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Nest Thermostat's Internal load presented to furnace; starts furnace

MichaelWR
Community Member

My older (25 yrs) forced air furnace is turned on by shorting the two wires (red & white) as in our old mechanical thermostat. I have about 27vac present on the two wires unloaded and it drops to zero when shorted (obviously). The Gen 3 thermostat loads the 27vac to 25vac, which seems good so far, it runs the Nest and keeps the batts charged. However, that appears to be enough of a load to trip the furnace and it starts heating. Result; the furnace runs continuously when the Nest Thermostat is connected... I've monitored the voltage across the control wires (red & white) while the Nest is connected and it drops to zero as expected when the Nest Thermostat temperature is raised enough and it calls for heat. So I believe the Thermostat is functioning as intended. My question would be, can I add a "Common" wire and connect it to the furnace "ground" to complete the power circuit and make things work as I've seen posted online? If I supply a Common conductor, does it provide a path to charge/run the Nest? How does it remove the load from the White (Call for heat) circuit? ~Curious user

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