This should be a simple yes or no question but I can't determine it myself. I have an AprilAire 550 Humidifier built into the ducting from my furnace. It appears that the humidifier isn't working due to water never spraying out of the line. I have narrowed it down to the solenoid. I can test the line going to the solenoid with a multimeter and confirm that 24 volts is going to it. Whenever the solenoid is connected, while monitoring the power, the power stays at 24 volts but the solenoid only opens the valve briefly when I connect it.

So I do know solenoids can be both intermittent and not. My question is, should this solenoid be an intermittent one? If the answer is no then of course I will replace it. I believe the answer is no since the humidistat and furnace circuit will determine whether it should be on or off.

  • This solenoid should not be making any decisions on its own. It you give it power it should stay on. – Paul Logan Jan 6 '18 at 18:54

The Aprilaire 4040 solenoid assembly appears to be a non-intermittent valve. As long as power is applied, it should be open.

Try thumping the solenoid while it is energized in an attempt to un-stick it. A sharp thump with something not too hard, like a block of lumber or a rubber mallet, might work.

If you have an ohmmeter available, check the coil resistance (disconnect the wires, measure ohms through the coil); should be something like 30 to 400 ohms. If it is infinite (open), replace the solenoid.

The owners manual hints that the solenoid stays energized to be on here (page 6).


This solenoid should not be making any decisions on its own. It you give it power it should stay on.


Solenoids can get dirty just like any other valve, if it is opening but only allowing a little water through it probably could use a cleaning. With the water and power turned off unscrew the nut on the top of the solenoid. Remove the nut and if a spring washer make sure you note how it is sitting you want to put it back together the same way. Pull the solenoid OFF. Now there are usually flats you can use a wrench to unscrew the body. As you unscrew use caution as some solenoidss have a small spring . once you have the valve opened you may see all kinds of debris stuck in the valve, clean it out. I usually inspect the small rubber seat that is embedded in the slug if it looks badly worn I will replace this seal. After cleaning out reassemble, put your coil back on and give a try. I can clean most of these type of valves in 2-3 minutes a bit longer to replace the seat. It's worth a try as these valves usually tip the scales at +$50 with some in the $300+ range. So a couple of minutes cleaning them out is well worth the time.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.