hvacthermostatthermostat-c-wire
Our old thermostat is working but I want to change to a new one (nest thermostat). I have two exact HVAC unit at home. I change one it works but when I change the second one it doesn't work because it can't detect power.
How do I fix this problem? Where can it possibly go wrong? Do I need to check my furnace?
First of all, don't make any assumptions about wire colors. There is no standard, everyone does it differently.
Secondly, there is a very clear C terminal in your picture, at the bottom right of the control board. It also looks like you have an air conditioner connected.
This "blue" wire you're talking about looks like it's not connected at the board. If my assumptions on the wires are correct, then all you need to do is connect the blue wire to the "C" terminal, and you'll have a C wire at the thermostat.
If I'm wrong about anything, or just for more background, you can refer to our go-to thermostat-c-wire question: How can I add a "C" wire to my thermostat? (or read any of the other questions with the thermostat-c-wire tag).
Based on the ladder diagram, it looks like the R terminal is only energized when all the limits (main limit and however many rollout limits there are) are closed. So if any of the limits open, the thermostat loses power (maybe).
I can't say for sure; since I'm not familiar with that board, but if that's how the furnace disables itself during a problem. Then bypassing that safety system can be quite dangerous. I'm not sure if the board monitors the limit circuit, or simply cuts power to the R terminal in the event of an open limit. If it's the latter, then connecting the thermostat directly to the transformer would be hazardous.
WARNING:
The following procedure requires working on energized equipment. If you're not comfortable with that, please contact a local licensed HVAC technician.
In this state the furnace will not work. But what you're looking for, is whether or not the thermostat has power. If not, then you're not going to want to bypass the IFC. It also means that whenever a limit opens, the thermostat is going to reset. Which is not a terrible thing, as it makes it obvious that there's a problem with the furnace.
Right now the thermostat is connected to the IFC terminals like this.
If instead you connected the thermostat directly to the transformer, then it would look more like this.
You'll notice that the limit circuit (highlighted in red), is completely bypassed by connecting the thermostat directly to the transformer. Which means even if one of those switches open, the thermostat will still be able to signal for heat/cooling.
Again, I'm not familiar with this IFC, I'm simply basing this on the diagram provided. The IFC may in fact monitor the limit circuit, which is why I recommend testing it.
Best Answer
With certain versions of the nest thermostat they require the C or common wire for operation. The nest thermostat needs 5 wires to accommodate the extra common wire. Check to see that how many wires the thermostat wire is. You may be lucky and have the extra wire tucked in behind the thermostat.
Another possible issue, if you do have 5 wires hooked up at the thermostat is the C wire may be disconnected at the furnace control board as it was not needed for the old thermostat. If this is the case you would have to open the furnace and add the C wire to the C terminal of the control board.