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A typical 2-stage heat pump requires at least 6 conductors between the thermostat and the air handler:

• R - 24VAC supply

• C - 24VAC common

• Y1 - Stage 1 call

• Y2 - Stage 2 call

• O - Reversing valve

• G - Circulating fan call

What if the chump who installed the wire for the thermostat (me!) only installed 5 conductor?  My HVAC contractor used something called "Add-a-wire".  It's a clever little device that uses a diode to split the positive part of the AC sine wave into one signal and the negative part into a different signal.  This allows you to use one conductor for two signals.  Unfortunately, the Ecobee doesn't like this.

Instead, I used the Power Extender Kit (PEK) that comes with the Ecobee to gain an extra signal.  This kit is typically used for installations that do not have a C wire.  I installed the PEK and then was able to use the former C wire conductor as another signal.

The before and after schematics are shown in the image gallery above.