I have a furnace from the 80's so no humidifier connector. I have a newer, dc wall-plug transformer powered humidifier. So it basically runs all the time because there's no control. I only want it on when heat is on. There is a humidistat, if the humidity is low it will run, but it doesn't know whether my furnace is heating or not. There's no intelligence to it. Can I not just simply buy a 24vAC relay switch, connect one end to the furnace to toggle the switch, then connect the other end between the dc supply and the humidifier motor? I don't know much about hvac but to me this sounds like it would work. Where do I connect wires from the furnace / thermostat to the relay switch? To me, white = heat and black = common seems like the proper choice, but I'm uncertain. I had a dc relay kicking around and tried that, but of course it didn't work because dc relays wont switch well or at all on ac. I did take my multimeter to it and white seems to throw 7ish volts down the line when I turn heat on. Those black and white wires between the thermostat and furnace terminate on the furnace's transformer, I can only assume that those are the ones I could connect the relay to. However I'm curious if that might short something, would I need to diode isolate it, or maybe find a different spot to ground? Thanks!