Should my Aprilaire bypass humidifier be installed on the supply or return duct?

Consumer question:

The isntallation instructions for the 400A humidifier states that the humidifier unit can be install on the return or supply side of a forced air heating system. All of the diagrams have the unit installed on the supply side - are there any disadvantages of hanging the unit on the return side?

Aprilaires' response:

Thank you for contacting us. A bypass humidifier gets its air movement through the Water Panel® by the differential pressure between the supply (hot air) plenum and the return (cold air) duct. The supply plenum is under positive pressure and the return duct is under negative pressure.

One of the advantages of a bypass humidifier is that it can be installed either on the supply plenum or on the return duct with equal effectiveness. No matter which duct the unit is mounted on, the airflow always goes from the supply to the return through the humidifier.

A bypass humidifier will also have no adverse effect on any component in the furnace, air cleaner or filter. Aprilaire® Humidifiers put “water vapor” into the air, not raw water. This is nature's way of getting relative humidity into the air. Please contact our Customer Service Department if we may be of further assistance.

Comments for Should my Aprilaire bypass humidifier be installed on the supply or return duct?

Sunday, November 25, 2012 by Ryan:: If you install the humidifier on the supply with the bypass connected to the return it will be pushing the air from the supply through the water panel to the return - correct? Then it will flow through the furnace drying out the air - reversing the humidifying process correct? So is the idea that the furnace can only remove so much moisture in a short amount of time that you are still better off than without the humidifing action?