Original Aprilaire #4040 replacement solenoid valve.  Note: Valve styles may vary.  24 VAC 2.3 Watts Replacement solenoid value for Aprilaire Humidifier models 220, 224, 400, 440, 500, 550, 558, 560, 568, 600, 700, 760, 768.  The #4040 soleniod valve includes a new inline strainer (pre-inserted in inlet side of valve), and a compression nut & washer for the inlet side only. Solenoid Valve Replacement Instructions For power units unplug electrical cord. For bypass units turn off power to furnace. Also turn the humidistat off. Shut off water supply at the saddle valve Disconnect inlet and outlet water compression nuts. Double wrench. Note, some water may drip on floor. Remove the orifice (which controls the water flow) from the plastic or copper tube and make sure there is no obstruction. If blocked clean or replace. Remove the wire nuts from the water solenoid valve wire connections. Set aside. Remove screws that secure valve to unit. Attach new valve with screws. Do not over-torque screws into plastic housing. Reconnect wires with wire nuts. For units with plastic feed tubes inspect for brittleness, hairline cracks, nicks or abrasions, etc.  Replace if necessary. Reconnect the water lines, double wrenching the compression nuts. Turn water back on and check system for leaks. Tighten compression nuts as necessary. Restore electrical service and activate furnace by turning up the thermostat. Turn up humidistat to verify operation of humidifier. Recheck for leaks. Turn down humidistat to make sure valve shuts off. Reset humidistat and thermostat to original levels.