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I have a Generalair drip humidifier attached to a Trane furnace, having the control circuit board d341396P05. Circuit Board d341396P05

The humidifier is wired to the heat cycle of the furnace, so it starts working only when the thermostat calls for heat. However, when I set the thermostat FAN ON (instead of AUTO) the humidifier does not kick in. It is still pretty dry in the house and my intention is to wire the humidifier such that it starts when the furnace fan starts (even without heat). I assume this has to do with changing the connections on the circuit board. As it is now, the 24V transformer of the humidifier has two wires connected to the HUM-N terminal (circled with RED in the picture) and to EAC-N terminal (circled with BLUE in the picture).

Could anyone please advise how to change the wire to achieve what I want, if at all possible? Your help is much appreciated, thank you!

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The terminal circled in red should be labeled HUM-H, which is the humidifier (hot) terminal. This terminal is energized to 120 V when the system is running in heating mode.

The terminal circled in blue is EAC-N, which is the electronic air cleaner neutral. This terminal should be tied to the other neutral terminals, as well as the system neutral.

If you move the humidifier transformer wire from HUM-H to EAC-H, the humidifier will run whenever the fan is on. However, this also includes when the system is in cooling mode, which is not recommend.

What you could do, is install an override switch, which will allow you to manually turn off the humidifier during the cooling season. So you'd run a wire from the EAC-H terminal, to one side of the switch. Then connect the humidifier transformer wire, to the other side of the switch. This way when the switch is closed, the humidifier will run with the fan. But when the switch is open, the humidifier will not run.

If you don't want a manual switch, you could mess around with a relay or other control circuitry.

Using a relay, it could be wire up like this.

Relay to control Humidifier

That way the humidifier will run whenever the fan is running, except when the thermostat is calling for cool.

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  • That was great, it worked as you said, thank you. I am not worried about the cooling mode since I do not have an air conditioner installed, rarelly need one where I live. Will keep in mind the wiring advice if I will install one. Just another quick question: would it make a difference in term of humidity output if I connect the water supply to hot water instead of cold, as it is now?
    – Theo
    Jan 28, 2017 at 0:22

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