hvacthermostatthermostat-c-wire
I'm trying to install a wireless thermostat for my church building's HVAC system. Currently there is no C wire, so I tried to see where the circuit board was so I could install one. I couldn't find anything that looked like a C wire though
If this isn't possible, I'm guessing a second solution would be to power it with an external transformer, but I'm trying to avoid that route if I can.
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.
It’s possible you don’t need a C (common neutral) wire when installing your Ecobee3. Check out the guide on using the Power Extension Kit that comes with your new thermostat: https://www.ecobee.com/faq/i-do-not-have-a-c-wire-or-common-wire-can-i-install-ecobee-residential-thermostats/
Best Answer
You may not have a C wire. The C is a common used to provide a constant power circuit (C is the Negative or Ground) for the thermostat to use.
In this furnace there are only 2 wires that a manual thermostat or battery operated on will connect which turn the furnace on.
Your only option is to run another wire (Or use another if there are extras) and connect it to the negative pole on the transformer (LOW VOLTAGE SIDE).
Its often the blue wire but I would need a better angle on the transformer to tell for sure on that particular one.
You should also check that the output is no more than 24v. If its anything else it could damage the thermostat.
CAUTION: There are live 110 lines into that transformer. Make sure your power is shut off at the breaker before touching anything in there. Make sure its covered when your done.