Introduction
If you need to check or replace your thermostat and decide to do it yourself like in this post - you do so at your own risk! Electricity and HVAC are no joke. If you’re not confident in your abilities, it’s better to call a professional. You’ll save time, stress, and possibly your health π
My family and I were on a week-long trip around the states. When we returned home, we discovered some damage. I suspect it was caused by a strong thunderstorm.
A GFCI outlet failed, the modem (coax to twisted pair - the LAN port died) broke, and my Google Nest E thermostat stopped working.
Diagnosis
I simply ordered a new modem and replaced the outlet. But with the thermostat, I had to do some troubleshooting to figure out exactly what was wrong. It could have been a problem with the HVAC control board, the transformer, or the wiring.
The Nest showed that it couldn’t detect the C wire - that’s the “Common” wire, basically the “neutral” for power. It also showed Von and Vin voltage of only 2 volts. It seemed to be trying to “pull” power from other signal wires - the HVAC would turn on from time to time as if the thermostat was sending a cooling signal.
First, I checked the voltage between R and C - the multimeter showed 27 volts. That’s good, but that’s without a load. To check if there’s any voltage drop under load, you can use a 1-2 kΞ© resistor.
Important: You need to set the multimeter to measure AC voltage. With the resistor, it still showed 27 volts - so I could conclude that the transformer and wiring were fine.
HVAC Control Test
You can also check if the HVAC control is working properly. To do this, carefully connect R and Y (cooling) or R and W (heating). The test showed that the control board was also working - cooling and heating turned on.
β Never connect R and C without a load! This will cause a short circuit! Be careful!
Final Check
I examined the base. I didn’t find anything unusual - the wiring wasn’t oxidized and was inserted correctly. The problem was most likely either in the base or in the Nest itself (it was already 5 years old), plus, as I mentioned above, there was a strong thunderstorm and no lightning rods/surge protection.
I ordered a new Google Nest Learning thermostat, connected the wiring - as shown in the photos below. I installed the thermostat - everything works! The voltage is also normal.
We won’t die from the Texas heat!
Conclusion
I need to install a lightning rod/surge protection π