Looks like you have a heat pump system (B wire is usually for a heat
pump to switch over from ac to heat) and also 2 wires on the heat
outputs W1 and W2. If you go to the indoor unit and take the panel off
the front take pics of the make and model nu...
You will need to go to the furnace and remove front panel. Hold the door
switch in with your finger when panel is off. Now look at the control
board see if any led lights are on. That will tell you if there is power
to the board. Then look for a fuse...
Is there another thermostat in this system or only this Nest? I see 2
thick thermostat cables, and 2 thin (2 conductor R W) entering the
furnace. Which thick cable comes from the Nest? Looks like the black
wire nut in the center of the pic is all wir...