How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Phil Your Own Question
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
Type Your HVAC Question Here...
Phil is online now
A new question is answered every 9 seconds

I have the Aprilaire 400 digital humidifier installed but

Customer Question

Customer: Rich. I have the Aprilaire 400 digital humidifier installed but not wired. The diagram provided is not clear to me as I am not versed in wiring. I need easy instructions. There are only a few wires so it can't be that confusing.
JA: Thanks. Can you give me any more details about your issue?
Customer: I am not sure where the transformer goes.
JA: OK got it. Last thing — JustAnswer charges a fee (generally around $18) to post your type of question to HVAC Experts (you only pay if satisfied). There are a couple customers ahead of you. We can help you for less if you're not in a rush. Are you willing to wait a bit?
Customer: Yes
JA: OK. Now I'm going to take you to a page to place a secure deposit with JustAnswer. Don't worry, this chat is saved. After that, we will finish helping you.
Submitted: 1 year ago.
Category: HVAC
Expert:  Matthew replied 1 year ago.

Are you using a current sensing relay?

Expert:  Matthew replied 1 year ago.

And are you going to use the humidistat that came with it to control humidity?

Customer: replied 1 year ago.
I'm not sure about the current sensing relay but I am using the didital controler provided.
Customer: replied 1 year ago.
There are threads on the transformer like it attaches to something but I don't know what
Expert:  Matthew replied 1 year ago.

great. The digital controller has the terminals


from your furnace control board where all the thermostat wires hook in you are going to run a wire from the R terminal of that board to the R terminal of the controller.

Run a wire from the C terminal of that board to the C terminal of the controller.

Your outdoor temperature sensor goes into the ODT terminals of controller.

Run a wire from the W terminal of the furnace board to the W terminal of the controler

At your furnace control board you will find a wire under G (should be green wire). You take that wire out of the G terminal and wire nut a wire onto it to extend that green wire up to the controller G terminal.

Then run a wire from the controller Gf terminal down to the furnace control board G terminal.

out of the transformer provided with the humidifier you have 2 low voltage wires. One wire will go up to the controller and hook to one of the H terminals. The other one will go up to one of the two yellow wires hanging off the humidifier itself. Then the other yellow wire on the humidifier will run up and hook to the other H terminal on the controller.

Customer: replied 1 year ago.
what does the transformer screw/attach into ?
Expert:  Matthew replied 1 year ago.

you can mount it just about anywhere you please. It has to hook into 120v somewhere so most often it just mounts right inside the furnace cabinet somewhere so that you can splice off the incoming 120v feed that supply's the furnace power. Will be a white and black wire that is wire nutted into the furnace high voltage feeds.

Expert:  Matthew replied 1 year ago.

You will want to make sure high voltage is off to the furnace. Normally you will have to provide larger wire nuts now that you have 3 wires being put together instead of just the two that are already together in the furnace now for high voltage.

Customer: replied 1 year ago.
Okay thanks, ***** ***** be helpful. I'll let you know if I need more assistance.
Expert:  Matthew replied 1 year ago.

Sure thing, I check back in periodically throughout the day.

Customer: replied 1 year ago.
I take it the low voltage wires connect to the terminals on the transformer. I have the transformer mounted but having trouble identifying the high voltage feeds from the furnace.
Expert:  Matthew replied 1 year ago.

There should be a high voltage wire/whip that runs into the side of the furnace. This wire comes from either a toggle switch next to the furnace or if the furnace is within 6' of the breaker box it will run directly from the box into the furnace. Depending on the model furnace you have the wires will lead into an area where they then wire nut into two wires leading off the furnace. The hot lead (black wire) normally comes off the door safety switch. This switch is pressed in when the furnace doors are securely put on. The white lead (neutral) will lead from the furnace control board up to the wire coming into the furnace either from said toggle switch or from breaker box.

Most often these wire nut connections are behind a little metal box that covers them up. You have to take the little screw out of the little metal box so the side of it pulls off and exposes these wire nut connections.

Expert:  Matthew replied 1 year ago. in this picture it's on the right side. The metal conduit outside the furnace has the high voltage wire in it and leads into this box where the wire nut connections are made

Expert:  Matthew replied 1 year ago.

And correct. If your particular transformer has two little screw terminals on it and not wires then that is where the low voltage ties in.

Customer: replied 1 year ago.
I located the box and attached a photo of it. There are three black wires inside of the box. One comes from outside of the furnace and I suspect it is the lead wire. Of the other two black wires that come from inside of the furnace, one is marked HUM and the other EAC and they are loose and capped off. Also Inside of the box there are two white wires that are fastened together and a blue wire I suspect is the ground. The black wire marked HUM is connected to what looks like a transformer similar to the one provided with the humidifier. Is this unit pre-wired for a humidifier? I'M confused.

Related HVAC Questions