I have central air and heat, with a Nest thermostat. My system has Y, G, R, C and W wires coming out of it:
which are wired into my Nest like so:
In between, there is a splice where the C wire is spliced with electric and another wire runs from that splice back to the C hookup:
And then the other wires are spliced with a new set of the same color:
The heat and cooling work fine. I just can't get the fan to run on its own. What could be at fault and how can I diagnose it?
I would first try touching the R and G together at the thermostat board (the wires coming out of the wall) and seeing if the fan turns on. If it does, then all the wiring is correct. If it doesn't, then I would try to do the same thing with two short wires that have been attached to the thermostat control board R and G terminals. If it still doesn't, then the problem is in the furnace. If it does, then you have a loss of continuity somewhere between your thermostat and your furnace on the G wire (because the heat and A/C work, so the R works). I don't have a Nest myself, but I know that not all thermostat systems make it obvious how to run the fan without cooling or heating.
It's also possible that if you hear a clicking sound, the relay is activating but the motor isn't connected to the switch. My blower motor had multiple lines for various speeds. When the Nest activates the heat, the fan would turn on after 30seconds, but that circuit is independent of the relay. So I had to choose another cable to run to the relay.
Then when I shorted the wires on the wall, it worked!
