I have novice electrical experience, but I'm unfamiliar with HVAC setups. Last year I purchased a new thermostat for an older house, but I'm only now getting around to hooking it up. As best as I can tell one of the previous owners had a unit that was 5-wire, but the current unit is only four:
I've confirmed all five wires (blue, yellow, green, red, and white) feed through the house and to the thermostat - except the blue. It's not connected at the unit or the current thermostat. Here's the diagram for the unit:
The thermostat is a Insteon 2441TH, which uses five wires (I believe I'm only missing the Common). I also have a Venstar Add-A-Wire, but I'm not sure how to wire it up (if it is even needed). All the Venstar examples refer to wire colors or designations that aren't obvious in this setup. I'm stumped with what I need to wire together; any help with what I actually need to connect would be appreciated.