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I just got a Trane XC95 communicating unit with a Nexia XL1050 thermostat. I want to connect my Aprilaire 700 humidifier. Since it's a communicating setup there's no way (that I know of) to connect the Aprilaire's humidistat because the terminals on the furnace control board are disabled in communicating mode. But there are connections for an external humidifier on the control board that supply 120V when the communicating thermostat (Nexia XL1050) calls for humidity. I was considering connecting these to the primary of a 120/24 transformer whose secondary is wired directly to the humidifier. That way, when the wires go hot the fan and water solenoids in the Aprilaire will be energized. Trane and Aprilaire offer little advice and the installer was unfamiliar with hooking up a humidifier with a communicating thermostat so I'm hoping someone here can tell me if this is the correct way or if there is another.


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Appreciate the advice but I got it working. The HUM (120V) line on the board does indeed go hot during a call for humidity from the communicating thermostat. The Trane brand humidifier is identical to the Aprilaire I have so wiring the HUM line, and it's return, to a 120/24V transformer properly controls the solenoids for the humidifier's water valve and blower fan. The AC power line are wired directly to the furnace power block although they could have just been plugged into a wall socket.

The distressing part was that the certified Trane installer had no clue about how to do this. He said it wouldn't be a problem when we bought the unit but then didn't want to be bothered. When he did finally get around to it he wired it all wrong (didn't even know about the on-board circuits). When it didn't work he told me I need to get a some sort of airflow proving relay and pretty left it at that. Fed up with his elaborate but ineffective wiring scheme I found about the on-board circuit in an internet discussion. There's nothing about it from Trane. Arpilaire did try hard to help but their wiring plans are for their own humidistats.

In the end it was super easy, once I found out how. Now I know more than the Trane installer :) Thank you to all those who share information.
 
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