The humidistat on my furnace is not working properly.
A humidistat either works as a separate device to control the humidifier or is part of the central heating and air conditioning system.
Generally, set the thermostat for the optimal relative humidity level for comfort and for avoiding health effects to between 35 and 60 %.
Excess humidity is a breeding ground for mold, pests and rot in homes.
Find the control panel for the humidistat, usually near the furnace. With the furnace on, turn the humidistat very high, say 80%. If water flows, the solenoid works.
If no water, use your multimeter to test the voltage at its connection with the solenoid, then the humidistat is operating correctly. Check for 24-28 volts at your solenoid valve. If you have between 24-28 volts present, and water is not coming out of your solenoid valve, then the valve has failed and must be replaced.
There is no polarity when connecting the new humidistat. Attach either wire to either of the connections.