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I hooked up a Honeywell WiFi thermostat last fall. There is no C wire in the furnace & the Honeywell rep suggested I wire up a small separate transformer with new wires going to the C and RC terminals (no jumper between R and RC) on the new thermostat. Heat worked fine all winter & I just switched the thermostat over to cool, but the AC will not come on. "Cool on" is flashing on the thermostat but there is no fan or compressor action.

The furnace is a 1986 Trane model BLU162E960B1. Natural gas - forced air. AC is much newer. The transformer is wired up for 24 volts.

The existing wiring includes a G, W, Y, and R. The C and RC wires go to the small (doorbell-type) transformer. There is no C wire marked on the furnace wires or on the schematic.

What am I missing? No help from Honeywell.

furnace schematic

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  • We have lift off ! Stat powered up fine & AC works ! Thank you again for all your help. Will use external transformer for fishing weight. Steve
    – Steve Stitle
    Commented Jun 12, 2015 at 16:56
  • Great! Glad I could help. I'll send you the bill. Ha-ha.
    – Tester101
    Commented Jun 12, 2015 at 16:58

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Get rid of the external transformer, and pull a proper C wire.

It's not working because you have one side of the contactor coil connected to one transformer, and the other side connected to another transformer. There's not a complete circuit.

When the thermostat is calling for cool, your circuit looks like this

external transformer diagram

If you follow the circuit, you can see that it's not a complete circuit.

external transformer diagram highlighted

I've highlighted the R wire in red, and the C wire in blue.

Highlighted image

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  • Can't I use one of the two devices (furnace and AC) as the source of power for the thermostat year-round?
    – Mikhail T.
    Commented Jun 28, 2020 at 0:44

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