Need Switched 120 VAC From Furnace For Humidifier Fan

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Old 10-22-15, 07:58 AM
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Need Switched 120 VAC From Furnace For Humidifier Fan

I have an Aprilaire 760, connected to a Rheem Criterion furnace (14 years old?), and the humidifier/humidistat is wired directly to the furnace to operate the solenoid. The fan motor, however, isn't wired to anything, so it doesn't operate. It has been tested good. I need to find a way to provide it with 120VAC, only when the furnace blower is on. Any ideas? I don't want to have to put in the current sensing relay, but I will if I have to. Thanks in advance.
 
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Old 10-22-15, 10:08 AM
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Welcome to the forums.

There are two 760's. There is the 760 and 760A.

Does your unit have an AC power cord attached to it ?
If it does.... there was supposed to be a control board in it for the fan.... is it gone ?
 
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Old 10-22-15, 12:38 PM
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It's a 760 with a power cord, and there is a control board on the fan motor.
 
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Old 10-22-15, 02:07 PM
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What are you using for a humidistat ? Model number ? .... or just basic two wire one ?

This should be the unit you have. It needs the 120v to open the water valve. Is your valve opening now ?

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Old 10-23-15, 06:17 AM
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The water valve opens properly. I just need to power the blower. The motor was just replaced, and tested as good. The previous own of this home wired the humidifier directly to the furnace control board, and also damaged the motor.
 
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Old 10-23-15, 10:19 AM
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There should be a 24vac control relay in the unit. That may be missing or damaged too.
 
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Old 10-23-15, 12:07 PM
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Where would I find it? I'm probably looking right at it.

Thanks for all your help.
 
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Old 10-23-15, 07:04 PM
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For some reason the 760, with the multiple issues, always causes problems.

You have a slightly newer version. The one I posted above uses a 120v solenoid. When I re-read I see yours has a 24v solenoid. Evidently the control board was removed from that unit. You need an Aprilaire 4238 control board. This board supplies 24vac to the control circuit and the solenoid. It does not get connected to the furnaces 24v which is why the original was fried by the previous homeowner. Shop around for best price.

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However....
If you want to forgo installing a new board you can just get a 24vac N.O. relay and use that to control the fan. This route would be the easiest but it would be up to you to mount the relay in a safe manner inside the unit. 90-290, 90-290Q, 90-293, 6AZU2 are all basically similar and will work fine.

White Rodgers 24-Volt Coil-Voltage SPNO RBM Type Relay-90-290Q - The Home Depot

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