This is the second post in a series for repairing the NEST.  This second method should not be attempted by anyone who does not understand electronics.  If you feel unsure of your capabilities or your understanding of this repair, please call a professional repair person who understands NEST products.  It is strongly encouraged that you shut down all power to your heater and air conditioner prior to making this repair!  Typically this is done by turning off your circuit breakers for these devices.   Please call a HVAC qualified repair person to check your wiring prior to energizing the system after this repair is made.

A few months after putting up a post describing my repair of the NEST thermostat we encountered another problem. We started getting errors showing the NEST was not connecting to our WiFi.

In the evenings after the AC has been on for hours we begin to see battery error. (This error is because we had NO C wire connected which I will explain to you shortly).  We only had power from furnace and air conditioning wires powering the thermostat.

At night, after the air conditioning runs for a long time during the hot months, we get low battery resulting in loss of WiFi connection. Battery should be 3.8 or higher.

 

Note in the image above that the battery voltage is a bit too low (3.848 V).  We still have a problem.  The battery should be 3.9 or higher.

In the morning, after the AC has been off for hours, the battery is back to normal as seen in the image below (3.926V) and the system works properly.

This is obviously a power supply problem. My guess is power from the heater and air conditioning wires is not providing enough power to the thermostat. We need a better solution. I then added my own 24 volt transformer as the supply source but that failed because the wiring did not “see” a complete circuit. The final and best solution was to connect the C wire! I found this was the problem by finding this web site: https://smartthermostatguide.com/thermostat-c-wire-explained/

 

Prior wiring that failed to work properly as seen in the picture above was because the NEST was getting power from the heat and cool circuit wires and not from a dedicated power source.

I then turned OFF the whole furnace and AC unit at the circuit breaker box. I know electronics but I am NOT an authority for HVAC systems. I really do not want to take down our HVAC system by making any mistake.  I will only add new circuit wires to the system when the system has been turned off!

I looked carefully at the thermostat wires and found an unused wire at the thermostat and at the furnace unit. Prior installers had spun this extra wire backwards around the the other wires to keep it out of the way.  It took me a while to find this unused wire.  At the thermostat end, I had to pull the whole set of wires out of the wall to find this unused wire.  I stripped the thermostat end of this prior unused (blue) wire and attached it to the C connection at the NEST thermostat as seen in the image below.

 

 

I then went to the furnace and found that unconnected same wire color and connected it to the “C” terminal at the furnace.

ALWAYS MAKE SURE THE SAME WIRE COLOR CORRESPONDS TO THE CORRECT CONNECTOR AT THE FURNACE END AND AT THE THERMOSTAT END!

I then called a HVAC repair person that knows NEST products and told him I would pay to have him make sure this new thermostat wiring was good to go. He came over and took a look and said it was good. I turned the circuit breakers back on. I then did a thermostat reset.  We have not had a problem yet with the system.

Summary: This method adds a single wire connection between the thermostat and your heater/air conditioning units to complete a new “C” or common circuit between the two devices.

If you do not have your NEST wired for C and are having problems with your NEST, research this option by going to this web site: https://smartthermostatguide.com/thermostat-c-wire-explained/ This web site is a bit technical.

When I tried to find a HVAC person to come over and check my wiring I found the Internet did show vendors that service NEST thermostats and I did call that qualified person.

I find it rather odd that the NEST Thermostat worked for over a year before having power supply problems.

I just love the NEST as I can see the house temperatures and adjust them even when on vacation.

 

Posted December 5, 2019