We bought a home with an Aprilaire model 440 and have no idea how it works. Please help!

Consumer question:

We bought a home with an Aprilaire model 440 and have no idea how it works. It has a dial with a red line and a white line. On top of the dial it says humidity and on the lower half it says outside temp. Where should the red line be and where should the white line be? Thanks for your help.


Aprilaires' response:

Thank you for contacting us regarding the control for your 440. The white line points to the upper numbers, which are the percentage of humidity settings, and the red line points to the lower numbers, which are the outside temperatures. To set this style of humidistat, check early in the day what the anticipated low temperature will be for the night and line up the red line with that temperature. The white line will then be pointing to the correct percentage of humidity. We hope this information is helpful to you.

If you have any further questions regarding this or any of our products, please feel free to contact our customer service department at your earliest convenience.

Aprilaire – Division of Research Products Corporation
Visit www.aprilaire.com
Call center hours are M-F 7am-5pm CST


2 thoughts on “We bought a home with an Aprilaire model 440 and have no idea how it works. Please help!

  1. shelley staffig

    I bought my house and it has a Aprilaire I believe it’s a 550 because that’s the number of the filter I bought. It wasn’t hard changing the filter, I couldn’t take it apart to clean it, I was afraid I’d break something. My question is, how do I know it’s working? The plastic tube drips water in the laundry tub but do I have to put water in it when it’s low? I really don’t know how to take proper care of my humidifier and I would like to know. My last house didn’t have one and it was so dry! I appreciate any advice you can give me. I have a book but it doesn’t say much in it, The book is from 1992 so it’s 22 years old quite but furnace and air conditioner work fine. Thank you Shelley

    1. aprilaireadmin

      Dear Shelley,

      Thank you for your inquiry regarding the operation of your Aprilaire model 550 humidifier. Your Aprilaire humidifier has a water line which constantly brings water to the humidifier. When there is a ned for humidity to be added to your home and your HVAC system fan is moving air, the humidistat sends a signal to the water valve to open. Water then flows in and through your humidifier. This continues until the humidity set point on the humidistat has been reached, or your HVAC system fan stops running. Each season the water panel needs to be changed as minerals not flushed down the drain during the operation will be trapped in the water panel. To better assists you we need to know what your humidifier control looks like. If it look like the control on the left, you can determine you humidity by turning the dial from 15-45% and back to 15%. There should be a high click and low click . Halfway between the clicks is the current humidity in your home. The control on the right has a test positon. Provided your HVAC system is making a call for heat the control will operate the humidifier for one minute in the test position. After the minute is up you will want to set the control somewhere between 1-7. If the control is left in test it can never operate the humidifier.

      Please visit aprilaire.com to view the owner’s manual.



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