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I have an Aprilaire 600a humidifier from a few years ago and currently it's wired into my furnace with two control wires labeled "W/G" and "Cf". The "W/G" is connected to the W terminal on the furnace (call for heat). The "Cf" is connected to "C". My understanding is that any time call for heat occurs, W is shorted to C, which is detected by my humidifier and it enables the flow of water.

I would like the humidifier to run any time the fan is on, not just when there's a call for heat. The fan control is determined by the "G" terminal. Is there a way to change my wiring to do this?

Edit: Furnace model is Bryant 925t. Per the manual:

The HUM terminal is a 24 VAC output, energized when the gas valve relay is operating during a call for heat.

This seems like it wouldn't help since it's only available during the call for heat.

Edit 2: There is also a "EAC-1" terminal that is energized "whenever the blower operates". However, this operates at 115 VAC, not 24VAC. Is there a way I could use this?

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  • Also note that you may not get the expected results. Warmer air is able to carry more moisture. Blowing cooler air over the humidifier, may pick up little to no additional moisture.
    – Tester101
    Commented Jan 23, 2016 at 17:48
  • @Tester101 Understood. If I can get it hooked up with the fan-only mode I will measure the evaporation rate to ensure it's working. The input water is hot, so I do expect it to work OK.
    – KyleL
    Commented Jan 23, 2016 at 18:35
  • @KyleL you commented the input water is hot... it should not be. 2 reasons: 1, you'll be running off of your water heater all day, every day, this increases your energy expense and your cost. 2, because unless you have the water heater set at above 150°F (already above the required safe temp of 140°F), then you don't risk bacteria developing in your humidifier, causing legionella. Safe practice requires you to use cold water input at all times. I strongly suggest you change your input. My 2¢!
    – couzin2000
    Commented Dec 6, 2016 at 21:55

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You could use the EAC-1 terminal to power a step-down transformer. This is common with furnaces that energize the HUM terminal to line voltage.

Check the humidifier manufacturer, they should have a properly sized transformer to fit this exact need. They should also have a wiring diagram for this type of setup, in the installation instructions.

Basically, the primary side of the transformer will connect to the EAC-1 terminal, and a neutral terminal on the furnace control board. Then the secondary side will power the humidifier.

Whenever EAC-1 is energized, it will power the transformer, and the humidifier will turn on.

As noted in the comments on the question, you may not get the expected results.

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  • Do you know to connect a wire to the HUM or EAC-1 terminal? They are not the screw-based connection type like the W and G connectors and look more like a plate with a hole on them. Will it require soldering?
    – KyleL
    Commented Jan 23, 2016 at 19:14
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    @KyleL You'll need a properly sized "insulated female terminal connector" (a.k.a. "insulated femal quick connect terminal connector", "insulated female connector", etc.).
    – Tester101
    Commented Jan 23, 2016 at 19:19

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