Room temp on Nest reads higher than actual room temperature

[sid7198]

Greetings. 
I recently purchased a Nest Learning Thermostat (3rd gen) Jan 18, 2024 to replace Vivint thermostat. It was fine for about a couple hours until I noticed it was getting cold in the house. I went downstairs and Nest room temperature displayed @72 and room temp was 66 at that time. I went through the process of resetting and starting a fresh setup. No problem. Couple hours later I started getting E74: No power to Rh wire error in app. So I did the following:

1. Accesses Equipment detected area on Nest —> Y, G, W1, and Rh did not display red indicating issues

2. I pulled one wire out at a time to see if there would be a change to indicate a problem and each wire displayed red to show problem. 
3. Restarted thermostat —> correct temperature displayed (67 degrees) after reboot for about 15 mins before jumping back to the standard 72 or 73 degree room temp

4. Performed a reset again to get the correct temperature for 15 mins before jumping room temp up by 6 to 7 degrees

5. Continually got E74 errors throughout days to current day Jan 25th

6. Wanted to verify it wasn’t the device, bought a second one and ran into the same problem 

7. Had an HVAC check and verified there was no issue with furnace (forced air)

8. Software version - 6.2-27

9. Battery 3.7468 V, Vic 37.92, Vin 37.25, Lin 20 mA

10. Fabric ID (given) Status - Connected

11. Network - 85 signal strength 

12. Equipment sensed on - W1 Y1 G Rh

13. Software version 2.3.51/3.1, Model backplate-5.26

14. The HVAC tech put duct compound around the hole on the wall the 4 wires are coming out of as that was one of the suggestions

15. Reset the 2nd device I had purchased and connected it to mini usb to charge and left in the room where 1st Nest is located and then installed it 24 hrs later and the room temp read 66 which was the real temp vs the 1st Nest read 73.

Ive gone through all the step from various similar post and continue to get the E74 error. I’ve purchased the Nest Power Connector to that next but wanted to post just to see if there was anything else I should try as I have to set thermostat to 75 now just for it to come on as it will read room as 73.

Thank you for your help.

[zoeuvre]

Hi sid7198, 

Yikes! I'm sorry to hear about the issues you're experiencing with your Nest Learning Thermostat. It sounds like you've already performed several troubleshooting steps and even tried a second device, but the E74 error and temperature inconsistencies persist. Here are a few additional suggestions you can try (you may skip the ones you already did):

1. Make sure the breakers are on. 
2. If you used AC recently, check your system’s drip pan or drain tubes (skip this step if you only have a heating system). 
3. Check the condition of the HVAC fuse.
   • Turn off the main power to the HVAC system at the breaker box. Note: The HVAC system can have multiple breakers, so make sure the customer turns them all off.
   • Check the furnace and fan and make sure there are no lights that might indicate the system still has power. Note: If there are still lights on the furnace or fan, the system might still have power, and you need to turn off the main power to their home at the breaker.
   • Search for the HVAC fuse. It’s usually on the furnace control board. Note: If you need help to find their control board, check your owner’s manual. Fuses can be different for each type of system. You might find a small U-shaped fuse with a number 3 label and is pink or purple colored. You might also find a round fuse. 
   • Remove and inspect the fuse for damage. If the fuse seems burnt or has discoloration, consider a fuse replacement. To learn more about fuse replacement, consult the user guide for the HVAC system or contact a Nest Pro.
   • Make sure to reattach any panels on the furnace. Note: The furnace might not work if the customer doesn't reattach the panels.
   • If you're not comfortable checking the HVAC fuse, the replacement fuse doesn’t fix the issue, or the you can’t find the fuse, proceed to the next step.
4. Check you have an energy savings box. Some utility companies install boxes that cut off the power during high-demand (peak) times. These energy savings boxes can cause the error.
   
   • Check if there’s an energy savings box in your house.
     1. Search for an energy savings box near the electric meter (possibly near their air conditioner).
     2. Search for a small box with 2 lights on it (one light is red, the other is usually green). This box allows the utility company to cycle the your air conditioner off and on during certain peak times of the day.
   • If you have an energy savings box:
     1. Wait an hour or two for the rush hour to end, and then check if the error disappears.
     2. If the error disappears, contact their utility company, landlord, and/or Homeowners Association (HOA) for details about the program and your home's participation in it.
   • If you doesn’t have an energy savings box, proceed to the next step.
5. Check your air filter. You HVAC system needs to have a good airflow to work properly. A clog in the air filter can limit how much air flows over the cooling coils. If the clog persists, the cooling coils can freeze and cause your system to shut off.
6. Find your HVAC system’s air filter. The air filter is usually behind a grate in the hallway.
   • Check your walls and ceiling.
   • If you can’t find the air filter, search for it inside the furnace near the fan.
7. If the air filter seems dirty or there seems to be a clog, replace the filter with a new one. If the issue is because of frozen cooling coils, you need to wait a while for the HVAC system to thaw before they can use it again.

Let us know how it goes.

Best,

Zoe

[sid7198]

Thanks for the steps.

Unfortunately after replacing the wires to include the c-wire, changing fuses even though they were good, adding a pressure regulator, checking the board to verify it was still good, moving the thermostat, and checking the speed of the fan, the furnace per the HVAC person says the furnace is overheating and shutting down thus producing the power error message.  Any other thoughts would be greatly appreciated.

Thanks.

[sid7198]

Just wanted to provide another update. 

As stated earlier HVAC said filter was clean just slightly off white. What he did was pull the filter out while the furnace was running. Unlike before the furnace would shut down and the Nest would report the error message when temperature was increased, now it kept running. The air filter is a Filtrete MPR 1500 [purple label] that is supposed to changed every 3 months - I change the filter back at the end of December 2023. I changed out the filter for a new one and ran the furnace. It successfully ran without error up to the set temperature. Even though everything on the furnace checked out, HVAC believes due to the age of the furnace Thame thickness of the Filtrete 1500 MPR filter was too much for it and if I am going to continue to use that version of the filter it will need to be changed sooner than 3 months.

So as of now I have not had an issue since replacing the filter which would have been replaced in this month anyway 🙂 just had to do it a little sooner.

If the problem persists after this I will post again. If not thank you for your assistance Zoe.

😊 

[zoeuvre]

Hi there, 

Alright! Sounds like a plan. I'll keep this thread open and will wait for your update while you monitor the behavior of your thermostat and HVAC system. 

Thanks,

Zoe