Aprilaire 400
Aprilaire Drainless FlowThru Humidifier w/ Digital Humidistat 17 gpd
  17 GPD 12 GPD 17 GPD 18 GPD

Aprilaire 400 drainless flow-thru bypass whole-house humidifier. The Model 400 humidifier utilizes new evaporative technology to minimize water use. 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.

The Model 400 is a complete system and includes the humidifier with a built-in bypass damper, two water panels (#45), digital automatic humidistat (Model 60) with outdoor temperature sensor, self-piercing saddle valve, and 24 vac transformer (4010). Our photos (View More Images) for a view of all the items included. Items not included is 6" bypass duct, copper tubing, drain tubing, and thermostat wire.

Evaporative Capacity 0.7 gph
Unit Size 15-3/8"W x 15¾"H xn 10¼"D
Plenum Opening 9-7/8"W x 12-5/8"H
Electrical data 24 Vac, 60 Hz, 1/2A

Aprilaire 400 Measurement

Aprilaire 400 Humidifier Dimensions: 15x15x10 inchesThe Aprilaire 400 can be mounted on the return or supply duct. The air flow can run in either direction, in to the 6" bypass opening or out of the bypass opening.

The 400 can be configured with the 6" bypass duct connection on the right or left side.

Aprilaire Model 60 Digital Humidistat

Aprilaire 60 Humidistat New FeaturesThe Aprilaire 500 operates best when the furnace blower is turned on.

The Model 60 digital humidistat can turn on your HVAC blower when there's no need for humidification or heat. It works in conjuction with your heating, ventilating, ait cinditioning equipment (HVAC) to control the operation of the humidifier.

A new feature of the Model 60 is a simple onboard switch for manual mode or automatic mode. Automatic mode adjusts the humidity based on the outdoor tempreature. This feature can be disabled by a click of a switch. The previous Aprilaire humidistats required installing a manual mode resitor (see Model 60 Owners Manual).

Drainless Flow-Thru Humidifier

The term Flow-Thru refers to water trickling through a water panel and out a drain. The Aprilaire 400 series has the water trickle down the water panel an in to a reservoir. The reservoir has a sensor that detects the water level. When more water is needed the solenoid is turned on, and more water trickles down the water panel and refills the reservoir.

The water panel (Stock #45) that is used in the 400 series is different than conventional Flow-Thru models. This water panel is an absorbent paper mesh that can wick up the water from the reservoir. Conventional Flow-Thrumodels have water panels made of an aluminum mesh with a porous pumice-like coating that absorbs water allowing the water to evaporate but will not wick up water.

Conventional Flow-Thru models flush out minerals when the water trickles down the water panel and out a drain (3 gph). The Aprilaire 400 will have some slime and mineral build-up from standing water. The Aprilaire 400 water panel needs to be replaced more often than conventional water panels (such as Stock #35). Aprilaire recommends replacing the 400 series water panel twice a season.

The model 400 should still have a drain line connected in the event of a malfunction.

The Flow-Thru is an evaporative humidifier and requires hot air. If not using hot air then hot water should be used. We see little humidity gain if our Flow-Thru humidifier is operating with cold air (and cold water). We run our blower on constant to best utilize our electronic air cleaner, but our humidifier is wired to only operate only when there is hot air and works very well.

Flow-thru Humidifier Type Demostration

By-Pass Versus Power Humidifier

By-pass Flow-Through Humidifier Demostration
Power Flow-Through Humidifier Demostration

The term By-Pass or Power refers to the driving force behind the air flow. The Power humidifier (aka Fan Power) uses a fan to draw the air thru the humidifier. The ByPass humidifier uses a 6" diameter duct that connects the supply to the return duct.

In the ByPass configuration the air leaving the furnace is routed back to the supply. The warm air leaving the furnace takes this short-cut of least resistance and bypasses the long trip down the duct work, in to the house, and back through the return. The ByPass is the 6" round duct work shown below. During the the hot humid A/C season the ByPass duct should be closed with the butterfly valve since the air flow through the humidifier would not be needed.

The Power humidifier uses a fan to draw air in from the duct and through the humidifier and exhausts back out in to the duct. The air is drawn in through duct at the center of the humidifier and back out in to the duct at the sides. The power humidifier should be mounted in vertical warm air flow.

The Model 400500, and 600 are ByPass humidifiers. The 700 is fan powered.

Manual Versus Automatic Humidistat

The indoor humidity level needs to be set based on the outside temperature. This is to prevent water from running down the windows. The cool air near the windows can not hold the humidity and water will condense on the glass. To prevent this condensation, the indoor humidity level is sacrificed by lowering the humidistat.

An automatic humidistat (Aprilaire 60) adjusts the humidity by using an outside temperature sensor.

The Aprilaire manual humidistat as used in earlier Aprilaire humidifiers (Model 4655) requires adjusting the relative humidity based on the outside temperature. A manual humidistat needs to be set back as the season gets colder. The chart below shows the humidity setting for a range of outside temperatures.

When The Outdoor
Temperature is
This Relative Humidity
Should Be Maintained

The New Aprilaire 400 Replaces 400A

In 2009 Aprilaire consolidated the Models 400M and 400A to the Model 400. The new Model 400 is identical to the earlier 400A, except the new 400 does not include the DVD Video.

The difference between the 400 (automatic) and 400M (manual) was the 400M included a low cost manual humidistat (#4655) instead of the Automatic Digital humidistat (#58) that was included with the 400A.

The new 400 now includes the humidistat (Model #60) that can be operated as a Manual or Automatic humidistat.

Previous to the 400A & 400M was the Model 400 Series One. This early Model 400 was Cream in color whereas since the 400A & 400M (2008) the color has been a light Slate.

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