Wiring an ecobee3 without the G wire (G-wire, fan wire)

Note: this post has nothing to do with the dreaded "C-wire" that is required to run an ecobee.  If you're looking for that, go elsewhere.


One of the more frustrating items for a heating/cooling system is the sheer number of possible ways to set it up.  Sure, there are "standard" ways, but there are also always other ways.

We had a new furnace installed not long ago.  However, there were not enough wires run to the thermostat to support the additional A/C system.  The cable had only enough wires for heat-only operation.

To make it work, the installers should have run a new cable, but they didn't.  Instead, they disconnected the G (fan) wire, and used it for Y (cold call) instead.  This led to an atypical installation that lacked a G (fan) wire at the thermostat.

Note it still worked.  The thermostat sent the W (heat) and Y (cold) signals, and the furnace controlled its own fan.  No worries, thanks to them thar new-fangled furnace, y'all.

However, when I installed the ecobee3, it said "To use the Y1 wire you must install the G wire".  Since I didn't have one, that was a bit of a poser.


When I described this to to ecobee tech support, they could not wrap their head around the idea that the furnace controlled its own fan.  They actually told me to rewire the entire furnace to include a G wire, otherwise they wouldn't help. 

Note I said "wouldn't" there.  I don't know if they could have because - basically - they just gave up.  The tech support rep said - and I quote - "I have been instructed not to proceed further."

Now, I don't particularly blame ecobee for this, owing to the problems noted at the top, but the workaround is actually built into the ecobee.  There was no reason to tell me to go hang; it was rather disappointing and reflected a lack of understanding of their own product.

Fortunately, in the first 5 minutes of being on hold, I figured it out for myself.  The actual correct workaround is as follows:

-  Run the ecobee setup.
-  Tap "Configure" to set wiring manually.
-  Tap "G" to tell the ecobee the G wire is connected (though it is not).
-  Later on, it will ask if the fan is to be controlled by the ecobee or the HVAC equipment.  Select HVAC.

That's it, it should work as it did before.

Now, OBVIOUSLY, you do not have manual control over the furnace fan at the thermostat with this setup.  But I didn't have that before, did I?  So no change there.

Also, I would not recommend anyone switching existing wires around to get a C wire (or any other wire) by sacrificing the G wire.  My furnace works this was because it was originally set up and configured to operate in the absence of a thermostat fan wire.  A furnace that has been set up otherwise, only to have the fan wire removed, is highly unlikely to function correctly afterwards.

This workaround applies to furnaces that have a manual fan switch installed somewhere else.  This is sometimes called the "summer fan" configuration, and often involves putting a manual switch on the furnace to provide for fan control that is independent of the thermostat.  Not a "normal" situation - unless your installer was too lazy to run new wiring when you got your additional cooling/heating unit(s) installed.

So, if you're unlucky enough to be missing a G wire at your thermostat, you can still make the ecobee3 work.  Hope this saves someone a few hours of their life.