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Aprilaire Model 400 Humidifier

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Click here to see all replacement parts for Aprilaire Model 400 humidifier

NOTE: The model 400 replaces the model 400A

The Aprilaire Model 400 bypass humidifier is the newest addition to the line. It uses new evaporative technology to minimize water use, eliminate standing water and provide comfortable humidified air throughout your home. Gravity pulls water down while the wicking action of the water panel draws it back to the top, using 100% of the water delivered to the unit. It's the perfect choice for homes with septic systems, locations where a floor drain is not available or families interested in water conservation. Plus, its computer technology and innovative design makes it effortless to operate and simple to maintain. The Model 400 bypass humidifier is a high-capacity, water-conserving humidifier (0.70 gallons evaporated per hour), well suited to handle up to a 4,000 square foot, tightly constructed home.

Closeup of model 400 float mechanism

Included with unit:
  • 1 installed # 45 Water Panel
  • Self-piercing saddle valve 
  • # 60 automatic humdistat with outdoor sensing bulb 
  • 110 VAC to 24-Volt Transformer
  • Installation instructions/plenum cut-out template 
  • Owners Manual 

Humidifier installation will require the following supplies (not included):
- #8 sheet metal screws
- 1/4" copper water line 
- Small wire nuts
- 6" start collar for bypass connection 
- 6" 90 degree elbow for bypass connection
-6" pipe for bypass connection 
- 18-2 thermostat wire
- If furnace circuit board does not include humidifier accessory terminals - an Aprilaire #50 Current Sensing Relay will be needed.

What is the difference between the 400 and 400M models?
The new 400 model includes a new # 60 automatic control.  The model 400M uses a manual control

Does the outdoor sensor need to be installed in order for an automatic humidifier to work?
If the humidifier control is installed and is operating in the automatic mode (as referenced by the faceplate on the humidifier control) the outdoor sensor is required for proper operation. The sensor is the component which sends the temperature reading back to the humidifier control and allows it to adjust the humidity level in the home accordingly.

How often should I replace the water panel?
For the best performance we recommend the water panel on most of our humidifiers be replaced once a year, either prior to the start of or at the end of the heating season. (Model 400 should be changed twice during the heating season.)

Should I regulate the water flow to my humidifier at the saddle valve?
No. The saddle valve is not designed to regulate the water flow. The humidifier has a water-metering orifice that is designed to regulate the water flow to the water panel.

Should our humidifier be connected with hot or cold water?
All of our flow-through units as well as our Model 400 humidifier can be connected to hot or cold water. Hot water increases the evaporative capacity of your humidifier, provides more humidity to the home and offers more flexibility in the operation of the humidifier. We would recommend that if our power units are installed on the return ductwork, that they be connected to hot water, as this is their only heat source. With any drain-through Aprilaire humidifier connected to hot water, the heat in the water is used in the evaporation process and the water coming out of the drain will be cold to the touch.

Should we use soft water or hard water?
Aprilaire offers total flexibility in the installation of our humidifiers in that they will operate properly with either hard or soft water. Connecting to hard water may leave mineral deposits on the water panel that are scaly and hard in nature. Connecting to soft water may leave mineral deposits that are fluffier in nature. The bottom line is the water panel will typically last a full heating season using either type of water.

Should the humidifier be installed on the return (cold air) or on the supply (hot air) ductwork?
Any of our furnace-mounted units can be installed on either the cold air or hot air ductwork. We do recommend that if a power unit (#700) is used and is installed on the cold air ductwork, it be connected to hot water to maximize evaporation.

Will my Aprilaire Humidifier affect my Aprilaire Air Cleaner?
The Aprilaire humidifier uses a natural evaporation process to generate humidity. Because the humidity is a gas it passes through any air cleaner with no adverse effects.

What causes condensation on my windows and how can I eliminate it?
Cold air outside your window cools the air just inside your window more that the air in the rest of the room. If the well-humidified air just inside the window cools too much or too quickly, it can't hold as much moisture as before and water appears on the window. On the 400A - the automatic humidifier control (#58) knows how cold it is outside and will always maintains proper levels of humidity with no adjustment needed.

How do I properly install my replacement Aprilaire Water Panel?

The orientation of your water panel is important for maximum evaporation. Most genuine Aprilaire Replacement Water Panels you purchase will have a paint dot on one of the short ends. This paint dot should be placed up, towards the water distribution tray, for proper operation. If your humidifiers requires the #45 water panel, there will be no paint dot and the panel can be installed in either orientation.

5-Year Manufacturer's Warranty

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