I'm trying to install Ecobee, and like many others, I do not have C wire at the thermostat, thus need to find it on my air handler. My air handler is from Allstyle and the model number is HFD36-42840G-10KW (with heating). Below are pictures of the original wiring diagram on the unit and the newer wiring diagram on the company's website. Comparing the two, they are exactly the same. The only difference is the Thermostat connection part. (???)

On the original one, the "purple" wire is the Y wire, it will go to the outdoor compressor and come back to the thermostat, which seems reasonable, as Y wire controls cooling (right?). This match my current connection, the wires are connect to W G R Y on my current thermostat.

But on their new website version, the diagram connects the "purple" wire to "C" on Thermostat without any mention of cooling. (???so that the air handler works as standalone heater and air blower?)

So, in my case, is both Y wire and C wire the same wire (the purple wire) in my case? Is it right/ok for me add one extra wire to connect the 'purple' wire to Ecobee C wire socket?

Orignal Wiring diagram New Wiring diagram from website

  • Purple is C in both diagrams. The Y wire from the outdoor unit in your 1st diagram is not purple.
    – brhans
    Apr 30, 2022 at 13:32

1 Answer 1


Connect your C to the purple wire

The reason the first diagram seems confusing is because they dot the Y wire in from the thermostat to the outdoor unit, but do not specify a color for it. This is because there's no need for a corresponding wire or terminal in your air handler, as it relies on the thermostat to call for fan during heating or cooling calls.

So, in both diagrams, the purple wire is where you should connect C to. (It's also the other wire the outdoor unit needs to connect to, so that's why there's a dotted line from it to the outdoor unit in the first diagram.)

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