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?
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
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
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
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
Strongly suggest you run the units water drain on a dedicated line as well,
and NOT in the AC condensate drain.
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.
Contr>>aprilaire control. But before I turn everything on, does anyone
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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