Aprilaire Fan Powered FlowThru Humidifier w/ Digital Humidistat 18 gpd

Aprilaire 700 Mounting Location

The Aprilaire 700 should be mounted in the warm air flow or if mounted in the return duct, hot water is needed. The air flow should be vertical as the 700 draws air from the center and returns it out the sides. See Installation Instructions.

Aprilaire Model 60 Digital Humidistat

The 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 conjunction with your heating, ventilating, air conditioning 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 temperature. This feature can be disabled by a click of a switch. The previous Aprilaire humidistats required installing a manual mode resistor (see Model 60 Owners Manual).

Drainless Flow-Thru Humidifier

The term Flow-Thru refers to water trickling through the water panel and out a drain. The advantages of a flow-thru design is reduced maintenance and no standing water. Scale from calcium and lime gets flushed out of the water panel and down the drain. The water panel is an aluminum mesh with a porous pumice-like coating that inhibits growth of slime and is easy to change (once a year).

The disadvantage of the Flow-Thru is water usage. When in operation water flows in to the 700 water panel 6 gph. We are in the northeast where this is not an issue. We get a flat rate water bill unless there is excessive usage then the rate goes up. The Flow-Thru humidifier does not cause excessive usage.

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.

By-Pass Versus Power Humidifier

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 400, 500, 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.

• Humidistat Manual
• Humidity Information

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.

Aprilaire 700 Series Two Wiring - 2008 to Present

The Aprilaire model 700A, 700M and 700 manufactured in 2008 and later are Series Two and designed primarily for an automatic humidistat. To turn-on these units require a humidistat that sends 24v to the humidifier.

The 700 series two has a controller relay (pn 4740) that turns on the fan when the humidistat sends 24v to the humidifier. The 24v is also routed to the solenoid valve. The diagram above shows the operation of this controller relay.

The operation of the series two controller relay is significantly different than that in the previous model 700. For simplicity the above diagram does not show the wiring to the humidistat necessary to sense when the furnace is running. Or the outdoor temperature sensor.

Aprilaire 700 Series One Wiring - Through 2008

The Aprilaire model 760, 768 and 700 manufactured before 2008 were Series One and originally designed for a manual humidistat. To turn-on the unit required a simple on-off switch type humidistat. An automatic humidistat was retrofitted. 

The 700 series one has a controller board (pn 4238) with a 24v transformer that powers the fan relay and solenoid valve. The diagram above shows the operation of this controller board.

The operation of the series one controller board is significantly different than that in the current model 700. For simplicity the above diagram does not show the wiring necessary to supply 120v only when the furnace is running. Or otherwise the addition of the model 50 current sensing relay.