E74 but Vin and Voc have power

[Bokidd]

I got a nest thermostat to control my heating (gas boiler). The thermostat has W and R wires. The thermostat is giving an E74 code but I have power on the Vin and Voc wires (28V and 37V, respectfully). I can perform a test heat cycle and it will turn on. I have no clue what the problem is

[maedacortez]

Hi Bokidd,

Thanks for reaching out. I understand that your Google Nest Thermostat shows the E74 error. I know this can be frustrating, but no worries, I am here to help! Before I can provide specific assistance, I need to ask some questions to better understand your situation:

• What Nest thermostat model do you have? 
• What type of heating, ventilation, and air conditioning (HVAC) system do you have? 
• What exactly does the error message say? 
• What troubleshooting steps have you tried?

I will be awaiting your response.

Regards, 

Manuel

[Bokidd]

• What Nest thermostat model do you have? 
  • Nest Learning Thermostat 3rd Gen
• What type of heating, ventilation, and air conditioning (HVAC) system do you have? 
  • Gas boiler with radiators (Heat only)
• What exactly does the error message say? 
  • E74 No power to Rh wire detected
• What troubleshooting steps have you tried?
  • Re-strip and insert wires (no change)
  • Cycle breakers (works for 60 seconds then error code returns)
  • Install replacement baseplate and display (no change)

[maedacortez]

Hi Bokidd

Thanks for your reply. If you're uncomfortable with the next steps due to high voltage, I recommend contacting your personal tech or a Nest Pro. If you feel comfortable, follow the next steps.
 

Check the condition of the HVAC fuse

• Turn off the main power to the HVAC system at the breaker box
• Check the furnace and fan and make sure there are no lights that might indicate the system still has power.
• Search for the HVAC fuse. It’s usually on the furnace control board
• remove and inspect the fuse has any damage ( if the fuse seems burnt or has discoloration you will need to fuse replacement,  to make a fuse replacement consult the user guide for the HVAC system or contact Nest Pro)

If you have an energy box, this is normally the cause you see the error code E74. You only need to wait for an hour or two for the rush hour to end, and then check if the error code disappears.

if the inconvenience persists after checking the fuse please answer the next questions:
 

1. When was the thermostat installed?"
2. Who installed the thermostat?
3. Did the issue happen during the installation or after the installation?

I will be awaiting your response.

Regards,

Manuel

[Bokidd]

Thanks, fuse is fine. There is no energy box.

The other questions seem to have the same answer.

• When was the thermostat installed?"
  • Now? This is an installation issue. It has never worked.
• Who installed the thermostat?
  • I am attempting to install it
• Did the issue happen during the installation or after the installation?
  • During...I am attempting to install it now and having this issue

[maedacortez]

Hi Bokidd,

Thanks for your reply. It's helpful to continue investigating the cause of the error code. As you mentioned, the issue arose during the installation of the Google Nest Thermostat. To verify compatibility, I'll need some additional information.

• Could you please send me a photo of the old and current wire setup?
• Do you have an older gas furnace built into the wall or the floor?
• What is the make and model of your HVAC equipment?
• Finally, please share the power readouts from the Google Nest Thermostat. You can find these by going to Settings > Technical Info > Power.

If you have a voltmeter and know how to use it, please check the wires on your Google Nest Thermostat to ensure the power is the correct voltage (20 to 30V). If you don't have a voltmeter, you can skip this step.

I look forward to your response.

Regards, 

Manuel