the air through or around the media and picking up the moisture from the media as a vapor.
The by pass humidifiers relies on the difference in pressure between the supply (warm/inlet) side of the furnace and the return (cool) side.
It may be mounted on either the supply plenum or duct or the cold air return plenum or duct. Piping must be run from the plenum or duct where it is mounted to the other plenum or duct.
If mounted in the supply duct, it must be piped to the cold air return.
The difference in pressure between the two plenums draws some air through the humidifier to the return duct and is distributed throughout the house.