Can somebody tell me if my wiring is compatible with Nest Learning thermostat?

[Pabsilar]

[CoolingWizard]

Your system is a heat pump. You do not have a C wire available.  You will need a Nest Power adaptor.

[Patrick_Caezza]

It is not a heat pump

[Patrick_Caezza]

Yes, it is compatible.

Try this wiring.

Black - B to C

Green - G to G

Yellow - Y to Y1

White - W to W1

Red - R to Rc (jumper to RC not needed)

[Ayknit2]

The black wire is not common. The black wire is for the reversing valve on the heat pump it cannot be the common

[Patrick_Caezza]

Nope, you are incorrect. The B terminal on several manufacturers' thermostats is the C terminal just like the X terminal is also a C terminal.

[CoolingWizard]

Patrick, first of all, I own an HVAC company and we work on systems everyday.  The B and O terminals are allocated to the Heat Pump Reversing Valve.  Some older thermostats do not have a Common terminal.  Common is in the system since the outdoor and indoor units require it.  

When it comes to smart (programmable) thermostats, the need for Common at the thermostat  was created. Otherwise, the thermostat simply ran on a couple AA batteries.  However, if you brought Common to the smart thermostat, the batteries became backup power.  The B and X you referred to have never been used as Common on a thermostat.  

[Patrick_Caezza]

I don't care if you own an HVAC company or not.

I know what I am talking about and I provide the best support on here.

You apparently don't know much about HVAC history. Do a simple Google search and you will learn a lot.

Trane is known for using X for the C terminal, and several other manufacturers use B for the C wire on many of their thermostats.

You need to know this if you are going to supply help to members here. It's better to provide no answer than to provide a wrong answer.

If you don;t know all of the

[CoolingWizard]

Now I know where ignorance comes from. Yes indeed when it comes to thermostats many manufacturers created unique programmable thermostat controls that were only purchasable by a licensed contractor from a distributor and installed and programmed by the technician. So yes they are like little computers and the technician would program each function, relate the function to a terminal and yes, there were terminals labeled X and * and even a Z.  However, other than proprietary digital interface communications, No air handler or packaged DX system control board had a X terminal that needed to be programmed.  So, as you say, go Google it and prove me wrong.  

[Patrick_Caezza]

First off, I don't have to prove you wrong, I have been answering people on here for some time now and I have shown that I know what I am talking about.

You on the other hand seem to think that you have to give an HVAC session with every answer and then provide wrong solutions.

[CoolingWizard]

This thermostat shown above is a Honeywell RTH2300 series. And the B terminal is only programmable as a heat pump as is the O terminal. It is not programmable as a Common as you imply. This thermostat runs on a 2 AA batteries only.  The fact it had a wire on the B terminal screw is wrong since this is not a heat pump system.  There remains a need to have a Common wire for the Nest Learning Thermostat , 3rd Generation. As I suggested, discard the black conductor, at the air handler, place the blue conductor on the C terminal with the White conductor going to the outdoor unit.  Then, find the blue wire in the wall, label it C, and attach it to C on the Nest back plate. After all wire are connected to the thermostat backplate, and before attaching the Nest thermostat to the back plate, turn on the power to the air handler. Verify the system does not have a short on a conductor causing the fuse to blow.  If OK, return to the thermostat and snap the Nest thermostat onto the back plate. Start the programming.

[Pabsilar]

 this is the 24VDC hvac connections. The black wire is just hanging. When I’ve connected it to the C connection, the 2A fuse was blown. Everything works well without it. Will a learning thermostat work with my hvac?

[CoolingWizard]

HVAC systems do not use Direct Current. They use Alternating current and the control systems typically use 24 Volts Alternating Current.  

Make sure you turn off the power to the air handler before hooking up or unhooking wires.   

[CoolingWizard]

Since the black wire is not in use, On that control board, connect the Blue wire to the C screw. This will bring common to your thermostat. I also notice that Blue an Orange and. Brown conductor are not being used along with the Black.  Look inside your wall behind the thermostat and find the blue wire.  Wrap the black around the cable an tuck it away.  Use the blue wire as the C wire.  You system is fully compatible with the Nest thermostat.  And, you do not seem to have a heat pump after all. 

[Patrick_Caezza]

Interesting that the black wire would cause the fuse to blow.

To answer your question. Yes, the Nest Learning Thermostat, 3rd Gen will work with your system.

Wire it like I posted earlier and just leave the black wire disconnected

Black - B to (leave disconnected)

Green - G to G

Yellow - Y to Y1

White - W to W1

Red - R to Rc (jumper to RC not needed)

[CoolingWizard]

Patrick you need a common to keep the battery charged in the nest learning thermostat 3rd Generation. This is why  suggested he hook up the blue wire to the C screw in the air handler.  Then he will have C at the nest thermostat. 

[Patrick_Caezza]

NO! A-C wire is not required when using a Nest Learning Thermostat. Most of the time they will work just fine without a C wire installed. I personally always recommend that a C wire is used, but it is not required.

[CoolingWizard]

Patrick, while this is true, it does not work with every system. The nest tries to use the Y and W conductor to create a path back to the transformer.  Only, that path must go through the coil of the compressor Contactor for Y, and through the IFCB in the case of the W wire.  When you have a C wire it goes directly to the transformer and you then have a much better return path and more efficient charging of the lithium battery. Now most control boards will not allow the return path to be established thought the Y and W paths. 

[Jeran]

Hey everyone,

Thank you for visiting the Chromecast Community! Since this thread hasn't had activity in a while, we're going to close it to keep content fresh. We hope you were able to get the help you need, but if you're still having issues, feel free to create a new thread, and provide as many details as possible so that others can lend a hand. Hope this helps!

Best regards,
Jeran