Humidifier coming on when blower turns off.

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Old 03-25-16, 08:26 PM
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Humidifier coming on when blower turns off.

I have an Aprilaire 700 and the control board was fried when we bought the house so I purchased a Aprilaire 4655 manual humidistat to replace it. I have it connected like the manual diagram says, but now the humidifier only turns on when the blower is not running. I have the wires coming out of the EAC terminals on my furnace board.
When the heater kicks on the humidifer turns off. Basically the circuit completes only if there is not 24 volts across the EAC terminals and the humifistat is triggered. But if there is 24 volts across the EAC terminals the circuit breaks. Am I wiring something wrong?
 
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Old 03-25-16, 09:10 PM
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Welcome to the forums.

I'm a little confused. Normally the EAC output is 120vac. It would supply switched 120v power and the neutral would be obtained from the common neutral point.

What's the model number on that control board ?

You have a newer style 700 humidifier that requires 24vac on the input wires to operate.... correct ?
 
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Old 03-25-16, 09:35 PM
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The model of the Furnace board is a White Rodgers 50a55-843. It has a EAC-H pin and a EAC-N pin. I get 120 across these so I have a transformer that steps it down to 24vac. The humidifier/solenoid kicks on when the two wires coming from the humidifier are shorted. So it does not appear to need 24 volts across them to run.

-edit
As I have been researching it appears that pre-2008 700s had a different type of energizing circuit. I am guessing that is what I have.
 

Last edited by Phillip Thomas; 03-25-16 at 09:57 PM.
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Old 03-25-16, 10:13 PM
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Ok. You didn't mention the transformer in your opening post.

That board has two 120vac outputs..... HUM and EAC. The HUM may be a better choice as it is active with heat rather than just the blower running.

WARNING: You've got an older 700. It would be almond color instead of gray.
In this case you need a 120v relay installed in place of the transformer you now have.
A RIB-U1C relay is the perfect choice .Functional-Devices-RIB-U1C-Relay

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Right now you have two 24vac transformers fighting each other. Probably why the control board fried in the past.
 
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Old 03-25-16, 10:33 PM
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This makes so much more sense now, thank you. I ended up using a current sensing relay that was already installed (probably how they ran it originally) and it now functions correctly. I had no idea that I didn't have to use the HUM/EAC terminals on the furnace board since every single wiring diagram out there for this humidifier has them included. Thank you again. I have been struggling with this one for a while now.

I know the current sensing relay isn't the best choice (as it triggers whenever the fan is running, not just heat), but it works for now and I can look into getting that relay you mentioned in the near future.
 
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Old 03-25-16, 10:38 PM
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What you have is ok. You'll just need to shut off the humidistat when done for the season or the humidifier could run in A/C mode.
 
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