Honeywell furnace circuit board

Hi, I just installed an Aprilaire whole house humidifier in my ductwork. All that's left is the wiring. I have a Comfortmaker Enviroplus90 gas furnace. It has a Honeywell St9120 fan timer control board. It has a humidifier connection plus a neutral connector. I wired from this to the aprilaire control. But before I turn everything on, does anyone know if these connections are 24vdc or 120vac? I can't find any documentation on that. I assume DC but maybe I have to wire the transformer in line? Thanks, Jeff
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Jeff Guay wrote:

I seriously doubt they'd be dc, but they very well might be 24V AC.
That's what meters are for.
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The connections from the board are 120v. You will need a transformer to cut the voltage down to 24v. Make sure you come off the humidifier tap. It powers up when the inducer fan comes on. The EAC tap energizes with the blower.

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You dont have enough information. You also, dont know a thing about the cost of that board, since that board was in several configuations for several different models. It is NOT a DC board, nothing on that unit as far as controls is. As far as the output....that would be something that your installers doc would tell you, or you could measure. However, on a gas unit like that, Id strongly suggest that you use a current sensing relay as you should have already been told in the AprilAire docs... And as a hint..I REALLY would not wire that to the Neutral on that board...if you did, and you have applied power in testing the unit, you have fried, or come real close to frying the solenoid on the humidifier. Since you ddint say what model Aprilaire you have, this could be hit or miss on what you should do, since different models come with different parts....but..assuming you bought the proper sized unit for your home, and assuming you got the cheapest one they offer:
You should have a Transformer, a humidistat, the bypass humidifier, and some assorted fittings. Use the transformer, and do NOT try to use the one with the unit, since should you fry the solenoid on the humidifier, its on a sep setup and you can still have heat.. Follow the instructions, mounting the transfomer on a J box as its shown, and run the wires to the unit, and the remote current sensing relay that didnt come with it that you have to buy separate from it, as shown. Remove the wire that goes from your board to the motor that is labeled HEAT, slide the relay over it, and reconnect it to the board. And assuming that you didnt spend the extra $20 on the model with the remote sensor, you dont have to worry about wiring that part...
Should take about an hour and you are done....assuming that you have your water hooked up, and your bypass duct ran, and supported, and your drain ran out. Strongly suggest you run the units water drain on a dedicated line as well, and NOT in the AC condensate drain.
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CBHVAC wrote:

Hi, This means it is 120V AC. You hook up the x-former which came with the humidifier kit here and use 24V AC output from x-former to control your humidifier/humidistat.. x-former derived 24V AC is floating, no such thing as hot and neutral. Trying to work on wiring without a multimeter of some sort? They cost very little you know. Good luck, Tony Contr>>aprilaire control. But before I turn everything on, does anyone know if

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Thank you for your help. The unit is the Aprilaire 600. It does have an outdoor temp sensor. I also found the info on Honeywell's site. The output is definitely 120v. Are you saying not to use the transformer that came with humidifier? I realize that there are two wiring configurations, one for a furnace without a dedicated humidifier control and one for a furnace with. Can I wire the 120 from the board's humidifier connection to the supplied transformer's input and run the output to the humidistat? The solenoid would only operate when the blower runs, right?

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Can I wire the 120 from the board's humidifier connection to the

Hi, You got it. Key is humidifier and electronic air cleaner only need to come on when blower is on. You're all set. Good luck, Tony

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