I purchased an Aprilaire 700 to install soon. I have been looking over the documents and it has an automatic setting with a sensor that goes outside and a digital control, so it will need continuous power. It has a plug to plug into a wall outlet, but I don't have an outlet convenient to plug into. They would require an extension cord which id rather not do. Can i some how tap into the power from the furnace?

I have a brand new construction home that I moved in this past April, it has a Lennox furnace, which gets its power from the box that the overhead ceiling light is connected to. There is also a switch connected to the box to cut the power to the furnace, but a separate switch to turn on and off the basement lights. There is also a transformer connected to it that either the doorbell connects to or the thermostat wire connects to.

Any information would be appreciated.


I will be putting on the right side here in the picture, as that's really the only place with room to do it on the supply. I don't want to do it on the return:

light box with current transformer:

Light box with transformer

Light box with transformer


enter image description here

Electrical supply to furnace:

electrical supply to furnace

Front of unit:
front of furnace

Inside unit:
inside furnace




Edit, inside light box: http://imageshack.com/a/img921/4453/p7sCdk.jpg http://imageshack.com/a/img921/1553/06sSY7.jpg

second update inside light box: http://imageshack.com/a/img921/6752/T8RPYD.jpg http://imageshack.com/a/img923/5521/5hKYYU.jpg http://imageshack.com/a/img924/5690/hC5ABS.jpg http://imageshack.com/a/img922/1246/TxVeJD.jpg

  • Can you post a photo of the inside of a) the furnace service switch box and b) the ceiling light box? – ThreePhaseEel Dec 4 '16 at 14:45
  • I updated with links to imageshack as my photos are too large to place inline here. Thanks! – eaglei22 Dec 4 '16 at 20:19
  • sighs -- I still need to see inside the box with the transformer attached...I don't have remote X-ray vision! – ThreePhaseEel Dec 4 '16 at 21:46
  • Oh, I thought since I did, everyone did! Misread what you first wrote.. updated. – eaglei22 Dec 5 '16 at 1:26
  • Holy cow that box is overstuffed! Can you pull the wires out some and get another photo? I can't tell heads from tails! – ThreePhaseEel Dec 5 '16 at 1:48

Alright -- while that box looks pretty stuffed, best I can tell, it has enough room for a couple more wires (being a big ol' 4-square with a mudring to boot). If you can get an EMT conduit run from where that box is to where you're sticking your humidifier (half-inch'll do quite nicely as all you're shoving through it are 2 14AWGs) and some black (or some other color than white, grey, or green) and white 14AWG THHN run through it, you're 90% of the way there.

From there, just get a self-grounding, single, 15A receptacle (not a duplex receptacle) and whack it in a surface mounted single gang "handy box" attached to the wall -- black to brass, white to silver, and let the EMT be your ground. In the ceiling box, with the power off of course, cut the orange wire in two, strip the ends, pull some slack out on the orange wires, connect your new black to the cut ends of the orange wires, and connect the new white in with the big bundle of white wires. Once everything's hooked up, flip the power back on, and you're good to go.

This arrangement is allowed by 422.12 Exception 1:

422.12 Central Heating Equipment. Central heating equipment other than fixed electric space-heating equipment shall be supplied by an individual branch circuit.

Exception No. 1: Auxiliary equipment, such as a pump, valve, humidifier, or electrostatic air cleaner directly associated with the heating equipment, shall be permitted to be connected to the same branch circuit.

Exception No.2: Permanently connected air-conditioning equipment shall be permitted to be connected to the same branch circuit.

  • Okay thanks! I will see what i can figure out. Seems like it's something doable within my comfort zone. I will pop back on here when start going with any other questions. Would it be easier if i moved the on/off switch to the furnace and used a combination box with one outlet, and hooked the switch as normal to the furnace but kept steady power to the outlet and just mounted to the furnace and fed it using the flexible tube anyway going to the furnace? If i did branch off – eaglei22 Dec 5 '16 at 4:42
  • Also using your approach, the red goes to the on/off terminal switch and then brings back an orange. Would i feed the black off at the on off switch? And should i give it constant power on the live feed terminal on on/off switch or dependent on switch? And then white would just connect in with the white in the ceiling light box correct? And everything is grounded through conduit. Sorry for poor terminology, I am a computer programmer by day.. – eaglei22 Dec 5 '16 at 4:49
  • Oh rereading you say to connect to the black and white the transformer is using. I am confused, what is that transformer for, and in this case what is the difference between the black and the red fed to the on/off switch that cuts power to furnace. Lots of comments i know, but I am exploring my options, and trying to understand the current setup. Thank you. – eaglei22 Dec 5 '16 at 4:55
  • @user1794106 Where does the orange wire go? Also, can you figure out where that transformer's secondary wires actually go, for that matter? – ThreePhaseEel Dec 5 '16 at 5:07
  • Sure, I will do it when I get back from work tomorrow evening. Thanks again for all the help. From what it looked like when i had the cover off earlier, the orange feeds to the furnace (the flex conduit going into the side). The on/off switch to the furnace is the switch you see in the picture across from the furnace attached to stud. All lights in basement turn on from a single switch at stairs. – eaglei22 Dec 5 '16 at 5:22

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