How To Turn On Furnace Humidifier. If this isn’t possible, set it to the lowest possible setting (the humidistat is usually installed on the return air plenum, right above the humidifier. Turn your furnace thermostat to heat and turn up to call for heat on the furnace.
Close the bypass duct damper or turn it to the summer position. The other wire will go to common or b/c terminal. Using the inlet fill tube located over the humidifier housing, open the water supply to the humidifier and turn the power on to the furnace.
When the furnace begins running, the humidistat should call for humidity, the solenoid valve should open, and water should come out of the water inlet tube and drain into the humidifier drain.
The humidistat control is on the air duct. Once you reach the water supply valve, turn it clockwise until it stops. Using the inlet fill tube located over the humidifier housing, open the water supply to the humidifier and turn the power on to the furnace. You set a thermostat for a specific temperature, and when that temperature is reached, the furnace turns off.