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Maintenance Tips for Aprilaire Model 400 / 400A Humidifier

Maintenance Tips for Aprilaire Model 400 / 400A

This article offers maintenance tip for this popular whole house humidifier from Aprilaire.


The Aprilaire model 400 whole house humidifier 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.  Unless there is an overflow situation - the model 400 will not need a floor drain. 

Maintenance tips:

Change your water panel AT LEAST 2 times each humidification season - unlike other humidifiers in the Aprilaire family - the model 400 does not use a rigid aluminum wick.  The wick material is more absorbent and has paper-like qualities.  Why?  Because the water panel on the 400 wicks up moisture from a reservoir as well as being wetted by water dripping from above.  Unlike other humidifiers - the model 400 has a water reservoir / float system where they others flush excess water down the drain.  The benefit to using the 400 is decreased water usage.  With this benefit comes additional maintenance.  The most important maintenance item is more frequently changing of the water panel, which is part # 45.

Troubleshooting proceedure - The two floats in the chambers at the base of the scale control insert activate switches in the water level sensor assembly, which control power to the solenoid valve.  The float on the right is the primary control mechanism.  When the water level in the chamber on the right rises, the float disconnects power to the solenoid valve.  Shutting off the water.  As the water level drops, the float activates the switch, energizing the solenoid valve, which opens and sends water to the distribution tray.  The float in the chamber on the left controls and over flow protection circuit.  If the primary float fails to shut off the solenoid valve, water will flow in to the chamber on the left, rasing that float and diconnecting power to the solenoid valve.  If both floats fail to disconnect ppower to the solenoid valve or if the valve fails in the open position., water fill flow into the safety overflow line.

Things to check:

Check both floats, the chambers, and the float cover.  Make sure the float cover is properly snapped into the place and that both floats move freely up and down.
Check  that the solenoid valve closes when de-engergized.
Check the electrical continuity of the water level sensor circuit.  With both floats down, the circuit should be closed.  With either float up, the circuite should be open.

Complete maintenance kits are available for the model 400 / 400A