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New 3rd gen Nest Thermostat is showing warmer temperature than it is

Thm22
Community Member

I had a 2nd gen thermostat that was working fine for many years, then it stopped working properly, without any changes on the wiring. It shows a room temperature that is 5 degrees warmer than it is, and won't turn on the heat. The thermostat is set to heat up to 72. It's 69 or 70 in the room (measured with other thermometers), but the thermostat says it is 75. Farsight is set to show the current temperature. There is no sun light shining on the thermostat. 

I replaced the 2nd with a new 3rd gen thermostat, and this one does the same thing. 

The HAVAC guys were here twice, and they don't know any further. They say the heating unit works as it should. Connecting the red to the white wire will turn on the heat, red and yellow turns on the cold, and red and green turns on the fan. There is a blue C wire, who delivers 28 V between C (blue) and red. I don't know how to troubleshoot that further. 

Any help appreciated. 

 

1 Recommended Answer

CoolingWizard
Platinum Product Expert
Platinum Product Expert

Thm22,

it sounds like you have a beginning understanding of your heater control system. I’m gonna give you a little more information about how it all operates OK.

A. When the thermostat “calls for heat “that is when the R is connected to W, the heating cycle begins. The furnace control first performs, a self check to verify the pressure switch contacts are open and starts the inducer motor.
a. Pre-purge—As the inducer motor comes up to speed, the pressure switch contacts close to begin the 15 second pre-purge period. 

b. Igniter warm up—at the end of the pre-purge, The igniter is energized for a 17 second igniter, warm up.

c. Ignition sequence—when the igniter warm up is completed, the gas valve opens, permitting gas flow to the burners, where it is ignited. After five seconds, the igniter is de-energized and a two second flame sensing period begins.
d. Flame-sensing— when burner flame is sensed the control begins to blower after a delay. And continues holding the gas valve open. If burner flame is not sensed, the control closes the gas valve and repeats the ignition cycle.

NOTE: The ignition sequence will repeat 3 additional times before a Lockout occurs. Lockout automatically resets after 3 hours, or can be manually reset by turning the circuit breaker off, for a minimum 3-seconds, and then back on.

e. Blower on delay— 45 seconds after burner flame is proven, blower motor is energized on heating speed. Simultaneously, the humidifier and electronic air cleaner ( EAC), terminals will be energized.

f. Blower off delay— when the thermostat is satisfied, the circuit between R and W is broken, de- energizing the gas valve and stopping for gas flow to the burners. The blower motor, and the EAC remain energize for approximately 135 seconds.

g. Post-purge—The inducer motor remains energize five seconds after the burners are extinguished.


It sounds like your flame sensor is dirty or faulty. This can cause some erratic burner operation. 

I hope this helps,

Ken, The AC Cooling Wizard

NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.

View Recommended Answer in original post

10 REPLIES 10

CoolingWizard
Platinum Product Expert
Platinum Product Expert

The nest thermostat has multiple thermistors in the base of the display that monitors the temperature physically around where the thermostat is located. Using the three thermistors it averages the values received to report the temperature that is detected. If there is no sunlight shining on that thermostat, then you should not have Farsight activated. 

manually turn on the fan, using the thermostat for say eight hours, and see if the reported temperature changes. As the air flows around the thermostat, this, assuming that the air return is near the thermostat, you should see the temperature change.

 

Ken, the AC Cooling Wizard

NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.

Hi there,

 

Chiming in — I hope you get the answer you're looking for provided by CoolingWizard. Please let me know if you have any questions or concerns, as I'll be willing to assist you from here.

 

I appreciate your help, @CoolingWizard.

 

Best,

Jenelyn

Thm22
Community Member

Thanks @CoolingWizard , the fan works, and the temperature is raising inside the thermostat. not outside. 

Update:  So the heat was running for a few days now as it should and then on Sunday around noon, it stopped heating. The current temperature on the thermostat again showed a higher temperature (74 and rising) than it was in the room (68 and falling) and the unit would not turn on. So I removed the display of the Nest and connected the red and white wire, which usually turned the unit on. It’s a Payne 373L Horizontal Heater/Cooling unit in the attic. I can hear a strong humming noise when the igniter gets hot and the gas should ignite. After a while, it stops and then after a second or so starts again, for I think 3 times. Then it is dead again and nothing happens.

 

Then I disconnected the power on the circuit breaker and reconnect the two wires. Now it also started up the fan for a short time, then stopped. Since the unit is in the attic I can not see if the gas ignites or not. Then the unit was just quiet and did not turn on. I left the wires connected. At about 11 PM (3-4 hours after the last try), the unit started to try to ignite again, without me doing anything but never started up completely.

