Possible Bad Humidistat

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Old 12-28-11, 07:54 AM
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Possible Bad Humidistat

I have an american standard humidifer (Model: AHUMD500APA00BA) with an Aprilaire Manual Humidistat (Model: ???).

Symptom: The solenoid valve and fan do not turn on when the furnace is on and humidistat "clicks on".

When I manually short the brown wires from the humidifer the solenoid valve and fan turn on and everything appears to work correctly.

Am I correct to assume that I have a bad humidistat?

If I need to replace the humidistat, which model # would I need to purchase?

Thanks!
 
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Old 12-28-11, 08:32 AM
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Take a photo of the Humidistat.. Apirlaire has many style/model of them.
 
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Old 12-28-11, 08:53 AM
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Thanks Jay.

I will upload a couple phptos as soon as possible. In the meantime, would you agree that it sounds like a humidistat problem?
 
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Old 12-28-11, 09:47 AM
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I am not sure why I am unable to post images, however, the casing looks exactly the same as the casing on this website page

http://www.filtersfast.com/ProdImage...655-Filter.jpg

As for underneath the casing there is a black box from dnagroup.com with MODEL 884021BS listed on it.

Hope this helps. If not please let me know and I will try again to upload the images.
 

Last edited by subydave; 12-28-11 at 10:04 AM.
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Old 12-28-11, 01:39 PM
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Images



 
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Old 12-29-11, 08:03 AM
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If you took the two wires at the humidistat and shorted it together, did it run?
 
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Old 12-29-11, 08:54 AM
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I have not tried that. I will give it a shot this evening when I get home.
 
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Old 12-29-11, 02:06 PM
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Let us know what happens.. if the unit runs then you have a bad humidistat.
 
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Old 12-29-11, 06:16 PM
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Yes it runs when I short the wires from the humidistat. Do you know which humidistat I should purchase? Would the Aprilaire 4655 do the trick?

Thanks.
 
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Old 12-29-11, 08:08 PM
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Yes that one will fine for you.
 
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Old 12-29-11, 08:25 PM
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Thanks Jay11J for all of your help. I will purchase it and let you know if it took care of the problem.
 
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Old 01-10-12, 08:16 PM
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It was the humidistat. Everything is working now. Thanks again for your help
 
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Old 01-10-12, 09:23 PM
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Thanks for the update, and hope you have a good New Year!
 
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Old 10-21-12, 06:47 PM
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non-zero resistance when calling for humidity?

I realize this is an old thread, but it describes exactly my problem. I confirmed good 24VAC from the transformer when fan comes on, but the only way I get the solenoid to activate is to short the two terminals on the humidistat. When open, multimeter shows infinite resistance as expected, but when "closed" shows 50-90 ohms, not zero as expected (measured with the humidistat out of the circuit). So much for "dust-proof" control?

Humidistat is duct-mounted and came with Aprilaire 600M humidifier. Part number on the control is 884021BS.

edit: Should have talked to my son (repairs vintage musical instruments) before posting! He suggested putting a 9v battery across the terminals (with the humidistat out of the circuit) to burn crud from the switch contacts. Worked like a charm. Switch now measures zero resistance when closed and solenoid activates when humidistat is calling for humidity.
 
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Old 10-21-12, 07:15 PM
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A 9volt battery across the switch terminals to clean contacts.....a new one for me. I have a supply of those mini type snap switches. I've taken them apart and cleaned them in a pinch. I believe your repair is temporary and will probably return.
 
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Old 10-21-12, 07:34 PM
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Agree but it might be a while before it fails again. My son was puzzled that the solenoid current was not enough to keep the contacts clean, but let's assume the switch is really not well sealed. So since last heating season, it's been in the "dirty" return air stream but not powered so maybe the crud was gradually building up. If this theory holds then I should not expect to see the problem until Fall 2013.
 
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Old 10-21-12, 07:52 PM
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The problem with those switches, and like almost any switch, is an inductive load like a solenoid cause a large spike that causes an arc at the switch when it opens. Your switch is not crudded up. It has a buildup of carbon on the contacts from that constant arcing.
 
 

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