cancel
Showing results for 
Search instead for 
Did you mean: 

Trying to install new Nest thermo with OLD furnace and only two wires

Jack_Hum
Community Member

Oil_burner.jpg

Hi, I'm trying to install the new Nest Smart Thermostat. I have an extremely old (30yrs+) furnace. I only have two wires going into the old thermostat. Also, no central air, just an oil fired furnace. I measure 24CDC across T1 and T2 when system is not calling for heat. What's strange is that it's actually -24VDC with respect to the white wire... I'd like to simply add a wall wart to add power and the 'C' common wire. But I'm concerned about the polarity... Is there a full wave bridge on the front end of the Nest? (I'm thinking there must be, since I see people adding the wall wart power supply with a 24VAC.) So if I do that, I assume I'd connect the wall wart between 'R' and 'C', and the old thermostat wires between 'W' and 'R'. BUT DOES THAT POLARITY MATTER? Like I said, the voltage across the Red and white wires is -24VDCXformer.jpg

1 REPLY 1

Jack_Hum
Community Member

TYPO CORRECTION: .... I measure 24VDC across T1 and T2 when system is not calling ...