I found the "Why doesn't the “fan on” mode work after I installed a new thermostat?" thread on this site and am wondering if the manual rewire steps would also work for a two wire system. The setup: older house with Trane XV80 furnace, two wire setup, thermostat/Rh to controller board/R; thermostat/W to controller board/W1; thermostat Rc to Rh jumper; controller board W1 to W2 jumper. As I understand it an additional green wire would be the best solution, but that's not an option for me. Can I get the fan to run continuously during the summer by manually moving the thermostat wire from Rh to G and the controller board end from W1 to G? Thanks.
Trane XV80 furnace, two wire setup, wanting continuous fan when no heat is needed
furnace
Related Solutions
After looking at all the photos, it looks like the fan is controlled by a fan and limit switch (the black thing with a silver dial and three black wires).
So basically, the thermostat calls for heat by energizing the TH W terminal on the gas and ignition controller (Black Honeywell box with all the wires connected to it). This opens the pilot, turns on the spark, and ignites the main burners. Once the temperature in the heat exchanger gets up to the FAN ON temp., set on the fan and limit switch, the blower comes on. When the thermostat is satisfied, it removes power from the TH W terminal. This tells the ignition and gas controller to turn off the main burner. When the temperature in the heat exchanger falls below the FAN OFF temperature setting, the blower turns off.
To control the fan via the G wire from the thermostat, you're going to have to use a relay to turn on the blower. You'll energize the coil of the relay using the G wire from the thermostat, and the 24V common terminal from the transformer. You'll have line voltage (120V or 240V) connected to the common terminal on the relay, and a wire to the blower connected to the normally open (NO) terminal on the relay.
If you don't understand the information provided in this answer, or don't feel comfortable making this type of modification to the furnace. Please don't hesitate to contact a local licensed HVAC technician.
Side Note: The gas and ignition controller is installed upside down
I believe you need to move the red wire at the Nest to the Rh terminal. Rh is 24v from the Heating system. The Rc terminal is 24 volts from the Cooling system. Change that around and see if your issue is fixed.
Best Answer
It's probably simpler to just wire a switch at the furnace wired R and G. In the summer you just hit the switch. You can switch the wires in your furnace and thermostat to have control at the stat. Or you could install a Fast Stat, which uses two wires to signal a full complement of wires.