 

This morning I again disconnected the two cables, waited a bit, also disconnected the yellow (cooling) cable from the thermostat, switched the circuit breaker on and off, then reconnected the red and with, and again the same thing. 3 times trying, a bit of fan, then nothing. 3 hours later the unit turned on by itself and is now heating.

 

Now I don’t have a clue about what the control board does. Could someone confirm that there is a delay built in to retry on its own after a few times trying? Why would it not ignite sometimes, then after a while work again? Can this be the igniter or the heat sensor, the gas valve, or the control board?

 

I called the Havac guys again, but when its working when they get here, they won't figure out what's wrong.

Any help to diagnose further is much appreciated.

 

Thomas

CoolingWizard
Platinum Product Expert
Platinum Product Expert

Thm22,

it sounds like you have a beginning understanding of your heater control system. I’m gonna give you a little more information about how it all operates OK.

A. When the thermostat “calls for heat “that is when the R is connected to W, the heating cycle begins. The furnace control first performs, a self check to verify the pressure switch contacts are open and starts the inducer motor.
a. Pre-purge—As the inducer motor comes up to speed, the pressure switch contacts close to begin the 15 second pre-purge period. 

b. Igniter warm up—at the end of the pre-purge, The igniter is energized for a 17 second igniter, warm up.

c. Ignition sequence—when the igniter warm up is completed, the gas valve opens, permitting gas flow to the burners, where it is ignited. After five seconds, the igniter is de-energized and a two second flame sensing period begins.
d. Flame-sensing— when burner flame is sensed the control begins to blower after a delay. And continues holding the gas valve open. If burner flame is not sensed, the control closes the gas valve and repeats the ignition cycle.

NOTE: The ignition sequence will repeat 3 additional times before a Lockout occurs. Lockout automatically resets after 3 hours, or can be manually reset by turning the circuit breaker off, for a minimum 3-seconds, and then back on.

e. Blower on delay— 45 seconds after burner flame is proven, blower motor is energized on heating speed. Simultaneously, the humidifier and electronic air cleaner ( EAC), terminals will be energized.

f. Blower off delay— when the thermostat is satisfied, the circuit between R and W is broken, de- energizing the gas valve and stopping for gas flow to the burners. The blower motor, and the EAC remain energize for approximately 135 seconds.

g. Post-purge—The inducer motor remains energize five seconds after the burners are extinguished.


It sounds like your flame sensor is dirty or faulty. This can cause some erratic burner operation. 

I hope this helps,

Ken, The AC Cooling Wizard

NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.

Hi there,

 

I hope you get the answer you're looking for, provided by CoolingWizard. Please let me know if you still have any questions or concerns, as I'll be willing to assist you further.

 

I appreciate  your help, @CoolingWizard.

 

Thanks,

Jenelyn

Thm22
Community Member

Thank you @CoolingWizard, That helps. I cleaned the flame sensor.  I think a  faulty flame sensor would shut down the gas, even if it has ignited.

Sometimes, for several cycles, the igniter glows 4 times, and the gas never starts burning. Looks like the gas valve never opens. Once there are actually flames in the burner, then the gas stays on until I shut off unit. It looks like the gas valve opens only sporadically.

https://photos.app.goo.gl/98EhVtGVKAPNKyvYA

https://photos.app.goo.gl/U4wSavTxxPUwBXAC6

 

 

Jake
Community Specialist
Community Specialist

Hey there,

 

I wanted to check in and see if you are still in need of any help. Please let me know, as I would be happy to assist, and answer any questions you may have.

Best regards,
Jake

EmersonB
Community Specialist
Community Specialist

Hey everyone,

@Thm22, just one quick final check in here since activity has slowed down. We'll be locking the thread in the next 24 hours, but if you still need help, we would be happy to keep it open. If there's more we can do, just let us know.

I appreciate the help, CoolingWizard, Jenelyn and Jake.
   
Regards,
Emerson

CoolingWizard
Platinum Product Expert
Platinum Product Expert

You are correct, if the flame sensor is faulty the gas valve will indeed be shut.  This would be a fault and would cause the system to reset.  
You need to have an HVAC service technician come look at your system.  If as you say, the hot surface igniter glows but the gas valve does not open, then you added that sometimes it does burn and you have to manually turn it off.  This looks like the IFC is faulty.  

The AC Cooling Wizard 

NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.

Hello there,

 

Checking in — were you able to get an HVAC technician to check your system? 

 

Thanks for your help, @CoolingWizard.

 

Best,

Jenelyn