How can I tell the difference between the old aprilaire 400 series humidifiers?

Consumer question:
How can I tell what model I have? (please add this question/answer to your FAS's).

My unit is probably at least 25 years old. I am having a problem with it (leaking water coming from unit's housing), and would like to understand which model I am dealing with.

I have a unit that is installed on ductwork associated with the furnace. The unit is drained by a flexible tube that goes into a pump (on the floor) that evacuates the excess water to a nearby sink. The unit uses warm water.

The rheostat/control is installed on ductwork as well, about 3-4 feet away from the unit. The Rheostat is a dark brown/black control knob, in the middle of a rectangular panel that is also dark brown/black. All the printing on the rheostat's panel (that was once a faint gold perhaps) is worn away, except the '25' at the top/middle of the dial, and – in the upper right- the lettering 'aire.'

Some pages from the original manual were pasted on the inside of a cabinet door in the basement by a previous owner (until today I had not known what they were for.). The instructions say that the Summer Shut-off instructions are to turn the dial to 25. I was unable to find a manual on your website that contains those instructions.

However, from the pictures in, it appears like the models shown on pages 13 and 14 (Models 440, 445-448).

How can I tell which model it is? thank you.

Aprilaires' response:

Thank you for your email regarding your Aprilaire humidifier.
The following is the best way to tell the difference between the model 440 and the model 445-448. The 445-448 will have a thumb screw on the bottom of the Humidifier to hold on the reservoir. The 440 does not have a thumb screw because it does not have a reservoir.
Turn your Humidistat knob clockwise until it stops (to the off position) to turn your Humidifier off. You will have to set it to the desired percent when your humidity levels begin to decrease.


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