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.

18 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?

    Reply
    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.

      Reply
      1. aprilaireadmin

        Trenton,

        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.

        Reply
    2. Alex

      Ryan, I was thinking the same thing, it seems like there is inefficient loop process, first humidifying the air and then running it through the furnace which dries it… I don’t get it.

      Aprilaire,
      Would mounting the bypass duct on the same supply side but below the humidifier be more advantageous to the humidification process?

      Reply
      1. aprilaireadmin

        Dear Alex,
        Thank you for contacting us with regards to our Aprilaire Bypass humidifiers. The bypass loop creates a conduit for the air to travel from the hot air side of the furnace (supply) to the cold air side (return). Installing the bypass duct on the same side of the furnace that the humidifier is on would not create air flow though that conduit.
        The air flow does pass through the furnace twice by this method, but humidity is not removed from the air by heating it. Once the air travels out into the home and cools to room temperature, all of the moisture will still be there to humidify the rooms.
        Please let us know if we can be of further assistance.

        Reply
  2. William

    I have a question – If the bypass duct goes from the return air to the supply side of the furnace, isn’t the furnace filter getting bypassed ? In other words, how can you insure that the bypassed air gets filtered before it reenters the home ?

    Reply
  3. John Glowa

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

    John

    Reply
  4. 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.

    Reply
  5. 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

    Reply
  6. 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.

    Reply
  7. C. Kiefer

    Am glad I found your posts. What I DON’T Understand … if a bypass humidifier is installed on the Return side of the furnace, isn’t it pulling “unwarm” air thru the humidifier and pad, and thus pushing colder air into the Supply duct? We just got a new one and I swear the air coming out around the house now cooler than before we installed the bypass unit. Common sense tells me that if you hook it up to Return side, it is going to decrease the temp of the resulting air that gets circulated throughout the house. What am I missing?

    Reply
    1. aprilaireadmin

      Hi.

      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. Thank you for your questions and have a great day!

      Reply
    2. Anthony

      The unit on the return cold air is blowing the air across the humidifier water pad, drum wheel,whichever and ducted,bypassed to above furnace return hot air duct, up to your home,he’s telling you this is safer in case of leak overflow it’s not dripping down onto ac coil then furnace,hopefully that makes sense!

      Reply
      1. aprilaireadmin

        Dear Anthony,

        Thank you for your inquiry regarding the operation of an Aprilaire bypass humidifier. 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. If normal maintenance is performed the humidifier should not leak. If water isn’t draining properly, water would only flow down the outside of the HVAC system and not inside it.

        Thank you for giving us the opportunity to be of assistance.

        Reply
  8. Ettore DeMichele

    I have a Byrant 80 Down flow furnace in my home. I only have the return air plenium available in the furnace room as the outflow hot air flow is below the floor and runs through ductwork under the house in a crawlspace. Can I mount a humidifier in the cold air duct plenium and have it be effective. Thank you

    Reply
    1. aprilaireadmin

      Dear Phil,

      Thank you for your inquiry regarding the operation of a bypass style humidifier. 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.

      Reply
  9. MikeS

    I just installed an Aprilaire 600 on my furnace supply side. It has been running 2 days now on full moisture (for test purposes) and not seeing any humidity increase. The water flows when heat is on 100%. I have checked it.

    I know I have it installed correctly with bypass model but express the same concern as earlier post about flow of vaporized air now running through the furnace that should seem to negate the moisture introduced.

    I have 3 hygrometers around my house and not seeing an increase.

    Reply
    1. aprilaireadmin

      Thank you for your inquiry regarding the operation of your Aprilaire Model 600 humidifier. Due to it now being April, if there was no humidifier in your home for the entire winter it will take time for the humidity level to increase in your home. This is because all the furnishings in your home will have had moisture extracted by the air in your home. As the humidifier operates these furnishings are reabsorbing water. It may take a few days or more before the humidity equalizes in your home while this process continues.

      Thank you for giving us the opportunity to be of assistance.

      Reply

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