Furace fuse blows when Aprilaire humdifier is turned on above 35%

Question

I live in a pretty old house and have a Carrier furnace with an Aprilaire humidifier attached to it. The device that controls the water flow, I guess it's a solenoid, is visible on the exterior of the contraption, yet it is very rusty. It is so rusty it just looks like a block made of rust that happens to have wires going into it. This is the 4th winter so finally I decided to see if it works anyway. Turned out it worked for a few weeks at 25%. Then it started getting "warmer", approaching 20 degrees outside, awesome. So I followed the chart and increased the humidity to 35%.

Increasing the setting to 35%, I found it blows the 3 amp fuse inside the furnace.

Would replacing the solenoid fix this? Or is there any other potential cause or incompatibility between the two systems?

If I replace the solenoid, I won't need it until next winter, assuming that it warms up between now and then.

Answer 1

The solenoid shouldn't blow the fuse, unless there is a short in it. It's not like a motor that has high locked rotor current, which could blow a fuse if the rotor was frozen. If the coil of a solenoid energizes, it doesn't matter if the plunger moves or not (electrically speaking).

I'd check the resistance of the coil, versus the specifications for the solenoid. This should tell you if there is a short. Depending on the price of a new solenoid, I might just replace it if it's in as bad a shape as you describe.

The other thing to check, would be the humidistat.

I'm not sure why you'd see a fuse blow at one humidity level, and not another. I'm guessing the unit didn't kick on until it was set above 35%, so the problem may or may not depend on the setting.