Another "don't have a C wire for thermostat" question. Been following along, and if I could identify the C connection at the furnace, I'd be all set. Unfortunately, it's not simply marked C or common. I have attached an image of the wiring diagram of my Utica MGB-2490 boiler. Am I correct in thinking that the blue wire terminating at the " PV 3" is what I'm looking for?
thermostat only had 2 wires connected to R and W (extremely old Robert Shaw mercury type), there is a 3rd wire tucked in wall, I followed bundle to basement, it is not connected to anything, could use for C if I am sure what to connect it to. Any and all responses are appreciated
Unsure where to find “C” connection on furnace
thermostat-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
The initial picture showing that I connected into the other blue nut was correct. The unit had a button that had to be pressed in to ensure that the cover was on. DOH!!!!
Best Answer
You want pin 5 instead
The connection you want is pin 5 of the connector you have marked, labeled "24V GND" on the diagram -- it has a black wire running form it. You can also tap the low-voltage junction between the white wire from the transformer and the black wires -- it's the same thing.