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We've just moved into a new home with an heat only, oil, forced hot air furnace. The current thermostats are very old and we'd like to update them to something like a nest. The furnace is currently wired to the thermostat via two wires (T T) and there is no common coming from the terminal to which the thermostat is connected.

The nest says you can run it with a two wire system, but I don't want the nest pulsing the furnace to recharge its battery all summer when the system isn't in use and so would prefer to power it via a common wire.

My current plan is to buy a transformer and wire that to the C and Rc terminals on the nest. BUT, I wanted to look into another option.

There is a box on my furnace that does have a terminal labeled C. I believe it is a fan relay. Currently there is something running to that C wire, but it disappears into the furnace (pictures of the furnace system is attached). Could I buy some 18/3 wire, run two of those wires from the T/T terminals and then connect up a second wire to the C on the fan relay? Something tells me that's a mistake and I should go with my first plan, but if this second plan works it seems like a cleaner solution.

Overview

Fan Relay

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  • Can you trace things out and provide us with a wiring diagram?
    – ThreePhaseEel
    May 12, 2017 at 22:17
  • Sadly, I don't know enough about wiring to do that and the wires on the pictured transformer go into and behind the furnace and I'm too new at messing with electrical to take it apart. . I'll try to draw something up that shows what the overall system looks like.
    – John Basl
    May 13, 2017 at 21:08
  • C on a relay is common. If it's a fan relay, it'll be at line voltage. So definitely don't use that to power your thermostat.
    – Tester101
    Mar 22, 2018 at 12:49

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Every furnace that has an R wire also has a C wire. Those are the two low-voltage terminals on the transformer.

Generally wiring is in a functional triangle. R goes from transformer to thermostat, W/G/Y and other circuits go from thermostat to relay, and C goes from relay back to transformer to complete the circuit.

You can grab C anywhere you find convenient. Usually the thermostat wire goes close to the transformer or control board so most people grab it there. But wherever!

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  • Thanks. That's helpful. Do you have a guess about how the pictured transformer is connected to what I think is called the CAD Relay. So, currently, the thermostat is connected via two wires to terminals labeled T T (on the CAD relay where there are also two terminals labeled F F which are for, I believe, fire detection and shutdown). The pictured transformer is above this and has, as you noted connected terminals R and C. I'd love to learn how those elements of the furnace are related, but I know this question might be too novice.
    – John Basl
    May 13, 2017 at 21:11
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    @Harper, not all furnaces have C wires. I had to add one by wiring in a 24V transformer onto a J box to make a C terminal.
    – TWS
    Oct 2, 2017 at 13:36
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    @TWS I have two furnaces that don't have C wires. One is millivolt and the other is 240V. However if a furnace uses 24V controls, it gets the 24V from somewhere. If that's a transformer, the side going to the thermostat is R and the other is C. Of course not all furnaces give you access to C. Others don't have excess capacity to power a smart 'stat. Adding a second transformer, make sure to phase it so you don't have 48V between C and W (when heat is not being called for).
    – Harper - Reinstate Monica
    Oct 2, 2017 at 14:54
  • @Harper, the 24V is separate.
    – TWS
    Oct 2, 2017 at 18:28
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    @TWS no, it's not. You have tied the two 24v supplies at the R wire on the 'stat.
    – Harper - Reinstate Monica
    Oct 2, 2017 at 18:39

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