How do I wire a thermostat with an external 24V transformer?

Question

My current wiring has 5 wires, R, Rc, Y, G, and W wires. No C wire. I've added an external 24V transformer and connected one wire to the C connector but don't know what to do with the other. If I connect it to Rc, then I have heat but no A/C or fan.

Any suggestions?

Answer 1

Don't use an external transformer. Find a way to run an extra wire from the furnace/air handler, and wire it as a proper C wire.