House heated to above 100 degrees due to E78 error

[JB82]

Hello -

I've been running the AC with the nest thermostat for over 2 months now. No issues. I usually keep the house around 72 with the AC on.

Yesterday I got an error. 'E78 Additional wires required.'

My friend went to check on it today and it turns out the yellow wire came loose. He secured it and the unit worked again.

But the  bigger issue was, the house  temp was over 100 degrees Fahrenheit. So when the yellow wire got loose it seems like the heat came on and ran continuously and never stopped.

My question is, if this was to happen again is there a setting or something that would not allow this to happen? 

Thank  you!

[aguilarmateo]

Hello,

I'm sorry to hear you're having trouble with your Nest thermostat. I'm happy to help!

Here are a couple of troubleshooting steps you can try:

1. Check Wire Connections: Make sure all the wires are securely plugged into your thermostat. You can refer to your Nest thermostat manual for specific instructions on wire placement for your system type (heat pump vs. conventional split system). If you have a heat pump system reseat the Y1 wire. Also, if you have a conventional split system move the W2 wire to the W1 terminal.
    
2. Contact Support: If you're still having issues after checking the connections, our support department is happy to assist you further.

Please let me know if you have any other questions.

Best regards, 

Chris

[JB82]

Hello - I have an electric heat pump. 

I have the yellow in Y1 and white in W2. 

Is that correct?

Also, if the yellow wire becomes loose again how can I avoid this scenario from happening again? 

thanks. 

[aguilarmateo]

Hi JB82, 

Just checking in to see if you still need assistance with this issue.  We'll leave this topic open for another 24 hrs in case there are any follow up questions or comments. Please let me know if you need further assistance.

Best regards, 

Chris

[JB82]

Hi - My AC stopped working. it seems that the thermostat malfunctioned and shorted my whole system. Had a HVAC tech over and needed replace many parts. $$$

[aguilarmateo]

Hello JB82,

I'm sorry to hear you're having trouble with your Nest thermostat. I'm happy to help!
 

You can contact our support department, they can give you different options. 

Please let me know if you have any other questions.

Best regards, 

Chris

[aguilarmateo]

Hello JB82,

Yes!, you are right. Electrical safety is paramount. If you're noticing loose wires, it's crucial to seek professional help from a licensed electrician to avoid potential hazards.

Please let me know if you have any other questions.

Best regards, 

Chris