As I mentioned earlier, the humidistat is like a thermostat, only instead of being a switch activated by temperature, it is activated by the humidity level of the air returning to your furnace. In the photos above, the humidistat is installed on the outside of the furnace’s return duct and its humidity measuring sensor is in the return air stream.
To check the operation of the humidistat using 24 volts proceed as follows:
- Temporarily set the thermostat higher than its current setting so that it calls for heat
- With the furnace running, attach one end of the multimeter probe to one of the wiring connections that goes from the multimeter to the solenoid, then attach the other probe to the other wiring connection the multimeter to the solenoid.
- Turn the humidistat to a very low setting or to “off.” The multimeter should register no voltage.
- Turn the humidistat to a very high setting such as 80% or until you hear a “click” sound. The multimeter should register about 24 volts.
- If you show 24 volts at the connection from the humidistat to the solenoid then the humidistat is operating correctly.
If the humidistat is operating correctly (you see 24 volts being displayed) and the solenoid is not allowing water to flow to the humidifier, then the solenoid is defective and needs to be replaced