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I have two old thermostats (one for heating, the other for cooling) that I would like to replace with a single Nest E unit. I have central air and gas heating.

Cooling thermostat has the following connections: R (Rh and Rc are jumped) G and Y

Heating thermostat has these connections: R (Rh and Rc are jumped) and W

Combining these two into a single unit like Nest E seems simple - Nest has G W Y and R connections, HOWEVER - there is only a single R connection and from these two thermostats I have two R wires.

My question is 1) Do I have two transformers and therefore I need a unit that can support two transformers with separate Rh and Rc connections? Is there any way for me to check if my HVAC system is running from a single or two transformers? 2) If I do have a single transformer (again, how to confirm this) - can I connect just one of the two red wires and cap the other one off?

Thanks for any advice!

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    Can you give us photographs of the wiring at the furnace? Are the indoor coils for the air conditioner mounted at the furnace for that matter, or are they in a different air handler? Oct 25, 2017 at 11:39
  • Is there a single air handler? e.g. Is the A/C evaporator coil attached to the furnace, and does it use the furnace blower to distribute cool air?
    – Tester101
    Oct 25, 2017 at 14:19
  • AC is a separate unit with air handler in the attic. Evaporator coil is not attached to the boiler (which is a hit water gas baseboard heating system)
    – John
    Oct 25, 2017 at 14:48
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    Having two totally separate systems like this certainly points to having two transformers, especially if one system is newer than the other (A/C was probably installed later?)
    – JPhi1618
    Oct 25, 2017 at 14:59
  • Thanks everyone, great advice. I have two separate systems/separate circuits. Diagram with detailed explanation was really appreciated.
    – John
    Nov 1, 2017 at 0:54

3 Answers 3

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If you have forced air A/C and radiant heat, you almost certainly have two separate transformers. So you'll have to use a thermostat that can work with separate transformers.

If the thermostat you choose requires a C wire, you'll likely want to supply the C from the A/C transformer. It's almost always easier to get the C wire from a forced air system, than from a boiler. Also, some thermostats actually require that the C wire come from the cooling system.

Once you've selected the thermostat you want, connect the wires as follows:

  • Remove any jumpers between Rc and Rh.
  • R from the A/C to Rc.
  • Y from the A/C to Y.
  • G from the A/C to G.
  • C from the A/C to C. (Where required)
  • R from the boiler to Rh.
  • W from the boiler to W.

Extra reading

With two separate transformers, the wiring should look something like this.

Two separate transformers

Notice there's no jumper between Rh and Rc.
The green line represents the equipment grounding in the building.

In this situation, everything works as expected.

If you have two separate transformers, but only connect one of the R wires. You could still have a complete circuit, if the transformers secondaries are bonded. However, since you're using the buildings grounding system to complete the circuit, you may find that there's not enough power to get things to work properly. You might find that the A/C works fine, but the heat doesn't (or vise versa, depending on how it's wired).

Two transformers one connected.

Two separate transformers, with only one supplying the thermostat.
Notice there's a jumper between Rh and Rc.

If you have one transformer that is supplying two R wires. It's the same as having one transformer, and using a jumper to connect Rh and Rc. It's just a really long jumper.

One transformer two wires

If you have two transformers, and you connect a jumper between Rh and Rc. You'd be connecting the transformers in parallel. If the transformers are identical (or close enough), this may not be a problem. As long as the transformers are in phase (and identical), everything should be at the same voltage potential. If the transformers are not in phase (or not identical), you'd be creating a short-circuit.

Paralleled transformers


If you have two R wires at the thermostat. The best bet is to remove the jumper between Rh and Rc, and connect the separate wires to separate R terminals.

