The process of disconnecting the water supply to a furnace involves removing the humidifier cover and closing the saddle valve located on the water supply line. For safety reasons, disconnect power to the furnace at the circuit breaker before performing any work on the unit.
After disconnecting power, remove the screws holding the humidifier cover in place in order to access the water supply line. Locate the wires attached to the humidifier's solenoid valve. Test the screws with a neon tester or multimeter to ensure there is no electrical current. Turn the terminal screws counter-clockwise for removal, and unhook the attached wires. Use the saddle valve located on the water supply line connected to the humidifier to disconnect the furnace's water supply.
Furnace humidifiers connect to the unit through a hot air supply duct as well as a cold water supply line. Water supplied to the furnace enters a circular drum pad located within the humidifier. The furnace blower heats the water until it evaporates to form water vapor. The furnace circulates this water vapor throughout the household to increase humidity levels.
Disconnect the furnace's water supply line for cleaning, to make repairs or when replacing the humidifier or other attached components.