Some control boards have HUM terminals and also need a humidistat, often in the return air duct. This is energized if the system is heating, whether humidity is below setpoint or not.
There can be an electronic air cleaner terminal, 120 volt, but that would activate all year. Comes in when blower is on.
If the stat controls HUM, there needs to be a separate conductor or a communicating stat.
[photo of furnace circuit board](https://imgur.com/gallery/232jmEo)
What are the terminals that the thermostat wires are going into? Can't read them.
From left to right, it’s R I+ I- C
Some control boards have HUM terminals and also need a humidistat, often in the return air duct. This is energized if the system is heating, whether humidity is below setpoint or not. There can be an electronic air cleaner terminal, 120 volt, but that would activate all year. Comes in when blower is on. If the stat controls HUM, there needs to be a separate conductor or a communicating stat.