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Had my system replaced last summer. When they hooked up the humidifier, they just went to the transformer output for power. Now anytime humidity is below the set point, the water runs even if the system is off. Doing some research I found out this unit has HUM terminals. How do I connect my humidifier to the s9v2 HUM terminals? I put a multi-meter across the terminals to see if its 120v or 24v but just get 0v. This was measured during a call for heat and with no call. Am I measuring correctly? Or is it from one terminal to somewhere else?

Model number S9V2B060U3PSAAA

Thanks, Scot

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    The HUM terminals may just provide contact closure, acting like a switch. If you find no voltage across the in AC or DC mode, then I would switch to Ohms and check to see see if it's closed only when the blower is running. – Tyson Jan 8 '18 at 1:40
  • There are about a bazillion ways to control a humidifier so it is difficult to tell from what you have written. Usually with a 24 volt humidifier I would run it through the W terminal so it will only have power when heat is called for. With Trane and American Standard at least, their HUM terminals are 120 volts. I almost never used them. – user76730 Jan 8 '18 at 8:08
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The HUM terminal on most furnaces is a 120VAC terminal. If you want to use the HUM terminal to control the humidifier, you're going to have to use a step down transformer, to drop that down to 24VAC.

To measure the voltage on the HUM terminal:

  1. Set your volt meter to measure AC volts.
  2. Place one probe on one HUM terminal.
  3. Place the other probe on the other HUM terminal.

When the furnace is running in heat mode, you should measure ~120 volts about 1 second after the blower motor starts.

"HUM relay closes on any heating call (HP/Gas) approximately 1 second after the blower motor starts"

- From manufacturer's literature

According to the manufacturer, the HUM terminal is rated for 1 ampere at 120 volts.


You may be able to simply connect the wire from the humidifier to the W terminal in the furnace, instead of directly to the transformer. That way, the humidifier will only get power when the thermostat is calling for heat. However, without knowing more details about your system, it's impossible to say for sure if that will work. Please provide the make and model of the humidifier, and a wiring diagram/photos, if you want a more accurate answer.

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Your HUM terminal is probably 24v DC. Check your schematics to confirm. You could then test for current when the furnace is providing heat.

If your HUM terminal is rated for enough current to power your humidifier (unless you have an unusual setup, they probably are) and if your humidifier uses 24v DC. You can probably skip using the transformer and wire your humidifier direct to the HUM and common terminals. This worked for me on my furnace. Your mileage may vary. Don’t make any changes unless you confirmed all the details and are confident in your changes.

  • HVAC controls are typically 24VAC, not DC. Also, most HUM terminals are line voltage, not low voltage. – Tester101 Jan 8 '18 at 12:15

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