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.

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

  1. 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?

    1. Trent

      Ryan, it doesn’t sound like you understand the bypass concept. The bypass duct serves to pass the humidified air flow around the furnace, so it is not heated. When the humidifying panel is mounted on the return side, humidified air passes through the bypass into the supply duct past the furnace. When the panel is on the supply side, regular air from the return passes through the bypass, and is humidified by the panel immediately before it goes into the supply duct. So humidified air doesn’t go through the furnace and lose humidity, aside from whatever humidity was already in the air through the return as usual.

      1. aprilaireadmin


        Your comment about the bypass principle is not accurate. As stated in our original answer the supply duct of a furnace is under positive pressure and the return duct is under negative pressure. As a result the air in a furnace always flows from positive pressure to negative pressure. The furnace blower pushes air through the supply duct out into the home and then pulls the air back from the home through the return. Air can only flow in this manner.

  2. John Glowa

    Does tyhe ducting material matter in a forced air HVAC system for your humidifiers – i.e., metal versus flex tubing?


  3. Kevin

    Thank you for contacting us. The ducting material would not be a factor with the installation of the humidifier as long as the humidifier and humidistat will stay on the duct and the humidistat is still able to sense the humidity level of the air flowing through the duct. The humidity output would be mildly reduced when using a flexible duct versus a straighter metal duct. Air flow may be reduced slightly in a flexible duct because the air would get caught in the ridges of the duct, whereas a straight duct would flow smoother.

  4. myron shellist

    I have an Aprilaire 600 series humidifier. My humidifier is installed on my furnace which is in the basement. The humidistat is installed on the furnace air duct return downstairs. Could it be installed on the wall upstairs, where we live, next to the thermostat

  5. Jason

    There is an advantage to return drop mounting over supply plenum mounting. If for some reason down the line your humidifier overflows due to lack of maintenance or another issue it will leak down your return air drop instead of through the inside of your equipment. Some manufacturers show the return plenum as the preferred mounting location with the supply as an acceptable alternative if servicing will not be possible on the return drop. Both locations work, but if there is space I choose the return every time for a mounting location. It would be nice if AprilAire’s manual showed an image of that type of installation, but the good money is always on having a professional complete the work.


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