Water supply off.
Solenoid not receiving electrical signal to open.
Outlet orifice clogged with mineral deposits.
Caveat: experience with another aprilaire unit, and I didn't look up your unit
There is also a fine screen filter on the incoming side of the
solenoid. But to get it out, you usually wind up trashing it, so you
should have a spare. Arnold Service Co on the internet sells them as
well as the orifice. I'd start by measuring the voltage on the
solenoid, or listening for it to click.
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