Nest 3rd gen

[Msdym]

My Nest 3rd all of a sudden stopped working yesterday, it is saying it isn’t getting power from the RC wire. Please help.

[MsDrasby]

I understand that your Nest 3rd is not getting power from the RC wire. Here are some things you can do to troubleshoot the problem:

1. Check the wiring. Make sure that the RC wire is properly connected to the Nest thermostat. The RC wire is usually red, but it may be different colors in some installations.
2. Check the fuse. There may be a blown fuse in your HVAC system. This can prevent power from reaching the Nest thermostat.
3. Check the contactor relay. The contactor relay is a switch that turns on and off the power to your HVAC system. If it is aging or damaged, it may not be able to provide power to the Nest thermostat.
4. Try a different wire. If you have another R wire, you can try connecting that to the Nest thermostat.
5. Reset the Nest thermostat. Press and hold the Nest thermostat's reset button for 10 seconds. This will reset the thermostat and may fix the problem.

If you have tried all of these things and the Nest thermostat is still not getting power, you may need to contact a professional HVAC technician for help.

Here are some additional things to keep in mind:

• Make sure that the power is turned off to your HVAC system before checking the wires.
• Be careful not to touch any live wires.
• If you are not comfortable working with electrical wires, you should contact a professional.

I hope this helps! Let me know if you have any other questions.

[CoolingWizard]

@Msdym ,

If you are not getting power from the Rc, then the HAVC transformer might have gone bad.  You need to get a Volt Ohm Meter, and check the AC Voltage between the Rc and C to ensure you are or are not receiving the 24VAC +/- 5VAC. 

AC Cooling Wizard

[Msdym]

Thanks, I’ll check the voltage as well. For some reason, it has started working but now says the battery is low after it charged overnight. Would you recommend purchasing a new thermostat?

[Msdym]

The air is now working. It has disconnected from the internet stating the battery is low. The thermostat charged overnight. Would you recommend purchasing another thermostat? 

[Therealduke]

Help me understand how adding a neutral, ie the C wire, allows the nest to charge.  When measuring the voltage do you mean, take the wires out of the back plate and use the meter to touch the wire ends that are usually inserted into the base plate

[Therealduke]

Also, would there be any reason that nest could cause my fan to have multiple momentary, 1/2 second, pauses during a cooling cycle.  I thought this could be cause by a missing C wire but after adding one it still occurs.  The other end of my C wire connects to a Honeywell 2 zone board.

[CoolingWizard]

If you have a little understanding of electricity, the C wire is the neutral and R is the Line. When checking the voltage, we check the AC voltage between Line and Neutral.  If you have a C wire in your thermostat, you indeed check the voltage at the thermostat base R wire to C wire.  When you Nest has a C wire, this allows the nest use the C wire with the R wire to attain maximum charging voltage and current. 

AC Cooling Wizard. 

[Therealduke]

So does the other end get connected to the c terminal on the zone board or the c terminal on the furnace board 

[CoolingWizard]

@Therealduke ,

If you have a Zone Controller, the thermostat will be connected to the zone controller.  All zone controllers have HVAC equipment connections, thermostat(s) connections and zone damper connections.  If there is a C terminal located at the thermostat connection terminals, then that will be were you connect the wire on C at the thermostat. 

AC Cooling Wizard 

[Therealduke]

So nothing gets connected to the  R, G Y W C 

terminals on the furnace board

[CoolingWizard]

Normally, the zone controller sits between the HVAC Equipment and the thermostat.   So the thermostat R, C, Y, G, W are connected to the zone controller’s thermostat terminal block, and the R,C,Y,W,G from the HVAC equipment control board connects to the zone controller equipment terminal block. 

If you can, post some pictures of the Zone Controller so I see how it is wired up. 

AC Cooling Wizard

[Therealduke]

I added a c wire to the screw marked c at the end of red line

I believe the board is connected to the furnace at the equipment terminal but there is no c connection screw

 

[Therealduke]

Full board pic
 

[CoolingWizard]

@Therealduke ,

The HVAC system connection terminals do not need a C since the common of that transformer is inside the air handler/furnace.  The Common at the thermostat connections is connected to the white common wire of the transformer P2 connector.  

AC Cooling Wizard

[Therealduke]

Thanks

must be okay based on these readings

•  

[Therealduke]

 

[Therealduke]

 I assume this is good

[CoolingWizard]

@Therealduke ,

there is one reading below the screen, called Lin, you should see 200mA.  That would be the maximum current for the Nest. 

AC Cooling Wizard

[Therealduke]

see pic

[Therealduke]

Today I noticed the fan pause occurred when the cooling called was satisfied when the furnace switched to purge cycle