Nest Thermostat back plate needs replacement

[Jmartindale]

The wiring on the back plate of the nest is not working. The system can get the cold air going but the heating setting just turns on the A/C as well.

I had a professional come out and check the wiring, and the furnace will turn on when he crosses the wires manually, so that just leaves the back plate with its wire connections.

Can someone help me understand what I can do to replace the back plate?

[CoolingWizard]

The nest back plate is simply a landing point for the thermostat wires and an interface between the thermostat cable wires to the nest display unit. If your system is a heat pump, the problem is not with the back plate the problem is most likely the programming in the display unit. Heat pumps have a reversing valve that must switch to change it from Cooling to heating or in the case of some units from the default heating to cooling.

If you have a forced air heating system, then we need to know if it is a separate system such as a gas furnace.

Please help me to help you by answering a few questions. 

What type of heating system do you have?

- Heat pump

- Gas Furnace

- Oil Fired Boiler

Please take a picture of the base plate and upload it 

thank you,

Ken

[Jmartindale]

Thanks for the help, it is a Gas Furnace.

[CoolingWizard]

The wiring looks correct, I see an unconnected blue wire in the background, perhaps that can be set up as a COMMON in the future.   The Nest thermostat should be configured with HEAT FUEL as GAS, HEAT TYPE as FORCED AIR.

if you wan to test the backplate, with the wires installed as they are, use a wire a connect the hole by R with the hole by W and see if the heat comes on.  I use a metal paper clip.  You just wan to touch the two for a few minutes to see if the heat cycle is initiated.  If it does, the backplate is fine and the problem lies in the thermostat itself; perhaps the software configuration. 

Ken

[Patrick_Caezza]

You need to install a C wire. The newish Google Nest Thermostat will act weird when it has low power.

Verify that that blue wire is not being used and connect it to the C terminal on both ends. Do a reset to factory defaults and set it up again.

[Jmartindale]

What do you mean by "connect it to the C terminal on both ends"? There is only a C on the left side. I will go ahead and give that a try for now.

Thanks all for the feedback. Been a busy weekend but I am back at it.

[Jmartindale]

I have reset the system, but that blue wire in the C slot is not being detected. Appears to not have anything happening with it

[Jmartindale]

Actually looks like the furnance is now working for my Nest, though it is running off just battery power still. That C wire seems to be a problem, but hey I have heat! Thanks for the help!

[CoolingWizard]

Jmartindale,

The HVAC control system operates on approximately 24 Volts AC power.  There is a step-down transformer in the HVAC air handler that steps the 240 VAC or 120 VAC down to 24 VAC.  Now AC power needs to have two paths to work. We call them LINE and Neutral. So the transformer outputs on two wires, LINE and Neutral. We connect the Neutral to all components of the control system, it is therefore called COMMON to everything.  The Line output from the transformer is sent to the Thermostat and is attached to R terminal.

There is a Y terminal that is connected to the Compressor Contactor.  The contactor has a magnetic coil that gets energized by the Y wire and the COMMON.  When the Thermostat calls for cooling, it connects R to Y and that completes the magnetic coil circuit and thus energizes the coil, closing the contacts and sends power to the compressor to start it up. 

Smart thermostats need power for their processor and memory.  That power comes from a couple batteries or, can come from the HVAC Step-down Transformer.  To do this, we need COMMON in the HVAC equipment to be brought to the smart thermostat and attached to the C terminal.  
The Blue wire you say is connected to the C terminal and not seen by the Nest Thermostat.  It is likely the blue wire is not connected in the HVAC equipment.  

Is your HVAC system a split system? That is Part outside and part inside?

Ken, The Cooling Wizard

[Jmartindale]

I appreciate all the help but it appears that the system is now working for me with the latest reset. Not sure what was changed when I did this but its all good for now. I will come back here if it stops working for some odd reason. Thank you again.

[Jeran]

Hey there Jmartindale,

I'm glad to hear that Patrick_Caezza and CoolingWizard were able to help you out! I'll leave the thread open for a few days, so you have time to report back with any more questions or if any more issues arise.

Best regards,
Jeran

[Jhonleanmel]

Hi everyone,

As we got our resolution here, I'm going to mark this thread as resolved. I'll be locking this thread if we won't hear back from you in 24 hrs. Should that happen, feel free to create a new one if you have more questions or have other concerns in the future.

Cheers,
Mel