Hi, I have a 1 stage heating/cooling forced air furnace/ air conditioning system. I had a RiteTemp thermostat that did not use the C wire, it was disconnected but present. I purchased a Nest learning thermostat and tried to install it. Like many others, i am having huge problems with it. When I connect Rh Y W G and C, there is no power to the Nest Thermostat. I tried using what some others suggested, connecting G to the C to bypass fan control, that also did not work. I am curious why connecting the C brings no power to the thermostat. Here is a picture of my actual furnace wiring board and a diagram sketch. The yellow C wire is present at the thermostat.
https://imgur.com/a/MQIgo8u







Edit: I should clarify I do get power without the C connected, but it wont call heat and short cycled the furnace.

 

You have a zoning board. Nest has never seemed to work well with zoning equipment.
I’d suggest returning the Nest. They are full of problems such as these. Honeywell and Ecobee make a superior product.
Start by testing for power between C and R with your meter.
Where is C connected? I see an unusual wiring setup there.

 

Thanks for the reply. I am actually not sure where it is connected, its connected to the zoning board obviously, but I havent looked in the furnace yet. I will take a look tonight, I am not much of an electrical guy so me trying to do this probably isnt the best idea, but thought Id give it a shot. I havent tested it yet, will try tonight.

 

You do not have a C to the thermostat. You need to connect the yellow wire of the thermostat cable to the top C connection. The wire currently on C goes to the equipment..... not the stat.
In your picture are a loose brown and yellow wire. That yellow goes to the stat.

The relays on the zone board don't require enough current to keep the nest charged. It may work for a short while but without C the stat will run dead quickly.

 

PJmax, thank you for the response. Are you saying I add the yellow C wire to the zoned board connection "equipment" C, or replace the current yellow wire that goes to "equipment" with the line from the stat? Is that ok to have 2 lines connected there or if I remove the existing line, which goes to furnace/AC, does that mess that up also?

 

The wire currently on C supplies common power to the zone board. It must remain there.
You can connect the other C wire to the same terminal so that you'll have two wires on there.

 

Thank you! I am going to give that a try tonight