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I'm installing a 5-wire smart thermostat replacing a 4-wire battery powered model. I don't have 24v from the furnace, so I've installed a 24v transformer. My furnace wires go to Green, Yellow, White, and Red on the old thermostat, I removed the red wire (left unconnected) and connected the transformer to Red and Common on the new transformer. Everything seems to work but no fan, whether in Auto or Manual. The R/RC jumper is connected. So far, the tech help from manufacturer is useless. Sound familiar to anyone?

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    Where did you install the new transformer? Is it near the thermostat, hence easier to run new wires?
    – jay613
    Feb 5 at 0:02
  • 2
    Why not post a photo Feb 5 at 1:55
  • Perhaps you meant to write that you connected the new transformer to Red and Common on the new thermostat?
    – Greg Hill
    Feb 5 at 2:49
  • Pics of the wiring as well as actual brand/model of all the equipment or we're just making guesses in the dark like you are.
    – FreeMan
    Feb 5 at 12:34

1 Answer 1

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My first choice would be to use an "add-a-wire," "common maker," "wire saver," type accessory. These devices play electrical tricks to gain the functionality of having more wires. Compared to a second transformer they are much simpler to install correctly.

If you do prefer to use the second transformer there are a few things you'll need to know.

  1. The usual 24 volt HVAC controls call one side of the transformer "C" (common) and the other side "R" (red). The thermostat operates the system by shorting white (heat), yellow (cool), or green (fan) to red. The thermostat works exactly like a familiar light switch: the furnace control is connected to common (aka neutral) and the thermostat switches on the power to cause the functions of the furnace/air conditioner to turn on. The thermostat certainly must be connected to the R wire in order to properly switch power from that wire onto W, Y, and G.
  2. Transformers have a "phase." With a single transformer the phase doesn't matter, but when there are two transformers that will be operating together in a system, the phasing matters.

First things first: get the transformer phasing worked out. Disconnect the thermostat, then go to the furnace and connect the red wire to R and one of the other wires to C (this is only temporary). At the thermostat location connect the red wire to one of the terminals of the new transformer.

Switch on the power to the furnace and the new transformer. Measure the voltage between the second terminal of the new transformer and the temporary C wire from the furnace. If you measure just a few volts or less then the phasing is correct; if you measure something like 48-60 volts then the phasing is reversed. If the phasing is reversed, swap the positions of the red wire and the meter lead on the new transformer and measure again to confirm that the transformers are within a few volts of each other.

With the transformer phasing worked out you can connect the new thermostat. Return to the furnace, remove the temporary C connection, and connect the white, yellow, and green wires in the usual way. Connect the new thermostat to the white, yellow, and green wires also.

The last step is to bring the shared/common R and the local C from the new transformer to the thermostat. Note that the R from the furnace remains connected to one terminal of the new transformer, and also connects to the R/Rh terminals of the thermostat. The second terminal of the new transformer connects to the thermostat C terminal.

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