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  • Thanks. Two what-if followups: 1) what if I have one transformer and utilize both Rh and Rc wires? 2) what if I have two transformers but use only one of the R wires and connect Rh and Rc through a jumper?
    – John
    Oct 26, 2017 at 0:09
  • @John If both R wires are connected to the same transformer terminal, connecting them to Rc and Rh terminals of the thermostat at the same time should make no difference. If the wires are both connected to the same transformer terminal, they'll be at the same voltage potential.
    – Tester101
    Oct 26, 2017 at 1:12
  • Thank you. What about the other scenario - two transformers, but only one of the two R wires is connected to a joined Rh/Rc terminal. Would the effect be that the system that doesn't have its R wire connected doesn't turn on, or would there be an adverse effect? I am guessing there could be the issue of two circuits not being in phase?
    – John
    Oct 26, 2017 at 2:17
  • @John If there are two transformers, and you only hook one up, the system that doesn't have the R connected will likely not work (or not work properly).
    – Tester101
    Oct 26, 2017 at 10:51
  • @John I've added some diagrams, and more information. Hope it helps.
    – Tester101
    Oct 26, 2017 at 11:59
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The reason why most thermostats have a Rh and Rc is for systems with two transformers. The furnace transformer red wire goes to Rh, and the cooling (A/C) transformer red wire goes to Rc. In a home with one transformer, there is a jumper wire to connect Rc and Rh. You have two thermostats, so you're not really using the Rc-Rh as designed. If you had one thermostat, you would have one red going to Rh, one going to Rc, and no jumper wire.

Now the Nest E installation manual specifically says:

IMPORTANT: If you have Rh and Rc wires, you have a dual transformer system. The Nest Thermostat E is not compatible with dual transformer systems, but you may be able to use the 3rd generation Nest Learning Thermostat.

So, if you have two transformers you are out of luck.

Now, the easiest way to test if there is one or two transformers is to use a continuity tester and touch both red wires. If the tester shows continuity, that means someone has run both red wires from the same transformer. If that is the case, you can just use one red and cap the other.

Another way, if the wires are all close enough to touch each other, would be to connect the A/C red wire to the heat thermostat (instead of the heat red wire, not with it) and turn on the heat. If the heat turns on, everything is running from the same thermostat. Even with two transformers this might work because if the transformers are in phase, the hot from one and the neutral of the other will give you 24v, but its probably not good to wire the thermostat that way. Stick to the continuity test.

Depending on how everything is installed and how the wires are run, you may be able to convert your system to use one transformer, but just getting the right thermostat is going to be your best bet.

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    Your tests may or may not be accurate, since one leg of the transformer is likely bonded to the chassis, which is likely bonded to ground. The safest way to check, is to trace the R wires back to the transformer, and see if they both go to the same one.
    – Tester101
    Oct 25, 2017 at 14:23
  • @Tester101, the R wire would possibly be bonded to the chassis? I didn't realize that.
    – JPhi1618
    Oct 25, 2017 at 14:24
  • I am unable to trace the red wire to the source unfortunately because all of the wiring is routed inside the walls. Doing a continuity test between the two red wires would be great, but it seems like there is now an agreement that it may not be a good test. Is there anything else I can do to help determine if I have one or two transformers? This is to help determine how to unite two separate thermostats into a single one, issue being what to do with the two separate red wires. Cap one and connect the other to jumped R, or connect both (Rh, Rc)
    – John
    Oct 25, 2017 at 14:48
  • Can you find one of the transformers? It's normally close to the air handler, sometimes even mounted to it. Also, you can try and turn off breakers to see if one thermostat looses power and the other one doesn't. Of course if they both loose power at the same time, that doesn't indicate anything - there could be one, or could be two on the same breaker.
    – JPhi1618
    Oct 25, 2017 at 14:56
  • @JPhi1618 No, not the R wire. But the other leg of the transformer may be bonded. See attached image.
    – Tester101
    Oct 25, 2017 at 15:31
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If there is no C wire anywhere, the usual way is to jumper the Y and G posts at the AC unit side then repurpose the G wire as C, at the cost of losing separate fan control.

If tying the two transformers via ground doesn't work, a 24V relay can be used. Again, this requires a C wire because the relay is energized by the first transformer.

 --Y-----#  o-----R----
Prim     #==|\       Sec
 --C-----#  o o---Y----

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