My Nest thermostat won’t turn OFF the heater

[Glebych]

I have had an old thermostat that controlled my heater and air conditioner just fine. The only thing was, I had to go downstairs to change the mode from HEAT to COOL or OFF manually on the thermostat’s keypad. I decided to switch for Nest to have a convenience of changing the modes remotely via Wi-Fi. I have installed the Nest thermostat over the summer and was using it only to control air conditioner. Note: after initial installation, the Nest activated heater in the COOL mode, so I switched the setting from B to O in the thermostat settings and the system was working just fine all summer long. The Nest would turn ON the air conditioner when the house was warm and would turn OFF the air conditioner once the target temperature in the house was reached. Now the season changed so I switched the mode on the thermostat from COOL to HEAT and set the target temperature. Nest turned ON the heater but, once the target temperature was reached, the furnace did not shut off. The Nest display changed its status from heating to just displaying current temperature and the target temperature but did not shut off the furnace. Once I noticed that the house warmed above the target temperature and that the furnace was still burning gas, I changed the mode on the Nest thermostat from HEAT to OFF. The status on the Nest display changed to OFF but the furnace kept burning gas and warming my house even more. The only way for me to shut off the furnace was to turn OFF the main circuit breaker on the furnace itself. That shut off the system. I chatted with Google Nest support from Google Home app and, after some troubleshooting steps (disconnecting some wires and switching the wires from one terminal to another) they told me that the problem was with the base. They sent me a new base. I removed the Nest thermostat from my Google Home account. I reset the Nest thermostat to the factory settings. I have replaced the thermostat base with the new one. I have connected all the wires to the new base. I have added the Nest thermostat to my Google Home account. I have set-up the thermostat from scratch as a new device. With the current temperature in the house at 70F, I chose the HEAT mode and set the Nest to heat up the house to 73F. Nest DID NOT shut off the furnace once the temperature in the house reached preset value at 73F. I kept waiting and when the temperature in the house rose to 75F, I set the Nest to OFF. The gas in the furnace kept burning warming the house even more. In order for me to shut down the furnace, I had to turn OFF the main circuit breaker on the furnace itself. Only this action stopped the gas from burning in the furnace and my house from getting hotter and hotter. I tested the Nest in controlling the air conditioner. I chose the COOLING mode and set-up the Nest to cool down the house to 70F. Air conditioner stopped running once the preset temperature was reached. Does anyone know how to solve this riddle?

 

 

[Patrick_Caezza]

@Glebych 

It sounds like you wired the Nest thermostat for a heat pump, and it looks like you only have a conventional furnace.

Move the white wire to the W terminal and set it up again.

[Glebych]

Thank you Patrick. I will try it right away. By the way, the picture that I submitted with the original post was the picture of the wiring of the old thermostat. I couldn’t find a way to attach two pictures at the same time with the second picture being the Nest thermostat wiring.

[Glebych]

Good afternoon Patrick. I removed the W-wire from the *O/B terminal and connected it to the W-terminal. With the current temperature in the house 65F, I set the Nest into the HEAT mode and set the target temperature at 69F. The display in the Nest turned orange and showed current temperature (65F) and the “heating up to 69F”. However, only the fan started to run. The furnace did not lit up the fire. After a couple of minutes I set the Nest to OFF mode. The display on the Nest showed OFF but the fan did NOT stop running. I removed the display from the Nest but the fan did NOT stop running. I had to turn off the main circuit breaker on the furnace. Any other suggestions? Note: I have connected all the wires on the Nest base to the same terminals as they were connected on the old thermostat. Such as if the green wire was connected to the G-terminal on the old thermostat, I have connected the same green wire to the G-terminal on the Nest thermostat. And so on with the remaining wires.

[Glebych]

Good afternoon Patrick. You were right. Thank you. Apparently, I misunderstood your instructions at first. I just swapped the white wire from *O/B-terminal into W-terminal. That didn’t work. I figured that I had not only to swap the wire from one terminal to another but also to set the thermostat from scratch. So I removed the Nest thermostat from my Google Home account. I reset the Nest thermostat to the factory settings. I have added the Nest thermostat to my Google Home account. I have set-up the thermostat from scratch as a new device. With the current temperature in the house at 70F, I chose the HEAT mode and set the Nest to heat up the house to 74F. Nest DID shut off the furnace once the temperature in the house reached preset value at 74F. The fan ran for another few seconds (up to a minute) and also stopped running. Which means I connected the Nest incorrectly in the first place and you were right and now everything is working fine. Thanks a lot!