Aprilaire 400 Humidifier from Bypass - 17 GPD with Automatic Digital Humidistat

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

The Aprilaire 400 ByPass FlowThru Humidifier is a whole-home solution that helps homeowners reach their optimal indoor comfort through humidity control. This Aprilaire 400 humidifier is designed to humidify up to 4,000 square feet and is mounted directly into your heating and cooling system.


What you get with the 400 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.


The 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!


This humidifier 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.


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 Aprilaire 400 bypass humidifier comes with an Aprilaire60 automatic humidistat with an outdoor sensor. The Aprilaire60 humidistat will mount directly to your heating and cooling system to accurately detect and deliver the appropriate amount of humidity to your home by receiving outdoor temperature and humidity information from the outdoor sensor. The level of humidity your humidistat predicts is based on a formula that factors in both the current temperature of the house and the temperature / humidity outside the home, accurately controlling the humidity of your home without any work or guessing by the homeowner. Sensing the outdoor temperature and the indoor humidity level, the automatic humidistat will call for the ideal amount of indoor humidification to ensure you and your family are as comfortable as possible.


The Aprilaire60 automatic humidistat is equipped with notification lights to quickly alert the homeowner if the system is in need of maintenance.

Red = The humidifier is not working and needs your attention.

Yellow = The humidifier's #45 Aprilaire Water Panel needs to be replaced.

Green = The humidifier is working properly.


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

  • 1 Aprilaire 400 humidifier with a built-in bypass damper
  • 2 #45 Water Panels
  • 1 Model 60 Digital Automatic Humidistat (includes an outdoor temperature sensor)
  • 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, and for electrical reference, this unit operates on 24 Volts at 60Hz with a .5 AMP requirement. *Important note: If the Aprilaire bypass 400 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 400 humidifier is operating optimally. Additionally, using a genuine Aprilaire #45 water panel is highly suggested. This water panel was designed specifically for the Aprilaire 400 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: Automatic Digital
  • 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.)
  • Contains: 24 VAC Transformer, Automatic Digital 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.
    Reviews Write a Review
    • N8mn

      We had an Aprilaire humidifier in our previous home, so I knew they worked great. In our new home, there is not a drain near the humidifier. I was concerned about the "drainless" model working as good as the "regular" model. I tried to get a deal from a friend at a local HVAC on this unit installed and I just couldn't justify the cost. I found it on HVAC.com and was able to install it myself. I ended up saving over $200. The unit works great and really helped our home be more comfortable during the winter.

    • kimb

      It works good but it uses a ton of electrical almost double our monthly bill. It is also hard to come up with the exact right setting and lastly we have had it for 1 month and already says it needs a new filter

    • bradp

      In NM the air is very dry in winter and the heater just makes it worse. Since I installed the Aprilaire 400 Bypass Humidifier the house humidity is at 40% when heater is on. Uses very little water and does not drain. Very happy with purchase. Does take time to run water line, outside sensor and cutting in unit. But at least I do not have to fill a humidifier up every day. Since I had very little space I used the metal 6" flex duct for air bypass to return. Similer to your typical dryer flex.

    • Pinol

      So far it has held the humidity in the house at the set point. Customer service from Aprilaire has been excellent on answering questions. So far the weather in SC has not been cold enough to evaluate the capacity under prolonged real winter conditions, but so far OK.

    • David

      Hey people I ordered the Aprilaire 400A Last Saturday, It arrived last night……. Great job shipping. Tonight I installed the 400A. (I should send you some photos) So after installing the unit, I bought it on-line and went to shower. Within an hour the house was up to 50% unbelievable. You guys ROCK, how nice to deal with a company that knows their $#@! I sent an E-Mail last week and one of your people helped me, wish I new their name. Any how, thanks people, fantastic job. I will tell my friends if they need help, to contact you.

    • Mike

      I have owned several homes over the past 30+ years all have had Aprilaire humidifiers that required a floor drain. My current home has a basement with no floor drain, so I was skeptical that the no-drain Aprilaire 400 would even work. Some of the reviews I read of the 400 were negative and mentioned that it did not generate enough humidity, but since I don't have a floor drain, I really didn't have any other choice but to install an Aprilaire 400. Well, I am glad I did. The 400 generates enough moisture to get the humidity up to +30% and keeps the humidity consistent regardless of the outside temperature, so much so that I have to dial back the humidity to prevent condensation on my windows when the temps dips into the single digits. I should mentioned that the 400 does come with an outside temp sensor that works with the humidistat to automatically dial back the humidity when the temp drops (so I don't have to do it manually), but I have yet to install it.

    • Art

      I just wanted to send you folks a quick THANK-YOU note. It was an absolute pleasure doing business with you. This is my second Aprilaire 400 (second home) and we just love the unit. I wanted to thank you for ease of ordering, quick shipping, and perfect packaging. It is always a pleasure to do business with a Christian organization. Others could learn a lesson from you folks. Thanks again. Much appreciation.

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    Aprilaire 400 Humidifier from Bypass - 17 GPD with Automatic Digital Humidistat