Hello, i have an older Airco forced hot air furnace that uses a Becket oil burner and Honeywell R8184G 1302 Oil Primary Control. I am at a loss as to where to install the power connector to use with our new Learning Tstat. There really is not 'circuit board' that i can see, the Honeywell is installed over a junction box, it also has a simple terminal on the outside of the box with TT (thermostat) and FF (CAD Cell). I've attached a few photos for reference.
Yes you could put a wall plug in transformer near the thermostat but if it is easy to pull a new thermostat cable up to the Nest you are better off just running a new 5 conductor cable.
If you run a new cable you dont need the extra transformer just put Red to top T, White to inner T, and Blue to bottom F screw (Common).
50 ft roll of 5 conductor cable at the big box stores is like 20 bucks.
Ok, i was confused because I thought the bottom 2 F's were only for CAD Cell wires, but you think based on the wiring diagram that the bottom F is also the Common? If using the 5 conductor wire as you mentioned, what would the wiring look like at the thermostat? Red > Rc?, White > W1, Blue > C, what are the other two wires?
In the diagram the transformer is shown in the bigger box it is the symbol with squiggly lines the top wire leaves transformer goes up to the top T terminal, the bottom wire leaves transformer goes down to bottom F terminal.
If you want to check use a multimeter set to AC volts (not DC) and put meter leads one to the top T terminal and other lead to the bottom F terminal and see if you read approx 24 - 30 Volts AC.
The extra color wires are just spare not used but in the future if you ever put in a AC system or a gas boiler etc all smart thermostats need the extra C wire so you will have the correct cable already run and not need extra transformers etc.
Thank you so much, my local HD sells 50' of 18/5 for $23, that should do the trick. The only thing i still need to figure out is the switch mounted to the outside of the furnace that turns on/off the thermostat. You can see in my first photo wires running into the a junction box on the inside right of the furnace, the switch box is on the outside. The thermostat cable runs from that switch to my old thermostat on the 1st floor.
The thermostat wires should not be connected to that switch that is for the 120 Volts power to the boiler.
They probably just passed thru the box with the wire but that is actually against the electrical code to have low voltage 24 V wires in the same box as high voltage 120 V wires.
I would separate the thermostat wiring out of that box when you run a new cable. Usually they wire tie it to the conduit leading to that switch box then find a existing hole into the furnace or you could drill a hole thru the sheetmetal and wire directly to the control terminal screws.
No switch should be in the thermostat circuit between the boiler and the Nest.
This photo is from a post I found online same situation you have.
Here is the link to the website where they discuss the wiring.
Awesome — glad to hear that your issue is fixed. It looks like we can consider this one complete, so I will lock the thread shortly unless I can help out with anything else.