The red wire is usually the 24vac hot lead. The thermostat connects that to one of the other wires to turn on the desired function. Keep in mind that any circuit requires two leads for a complete circuit, but it might be referenced to ground rather than being floating. The transformer takes the line voltage (120vac) in, and reduces it to the 24vac required to run the various control circuits. The HUM lead may only be on when the fan is called for, and not continuously, so may not be a good choice. You'd want to verify anything with a multimeter prior to connecting it to ensure you don't fry something.