Product review (5 stars): Aprilaire 700m, it works. I'm no hvac guy ...
Aprilaire 700m, it works. I'm no hvac guy and didn't expect to have to wire in a 24v transformer in a fairly simple loop. Transformer- Which I got for 12 bucks by the doorbells in any menards, lowes, Home Depot. As soon as I solved that mystery it started flowing.

So if it stops. It could be a transformer or the control unit itself. Or if installing a new one maybe order a 120v to  24v transformer w the humidistat.

Furnace, humidifier, humidistat, transformer. 2 little wires. Trouble shoot cheapest first.