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Old 11-07-2012, 04:54 PM  
shake
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Default How can I control a newer humidifier from an old furnace?

I have a furnace from the 80's so no humidifier connector.

I have a newer, dc wall-plug transformer powered humidifier. So it basically runs all the time because there's no control.

I only want it on when heat is on. There is a humidistat, if the humidity is low it will run, but it doesn't know whether my furnace is heating or not. There's no intelligence to it.

Can I not just simply buy a 24vAC relay switch, connect one end to the furnace to toggle the switch, then connect the other end between the dc supply and the humidifier motor?

I don't know much about hvac but to me this sounds like it would work.

Where do I connect wires from the furnace / thermostat to the relay switch? To me, white = heat and black = common seems like the proper choice, but I'm uncertain.

I had a dc relay kicking around and tried that, but of course it didn't work because dc relays wont switch well or at all on ac.

I did take my multimeter to it and white seems to throw 7ish volts down the line when I turn heat on. Those black and white wires between the thermostat and furnace terminate on the furnace's transformer, I can only assume that those are the ones I could connect the relay to. However I'm curious if that might short something, would I need to diode isolate it, or maybe find a different spot to ground?

Thanks!



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Old 11-20-2012, 12:26 PM  
nealtw
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I don't have any answers but I will pose another question. If the furnace fan is running on 120 volts could you not wire an outlet with that to plug in the adaptor and it will only have power when the fan is on?



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Old 11-20-2012, 03:56 PM  
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Quote:
Originally Posted by nealtw View Post
I don't have any answers but I will pose another question. If the furnace fan is running on 120 volts could you not wire an outlet with that to plug in the adaptor and it will only have power when the fan is on?
Provided you unplug when running the AC. Or use a sail switch in the duct.
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Old 11-30-2012, 06:00 PM  
seawood
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Default this is how

you need a volting senceing relay

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