The Aprilaire Model 550 humidifier is mounted on the warm side/supply plenum, with a 6 inch round duct connected to the cold air return. Is this installation correct?

Monday, January 10, 2011 by Aprilaire Team
Consumer question;

I recently bought a house with a model 550 humidifier already installed. The 550 is mounted on the warm side/supply plenum, with a 6 inch round duct connected to the cold air return. Is this installation correct? Or does the 550 need to be on the cold air return? Also, there is a Temperature Compensating Humidistat installed. The probe goes outside, but the unit is mounted in the cold air return, above where the 6 inch duct connects to it. Is this installed properly?


Aprilaires' response:

Thank you for contacting us regarding the Aprilaire product line. We are happy to assist you with questions relating to the installation and operation of the model 550 humidifier. A bypass humidifier gets its air movement through the Water Panel® by the differential pressure between the supply (hot air) plenum and the return (cold air) duct.  The supply plenum is under positive pressure and the return duct is under negative pressure.

One of the advantages of a bypass humidifier is that it can be installed either on the supply plenum or on the return duct with equal effectiveness.  No matter which duct the unit is mounted on, the airflow always goes from the supply to the return through the humidifier.

A bypass humidifier will also have no adverse effect on any component in the furnace, air cleaner or filter.  Aprilaire® Humidifiers put “water vapor” into the air, not raw water.  This is nature's way of getting relative humidity into the air.

In reference to the location of the humidistat, this component should be installed on the return duct at least 6 inches upstream from the humidifier or bypass arm. This location is a requirement for proper reading of relative humidity within the home as well as proper operation of the humidifier.

We would be happy to further assist you with any questions or concerns relating to the operation of the model 550 humidifier. If there are any questions please contact us at your earliest convenience.




Comments for The Aprilaire Model 550 humidifier is mounted on the warm side/supply plenum, with a 6 inch round duct connected to the cold air return. Is this installation correct?

Wednesday, September 14, 2011 by Patricia Freer:
I took the filter out of the model 550 and now I don't remember how to put it back in.
Tuesday, March 11, 2014 by Dennis:
Can anyone take pics of their Aprilaire 550 installed so that I can see how its suppose to look ...will send email for pics

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