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Advertised as works without a C-Wire, not the case. How to add to an old system w/o control board?

Community Member

When selecting a thermostat, I had three choices, ecobee, Sensi, and Nest.  The ecobee and sensi both required a C-wire, the Nest advertised that it doesn't.  The ecobee and sensi systems have an alternative that allows a power converter to be added to the control board of the furnace.  My furnace does not have a control board, so installation isn't that easy and it made my system seem incompatible with both.

Having gone with the nest, and realizing the false advertising has led me astray, I will need to provide a C-wire to get the unit to function properly.  Having a furnace that doesn't use a control board, none of the after market converters seem to work on furnaces that don't have a control board.

My system has a 120/24 transformer, a relay, and a gas valve - that's it.  Is it possible to add a wire from the other side of the 24 VAC transformer that runs the control relay?  Is the C-wire simply 24 VAC, or something else?  If it's just 24 VAC, would one leg of the transformer go to the Rh - power, and the other leg go to the C terminal?

What's the current draw from the thermostat?  I don't think it would be more Volts/Amps then the transformer can handle, but I want to double check first.

My system is a forced air, heat and cool.  Thermostat is Nest Learning, 3rd Gen.

Wires used are RH - power, W1 - heat, Y1 - cool, G - fan.

Thank you, happy new year.



Yes, the C wire is from the other side of the 24v transformer.  

Where do the Rh, W1, Y1 and G wires connect where your furnace and A/C are located?  If you have a fairly traditional system with an outside A/C compressor, you would also have a 2-wire bundle coming in, with one wire connected to your Y1 terminal, and the other wire in that bundle goes to the C or 'other' side of the 24v transformer.  Run a wire from there to the C in your Nest.