ST9120C thermostat wiring for independent fan control


Old 06-09-10, 09:24 AM
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ST9120C thermostat wiring for independent fan control

I have a gas heat only system (no A/C). I am replacing the basic Honeywell thermostat with only a temperature control with a new programmable thermostat. Currently only the red and white wires are connected at the thermostat and on the ST9120C controller board in the furnace.

The new thermostat has an independent fan switch (fan Auto/On) to allow the fan to run independently from the heat. I'd like to setup the new thermostat to take advantage of this to occasionally allow the fan to run without the heat in the summer months.

Do I just need to connect the green and yellow wires to the G and Y terminals at both the thermostat and the furnace?

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Old 06-10-10, 05:10 AM
Join Date: May 2010
Location: USA
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Connect the green wire to "G". This will operate the fan when you move the fan switch from auto to ON. If you install A/C, then you would connect yellow to "Y".
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