IMO, the Aprilaire is a great unit.
There are minimum things that can go wrong.
There are only 2 "moving parts" the
fan/motor and the water solinoid. The media
should be replaced. And, probably the
most likely problem is the orifice just
after
the solinoid. It is a small plastic
"plug" in the copper line with a
pinhole. The model
110 is the smaller of their units (at
that time) and the 112 is the larger. I
have had a
112 for 34 years in a 2400 square foot
house. I probably could have done with a
110, especially now that we have good
windows. It only runs now when we have
extended 0 degree cold days.

wrote:
I have an old, dead, Aprilaire 110 (or 100) that never worked since I
bought my house. I'd like to replace it with a working humidifier.
Any suggestions on what a good replacement model would be? The house
is about 2,500 sq ft. I'm hoping I can find something that will fit
the hole and connections I already have.

I understand that replacing a humidifier isn't rocket science. Too
bad. I'm pretty good at rocket science. HVAC on the other hand...