Is this system considered a Dual Transformer system? According to Nest since there is a R and RC cable it must be a dual transformer. But they were unsure and I don't know what that means! I don't have a C wire. Trying to figure out which smart thermostat is best suited for such a setup and hook it up without tearing up much or pulling new wires!

Hope the picture makes sense. I currently have a Honeywell Thermostat with the following wires hooked up.

R, RC, G, Y , W. No jumpers. I have a FirstCO Air Handler, Burnham Heat, and AC unit outside the house.

  • Current Thermostat: Honeywell TH6220D1002
  • FirstCo Air Handler: 30MBX-3HW 120V-1PH-60HZ

Google did not help me much with the installation or ideas. They said "learning system may work".

Any help would be much appreciated. Thanks

enter image description here

Current System

Wiring Diagram Air Handler

wires from Air Handler - three cables boiler, condenser, thermostat

  • Someone has latched onto the "Trying to figure out which smart thermostat to buy" portion to close this as a "purchasing recommendation" which is off-topic. If one were to interpret that as "which type" as opposed to "which brand/model" it seems to be perfectly reasonable to me. Maybe edit your post to remove that sentence and folks will be happy with it.
    – FreeMan
    Feb 25, 2021 at 17:04
  • How do I open this post again? Or it's up to the mods? @FreeMan
    – user113807
    Feb 25, 2021 at 17:16
  • It's not closed, it just has 1 vote (of 5 required) to do so. Whether closed or open, you just edit to update to meet community standards. In the case (not yet closed), the close vote will expire or the voter may retract it. In case a question is closed, you can still edit it, then it will go into the reopen queue and people will vote to reopen or leave it closed. Take the tour and browse through the help center to see how the system works here.
    – FreeMan
    Feb 25, 2021 at 17:21
  • Additionally, it would be good to edit to include the exact model numbers of all the hardware involved, as well as the make/model of the existing thermostat.
    – FreeMan
    Feb 25, 2021 at 17:22
  • Done. Added the model numbers and the brand for the thermostat. Couldn't find the model number of the Burnham boiler
    – user113807
    Feb 25, 2021 at 19:50

1 Answer 1


You have a two-transformer system most likely

From the way your existing thermostat is wired, you most likely indeed have a two-transformer system. This means that you will need a 3rd gen Nest, since the Nest E dropped two-transformer support, and the 2020 edition Nest supports the same physical connections as the Nest E.

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