What is a Bypass Humidifier ? BY-PASS humidifiers are water based. They will use the pressure difference between the furnace supply plenum  and the return plenum to move air through the humidifier. This is achieved by adding a 6 inch round connecting  duct between plenums (CATEGORY = GOOD) What is a Fan-Powered Humidifier ? POWERED humidifiers are water based. They will use their built-in fan to pull air through the humidifier, making them more efficient (CATEGORY = BETTER) What is a Steam Humidifier ? STEAM humidifiers will convert regular tap water into pure sterile steam  (CATEGORY = BEST) What is the difference between  MANUAL mode and AUTO mode ? Operating in MANUAL mode, your humidistat is configured to measure your indoor return air RH (relative humidity) level within your furnace return plenum. Operating in AUTO mode, your humidistat is configured to measure your outdoor air temperature by utilizing an outdoor sensor. What is the difference between MECHANICAL and DIGITAL humidistats ? A DIGITAL humidistat includes an outdoor sensor, and is capable of operating in both MANUAL or AUTO modes. The outdoor sensor installation is optional. Additional costs are usually incurred in order to run the sensor to the outside of your home. A MECHANICAL humidistat is only capable of operating in MANUAL mode.