Some tips for replacing a solenoid valve include turning off the water supply to the valve and removing the electrical connections. These steps are the first two steps to a full solenoid valve replacement.
When all water and electrical lines are off of the solenoid valve, remove all the posts and screws in order to lift the solenoid valve away. Attach the new solenoid valve to the body of the humidifier, and make sure the ends meet. The size of the union male ends of the solenoid valve must meet with the union nuts on the water lines.
Next, plumb the line by wrapping the male threads of the body with teflon tape. Seat the copper inlet tube from the shutoff valve into the solenoid valve, and then tighten the nut. One end of the feed tube must be in position at the top of the trough of the humidifier. Then, seat the line from the humidifier into the solenoid valve and tighten it. Turn the water back on and check for leaks. If the valve is not leaking, connect the electrical wires back to the valve and turn the furnace on.
Turn up the humidity to at least 80 percent or until the humidifier turns on. The water should be flowing into the humidifier; make sure that there are no leaks.