Question about Aprilaire 550 Humidifier

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Water keeps running into humidifier even furnance blower is not

Aprilaire model 550 with manual controller. Water continues to flow to humidifier even furnance blower is not running. I can stop it by turn humidity control down, then water stops flowing even furnance is on.
tried power off furnance, then no water flow at all (of cause)
What needs to be fixed?? soleniod problem? bad electric wiring??


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  • 30all Mar 07, 2009

    Ha, I finally fixed the problem of continuing water flow to humidifier even furnace blower is not on. After searching and studying around on-line. I found the transformer was hard-wired in junction box of furnace, and there is no relay sensor. It was installed by "professional contractor'" from Air Group in NJ in the first place years ago. Since my Trane XE79 has HUM connection, I re-wired transformer to it, problem solved.
    Can we trust any contractors in these days??

  • cholden Mar 08, 2009

    Aprilaire 550 Humidifier- 5.5 yrs. old, worsening constant flow through the humidfier. All worked great for first 2 winters, water now flows freely through humidifier when furnace not blowing if humistat is set above 5%. last winter I had to go from 15 to 10%, yr before that- 20% was the max. The first 2 years I could set the humistat according to the outside temp., as it should be.
    Problem not related to outside temp, or flow of water INTO the humidifier.
    I don't know whether to try to replace (or clean?) the solenoid or humistat first.... or call a repair person. Items are in a condo that was gut rehabbed- my inspector found that the solenoid was not working, it was replaced (still have the original one, new but disfunctional at the time, right here)

  • 30all Mar 09, 2009

    I replaced humidid control in my first try, did not work. but I left the new one in because the old one was dirty anyway. The new control panel will not fix the old one (too small), I had to gut the old one and replaced only inner parts. now I have a useless new stat frame and cover.

    I tested solenoid by disconnect wires to it, water stopped. I don't think it's it. so I decided to make sure the wiring is correct before spending another $50. btw, I was pretty knowing nothing about how furname and humidifier are wired together before this.



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This sounds like, the 550 transformer is not connected to the blower motor power connection (in parallel)

Posted on Mar 04, 2009


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