I installed the unit using hot water to speed up humidification. The outer vent in the photo is just to cool the blower motor and is separated from the air duct by an internal barrier. After installation, a loud whistling noise occured even when it was powered off. Seems that there are several small round holes in the barrier that once upon a time were used to mount something. Once they were plugged, it ran quietly. Aprilaire really needs to remove these holes.
Its built in fan drives air thru a honeycomb block that has alot of wet surface area. Water went in hot (120F) but came out (10F) colder than the room air. House humidity jumped 10% in a few hours and then stopped rising. It took nearly two weeks of continuous operation before the humidity rose again. I think that wood, carpets and fabrics must have been absorbing most of the moisture that it was producing. Now it runs an hour or two a day to maintain the set point.
The electronic control has a remote outdoor air temperature sensor that changes the units set point to prevent condensation on cold windows or glass doors. So if you want that feature a little more effort is needed to run the wire outdoors. The control works well and looks good.
The unit has been running for 3-months now and I am happy with its operation.