On most heatpumps, the reversing valve is in the heat position at rest, meaning with no power to the coil that moves it. It you want the heatpump to cool, the 24vac must be applied to the coil of the valve to move it to the cool position. This is where "O" on your thermostat come into play. So... with everything off, 24vac comes into the thermostat at the R terminal. Its just sitting there waiting for you to turn on the heat or the cool. Now lets say you turn it to cool. Points close between the R on the thermostat and the Y on the thermostat & also the R and the O on the thermostat. 24vac flows from the R terminal into the Y terminal, out the wire that is attached to the Y terminal to one side of the contactor(makes the compressor come on). Power also flows from the R terminal of the thermostat to the O terminal, out the wire that is attached to the O terminal to the reversing valve, making it change positions for cooling. Power then comes from both the contactor & reversing valve back to the thermostat into the C terminal. Mind you, this is a very simple explination of a complicated process since there are many different H.P. t-stats out there with all different kinds of labeling. Hope this helps

I climb up the attic open the door at furnace and find out there is a transformer that step down 120 v to 24 v . and two terminals at the second end (24v) red terminal is power 24v and the other terminal is use to connect to frame or chassis and serve as common ground so a c wire is any unused wire you can pick any color you want in the same bundle that goes from the furnace to the thermostat in my case those extra wires are trim flush to the bundle jacket on both end so I need to peel that jacket off about 2 inches on both end and connect a single wire about 2 foot long from the frame to the wire you selected (let say black) on the furnace end then the black wire on the thermostat end will be "C" wire and connect it to the "C" terminal on the new wifi fancy Honeywell or Nest. If I say anything wrong please correct me. I will do this at some other time. for now I just thinking. I signed up for smart (something) at gapower.com and they will sent me a nest gen.3 for free. I think this nest also will need "C" wire. I wait till I get the Nest then I will install both the Nest and the Honeywell at same time I will put the Nest down stair and Honeywell up stair.

Tlewis is correct in that you need to be sure that it goes back to the C terminal. The C terminal is the Comon or Neutral side of the equation. The Rh/Rc terminals are made the same terminal by a jumper wire that goes from one terminal to the other, the wire that goes to that termial is the hot lead coming from your transformer. The C terminal is your neutral side, or ground side-depending on the configuration of the transformer. The wire that goes to this terminal must go to the neutral side or ground side of the transformer. Ususualy you can ohm out the C terminal to Ground and ensure its the proper terminal. There will be continuity between C & ground but virtually no resistence if the Neutral and ground are the same. E.W.

I see thermostat wire coming in on the bottom right. Find what the brown connects to. It looks like a mess :\ Orange uncapped, blue hooked to white instead of the unused white. That could be cleaned up for sure. Use the white wire for the white, cap off the unused orange and blue. Connect black to the brown to get your common as the black. Cant see the rest. Only real risk is hooking up AC incorrectly. Dont put the white coming from the AC to the white at the board, that is meant for heat. Put the white from the AC to the yellow at the board. https://smartthermostatguide.com/thermostat-c-wire-explained/ There are no standards for wire color! Any wire can be used for any purpose. A previous owner or handyman may have been “creative”, so what you find behind your thermostat might vary from what you read about online Here are common uses for wire colors: Blue or Black – C – Common wire, may be unused by your existing thermostat. Enables continuous power flow from the Red wire. Red – R – 24VAC power from the furnace’s transformer Red – Rc – 24VAC (dedicated to heat call) Red – Rc – 24VAC (dedicated to cooling call) Green – G – Fan White – W – Heat Yellow – Y – Air conditioner