In previous summers, we had been experiencing the air conditioner turning on then quickly turning off again, frequently resulting in a tripped breaker and an uncomfortable warm house until we noticed. Neil diagnosed this, with the help of the internet, to a Nest related issue. Here’s a great post about the issue: https://smartthermostatguide.com/no-c-wire-install-a-nest-thermostat-at-your-own-risk/.
Basically, the C (Common) wire connection from the furnace to the Next provides a 24V supply that the Nest can use to keep its battery charged. If you don’t have a C wire, the Nest charges itself from the furnace or A/C wire when those systems are running. When those systems are not running but the Nest battery is running low, it draws a small amount of power from the furnace to recharge itself. That’s not supposed to be enough to trigger the furnace or A/C to turn on… but in our case it was causing the A/C to turn on again very shortly after it turned off. This stresses the A/C unit (any likely contributed to a costly compressor replacement we had recently!) and trips the breaker.
When we first installed the Nest, the C wire wasn’t hooked up at the furnace, and the Nest instructions suggested it wasn’t needed. Fortunately, there was an unused wire in the bundle running from the furnace, so we were able to use that as the C wire.
