I found info at this thread, and would have placed this question there, but I can't comment since I'm too new.
I have HUM terminals on my control board, but they're line voltage (120VAC). I don't have a 24v transformer, so I hooked up the humidifier using the W and C terminals. It does work, but only when the air is being actively heated. It turns off when the heat distinguishes. At this point, the fan still runs to circulate the heated air that's already in the ducts.
My question is - if I were using the HUM terminals, would the humidifier still run as the air is being circulated but the heat is actually not firing?
I'm wondering if it's worth it to wire in a new transformer. It's not expensive ($35) but if it's redundant, I'd skip it.