I'm installing a new thermostat and the previous one did not have a C wire connected. Tracing in back to the HVAC, I can't see a C output connection? Based on the wiring diagram (and reading other posts), i'm guessing i need to connect it directly to the transformer and the brown wire? Not sure if that's best, but looking for outside help. Bottom line – where should i connect my C wire and how should i connect it? Thank you!
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C Wire placement in HVAC
hvacthermostat-c-wire
Related Solutions
TR is Common on your unit, Every 24 volt device will have a Common leg as well as the 24 volt hot leg, just as a car battery has pos and neg, common is akin to neg in this case. Common is called this cause every 24 volt circuit terminates upon Common to complete the circuit. The last pic shown is the "time on" "temperature off" fan delay, the 2 lighter colored wires are low voltage, 1 id the heat circuits 24 volt hot leg being W or white, the other being Common or C. The gas valve has the same 2 24 volt leads as well, without completing the circuit nothing will work, every electric circuit requires 2 legs of power be it 1 hot leg and neutral/Common or 2 hot legs as in a 230 volt circuit. Google 40 VA HVAC transformer to see what they look like so you will be able to locate it. Your 2 wire thermostat will have only red and white shown on my diagram
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.
- Connect the thermostat as normal, with the system powered on, and the thermostat not calling for heat/cool/fan.
- Open the access panel for the furnace, and locate one of the limit switches.
- Remove one of the low voltage signal wires from the limit.
- If removing the panel cuts power to the system, replace the cover.
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
You are correct
The brown wire from the transformer to the relays in your unit is indeed the C wire you're looking for -- you can cut it and splice it with a bog-standard wirenut, or use a piggyback tab terminal crimped on the end of what you're using for a C wire to make the connection at a device. There may even be a spare terminal connected to it but nothing else -- the wiring diagram on your unit has provisions for a C wire from the unit to an external compressor contactor.