We got on Amazon and Plumber's Outpost carries Sioux 660-TCO Mini-Rester. It says it is for and ice maker. We have a 550 Aprilaire Humidifier and couldn't believe how easy it was to install! The smallest Home Depot carries is for 1/2 tubing. They gave me info on how to cut the copper pipe and hook the mini-rester to the humidifier. I had to buy a pipe cutter and small box of copper pipe, along with small brass washers for copper pipe (don't know what they are called). It cost me less than $50.00 for everything, and less that 30 minutes to attach it. I couldn't believe how easy it was! Not bad for a housewife!

Novice

Check the humidity

The humidifier should only turn on if the humidity is below a set level. Make sure the humidity is actually below this level.

Clean the inlet

Check the inlet (where the water comes in), and make sure it's not all gunked up.

Check the valve

If the humidifier supply line connects to the water supply via a saddle valve, close and reopen the valve (and consider replacing the valve, which will require shutting off the water since the valve pokes a hole in the pipe).

Set the humidistat

If a humidistat is used, make sure it's set properly.

Intermediate

Check for water at the solenoid

Close the valve supplying the humidifier, and remove the supply tube from the supply side of the solenoid. Open the valve just a bit, and make sure water is coming from the tube.

TIP: Wrap a rag/towel around the open end of the hose, and hold it tight when opening the valve. The rag/towel will prevent the water from spraying out the end of the tube. Also, don't forget to hold the rag/towel wrapped tube over a bucket.

Check for power at the humidistat

Open up the humidistat, and use a multimeter/voltmeter to make sure the humidistat has power (most likely 24 volts).

NOTE: Depending on the installation, the humidistat may only have power when the furnace is running.

Check for power at the solenoid

With the humidistat open, and the furnace running. Set the humidistat to the highest setting, and check to see if you have voltage going to the solenoid.