I wish to replace my Carrier humidifier with an Aprilaire 700. I have 3 questions related to properly connecting my Carrier Infinity 96 furnace's HUM terminal to Aprilaire 700 (24 VAC line):

1) I see only one HUM terminal on my Carrier Infinity 96 furnace circuit-board. I remember seeing a response to a question on "Home Improvement" that said that there should be 2 HUM terminals. Given my situation (one HUM), where should the other line to complete the circuit from the transformer be connected to complete the circuit? Should I connect it to the black/common terminal?

2) One comment in "Home Improvement" said "When the furnace is running in heat mode, you should measure ~120 volts...." So, does it mean that the HUM terminal drops to near zero volts when the heat is off? i.e. does HUM line function as a relay-switch or should I insert a relay between my Carrier furnace and Aprilaire humidifier?

3) What is the VAC on the black/common terminal in Carrier Infinity 96? If I connect the HUM terminal (120 Volts) to the black/common terminal (24 VAC as I read somewhere on the internet), would it burn my circuit board since the black/common line is really not a grounded line but one carrying 24 VAC.

Could someone help me please so that I don't mess up?


  • Can you post the model number of your furnace? – ThreePhaseEel Feb 22 '19 at 23:51
  • Model number for Carrier Infinity 96 is: 58MVB100-20; Product Number is: 58MVB100-F-10120 – user97485 Feb 23 '19 at 3:37

The simplest way to hook up is to the W terminal and the C terminal. Like this.

enter image description hereThe HUM terminal may be 120v or 24v if it's next to the low voltage connections (R C W G Y not necessarily exactly like this) then it will be 24v ALWAYS check with a volt meter. If HUM is off by itself or next to E. A. C. terminal then it's 120v. Either way it will only be live when the burner and the fan are on

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  • Thank you, Sir. I initially started off with the manner you suggested with W-B combo (Heat Relay). However, for some strange reason it did not work. As I was unsure, I connected my humidifier to the R terminal. Now the humidifier's solenoid & fan worked. However, the only problem was that they never stopped when the heat went off. I thought that this was because the R-terminal is not on a relay. That was when I pursued the HUM terminal approach. Now that you have provided me some REAL SOLID ideas, I will check both approaches once again. – user97485 Feb 23 '19 at 5:32
  • My pleasure. To test the HUM terminal you need to close the door switch, call for heat and wait for the fan to kick in. And yes R is always powered. – Joe Fala Feb 23 '19 at 5:39
  • HUM seems to be closer (2 inches) to the low voltage connections (A, B, C, D) AND (W2, Y1, DHUM, G, B etc.) Comparatively, EAC seems to be a little far away (6 inches). So high probability that HUM is 24 VAC. QUESTION: If HUM is 120v instead, where would I connect the other end coming back from the transformer. The reason I am asking is because Aprilaire provided a 120-24 transformer. Of course, if HUM is 24 v and has a relay, then that would be perfect. Thanks for your tip about closing the door (switch) when testing. Assume it is for safety reasons. Correct? – user97485 Feb 23 '19 at 13:46
  • Yes the door switch is for safety. Usually when the humidifier terminal is 120v it's a 1/4 spade and somewhere on the board there is a neutral bank. If there were no spots left simply connecting to neutral via wirenut would suffice. – Joe Fala Feb 23 '19 at 14:25
  • A big “THANK YOU, JOE.” It now works!! Yippee!!! Thanks for your guidance and for answering my questions most patiently, meaningfully and completely. After connecting the wires (HUM, Humidifier, Common circuit), I tested it as follows: 1) no heat/blower – humidifier was off 2) on high heat/blower – humidifier kicked in 3) turned down heat/blower – humidifier automatically turned off. So, I guess it works as it should. I’ll observe it for some more time Thank you, Sir!! Appreciate your help very much!! – user97485 Feb 23 '19 at 22:54

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