Aprilaire 400M - Bypass FlowThru Humidifier 17 GPD with Manual Humidistat

Aprilaire ByPass FlowThru Water Conserving Humidifier with Automatic Digital Humidistat that humidifies up to 4,000 sq feet.

The Aprilaire 400M ByPass FlowThru Humidifier is a whole-home solution that helps homeowners reach their optimal indoor comfort through humidity control. The Aprilaire 400M humidifier is an efficient, high capacity, water conserving, drainless humidifier that uses an evaporative technology to minimize the amount of water used to humidify your home.


This humidifier is designed to humidify up to 4,000 square feet and is mounted directly into your heating and cooling system.  It is installed on either the supply or return duct (plenum), although the return side is advised if possible, and is easily reversible for right or left hand installation.  


The 400M humidifier incorporates a recirculation of water by using a special kind of water panel within the system. In short, gravity pulls the water down to the bottom of the unit while the water panel uses a "wicking" action to draw the water back up to the top, recirculating it until 100% of the water deposited into the unit is used!  


Benefits of Evaporative Humidification:


  • Prevents Flu and Allergies
  • Neutralizes Static Electricity
  • Reduces Pet Dander and Discomfort
  • Protects Fine Furniture, Wood Floors, and Musical Instruments
  • Relieves Dry Skin and Scratchy Throats
  • Reduces Heating Costs 


The 400M bypass humidifier is a manual humidifier which means that this unit will not automatically adjust your home's humidity level with the changes in temperature outside like the Aprilaire 400 unit. The system comes standard with a manual Aprilaire 4655 humidistat that can be wall or duct mounted on the return plenum of your HVAC system.


The Aprilaire 400M is Shipped to you as a Complete System that Includes:

  • 1 Aprilaire 400M humidifier with a built-in bypass damper
  • 2 #45 Water Panels
  • 1 Model 4655 Manual Humidistat
  • 1 Self-Piercing Saddle Valve
  • 1 24 vac Transformer
  • Items that are not included: 6" bypass duct, copper tubing, drain tubing, and thermostat wire. 


A plenum opening of 10" W by 12 3/4" H is required for this humidifier.  For electrical reference, this unit operates on 24 Volts at 60Hz with a .5 AMP requirement.

*Important note: If the Aprilaire bypass 400M humidifier is installed on a heat pump system it is best to connect it to a hot water line. 


Aprilaire recommends a #4788 maintenance kit be used each humidification season to ensure that your 400M humidifier is operating optimally. Furthermore, using a genuine Aprilaire #45 water panel is highly suggested. This water panel was designed specifically for the Aprilaire 400M humidifier and provides the most comfortable and efficient output. Don't trust the aftermarket products to provide the same quality and overall experience as the genuine #45 Aprilaire water panel.


If you have any questions regarding this product, please use the HVAC.com Contractor Directory to connect with a trusted contractor in your area.

  • Brand: Aprilaire
  • Genuine OEM Part: Yes
  • Type: Bypass Humidifier
  • Control Type: Manual
  • Coverage: Up to 4,000 sq. ft.
  • Water Panel: Aprilaire #45
  • Evaporative Rate: .7 Gallons Per Hour
  • Bypass Diameter: 6"
  • Electrical: 24V
  • Product Measurements: 15.38" W x 15.75" H x 10.25" D
  • Plenum Opening: 10" W x 12.75" H
  • Drain Connection: .5" (I.D.)
  • Includes: 24 VAC Transformer, Manual Humidistat, Remote Temperature Sensor, Saddle Valve, 1 Aprilare #45 Water Panel
  • This product comes with a 5 year limited manufacturer's warranty from the date of installation.

    Aprilaire 400M - Bypass FlowThru Humidifier 17 GPD with Manual Humidistat