I have an Aprilaire Humidifier Model 600 (or maybe 600M) mounted on my furnace. The humidifier is not turning on (i.e. letting water flow over the filter) when the furnace is on.

The below images show:

  • The control dial with the faceplate on.
  • The control dial with the faceplace off.
  • The housing that contains the filter.
  • The water solenoid.

The control dial has three wire pairs connected to it:

  • One pair goes to the furance to get 24V AC power. I've confirmed this has power when the furnance is on.
  • One pair goes to the solenoid and passes on the 24V AC. I've confirmed when the control knob is moved to "Test" (and the furnace is on) that the solenoid opens and water flows to the filter. So the "water side" of the humidifier works.
  • One pair goes to an ourdoor temperature sensor mounted in the housing of my air conditioner unit. I ohm tested the temp sensor and got nothing. So I replaced the temp sensor with an Aprilaire Model 8052 sensor.

I thought for sure that replacing the temp sensor would resolve my problem. But after two days of running with the new sensor (and setting the control knob to "6"), a humidity sensor in my house hasn't really moved from 46%. As mentioned, when I turn the knob to Test I can hear the solenoid open and see water flow. But when the knob is at 6 and the furnace is on, I never hear the solenoid get activated.

What other steps can I take to diagnose the problem? I've always ohm tested the temp sensor out at the air conditioner. So I could try testing it at the control panel I suppose. The other possibility is that some electronics in the control panel have gone bad.

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4 Answers 4


It's likely functioning properly. A setting of 6, probably correlates to 40% relative humidity. According to the Owners Manual

Your Aprilaire Automatic Humidifier, is a high precision system that will accurately maintain the relative humidity in your home to a maximum of 45% RH and a minimum of 10% RH.

If the home is at a higher relative humidity, the unit should not come on.

  • After reading the manual link, I think you are correct. It is probably functioning correctly now that the temp sensor has been replaced. Thanks.
    – Dean Hill
    Commented Nov 19, 2013 at 3:23

I had the same issue, no water. Ran the troubleshooting test here and found that I also had a bad solenoid, it just wasn't opening to allow the incoming water to run over the metal filter. Try blowing air into it before you buy and install a new solenoid. I just removed the solenoid from the line, used my air compressor to gently blow the diaphragm open and close a few times by placing the tip of air hose fitting into both ends of the solenoid. I also blew out the little orifice's, in both water lines that connect to the solenoid before and after the solenoid. Ran fan and heat function of the furnace and whallah, water is running into the drain again :)

I hope this has been helpful!


2 things to check.

Is the distribution water tray on the top of the filter level? If the water is not making evenly across the entire water tray, you will not be using all the area of the pad, and output would be low.

A follow on would be to watch volume of the water fall into the distribution tray. The water only flows all the way down to the pad from a few holes, you are not using all the area of the humidifier.

The click will not let you know if the water valve or pipe is not opening completely or clogged/stuck in some way.


I would take off the duct tape where the Aprilaire unit connects to the ductwork. I put tape on that same area in my last house and it caused suction problems with the filter inside the Aprilaire unit. It began leaking and causing efficiency problems. You can tape the joints of the duct work but i wouldn't tape at the Aprilaire unit itself.

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