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Ecobee wiring
How to wire up a 2-stage geothermal heatpump to an Ecobee with only 5 wires
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 conductors? 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 the two images at the top of this page.