Wiring for Nest

[Honu143]

Hi,

I installed a Nest a week ago it works, but I get errors messages saying it needs a power wire. I'm posting two pictures. The first is the wiring of my old thermostat. The second is of the Nest as currently wired. The white wire in the first picture is a jumper, going from W to Y, where the blue cable is. I'm not sure what I need to do to correctly wire the Nest so I don't get error messages. Is a jumper even an option? Thanks for any help.Old thermostatNest wiring

[Anonymous]

Kindly follow the instructions from this link: https://support.google.com/googlenest/answer/10523126?hl=en
I believe you might need to install a common wire or nest power connector.

[Honu143]

Thanks for responding. I tried to install a nest power connector, but there's no wire marked C on the HVAC control panel. The installer's guide for the HVAC shows that "B" is the common wire. Here's a photo of the HVAC control panel. The long white connection area has a B on it, but there's no wire there. But just below is a terminal marked B, but it's a two wire connection. The Nest power connector diagrams don't show how to attach to that. I called an electrician who would charge $400-$500 dollars to run a line down to power the Nest. That's just too much money so I may have to put the old thermostat back on.HVAC wiring.

[Houptee]

Is this a Heat Pump system?

No do not use any jumpers with the Nest.

You need a C wire to make the error message go away. Is there an extra wire in the cable not used that can be connected to C?  Then on the system end you need to connect it to C there also.

[Honu143]

Yes, a Trane heat pump (model 4WCY4048B1000AA installed in 2009). And, no, there's no extra wire coming down from the HVAC. Thanks.

[Houptee]

Usually there are 5 wires in the cable and the extra is snipped off or wound around the cable in the wall. It should have a yellow wire in the cable not being used.

B is common on some units.

There should be a row of screw terminals on the unit where the thermostat wires connect not that plug in terminal in your picture.

Sometimes you can attach a new 5 conductor wire to the old one and gently pull it down the inside of the wall yourself. Use wire with 5 conductors. Not hard if they did not staple it to the studs inside the wall. Can you see where the wire goes starting from your system follow it and see if a new wire can be run fairly easily.

[Honu143]

Is this the row of screw terminal you mean? If so, I'm still not sure which wire is the right one and how to connect it, to the Nest connector or look for it in the wall.

[CoolingWizard]

Your picture is of the high voltage section.  See the group of wires going down that hole? Those are the control wires.  The wire nuts are used to connect the thermostat wires to the control wires.  

[Honu143]

Thanks! So, I went back up on the roof and found these under the unit, which I assume connect to the thermostat. The problem is there are still only four wires, and of the same colors as the ones inside in the wall attaching to the thermostat. Since there's no yellow wire there and there is one in the picture above, it must attach to something inside the unit. Does that sound right?

[CoolingWizard]

Your Trane Heat Pump is a packaged unit with return and supply ducts going from the outdoor heat pump to the inside of your house. So your thermostat cable goes from installation de the house out to the heat pump outside.  The picture below shows your connection between the thermostat cable conductors to the Trane terminal wires. 

Ken, The AC Cooling Wizard 

[Honu143]

Thanks! I have the wiring diagram, too (from the Installer's guide) but wasn't sure how to square it with what the actual wiring looks like in the unit. Also, you wrote "from installation de the house out." I'm guessing "de" means "in"?

[CoolingWizard]

Here is the wire connections and their colors inside the heat pump.

[CoolingWizard]

Ok, the contractor that installed your system seems to have a blue conductor for the Y and they used White for the the reversing valve. Green is for the Blower Fan, and Red is the Power. So there is no 5th conductor for the common. So in the Low Voltage Connection Box, the Yellow wire is wire nutted to the Blue Thermostat wire. And the white wire is wire nutted to the Orange wire.  I did not see a second electrical contactor installed, so I do not think you have electric heat strips inside your heat pump.  

To get you a common wire, you will need to install a Nest Power Connector and install in the unit on the roof. The only other choice would be to install a new thermostat cable from the outdoor to the thermostat indoors.  

Ken, The AC Cooling Wizard 

[Honu143]

Thanks, Ken. So let me make sure I understand before I make a mistake and destroy my HVAC. What do I connect my Nest Power Connector to (inside the roof unit)? If it's in the low voltage connection box, is that what this picture is of?

[CoolingWizard]

Follow the tan thermostat cable into the outdoor unit, those small conductors will be wire nut attached to the control wires on the inside.  

Ken, The AC Cooling Wizard

[Honu143]

Thanks, yep, I did that. See pictures below, in response to Hooptee.

[Houptee]

Your thermostat wire insulation looks like it is deteriorated from the sun. The wire nuts should not be outside of the unit like that exposed on the roof.

I would suggest run a new 5 conductor thermostat wire from the rooftop unit directly to your nest.

[Honu143]

Thanks, that concerned me, too. I thought the thermostat cable went into the roof, but I went back up and saw it goes into the ductwork.

I followed the cable back into the HVAC and found it connects with an 8 wire cable, with four wires connected...

And four wrapped around the cable.

It's hard making sense out of all this. I think I may be over my head!

[CoolingWizard]

This set up sure does seem to confuse me. It looks like when that four conductor wire goes inside the Plenum duct, it comes out inside the unit on a seven conductor cable.  There is not likely to be enough room inside the low-voltage cable box to put the power connector in there. The power connector will most likely be on the outside of that box with the conductors being pass-through to the inside the low-voltage box.

[Honu143]

I appreciate your help, Ken. Not sure how to hook up the connector given things like the white thermostat wire is nutted to orange unit wire. Also, can I attach the wires on the right side of the power connector to wires in the low voltage box, or do I have to run them to the panel as shown in the upper right side of the Nest power connector installation guide?

[CoolingWizard · Accepted solution]

Let me see if I can help add some light to it OK. You pass the power connector wire through a hole for the area you will mount the power connector. You’re going to separate the little blue thermostat wire from the yellow number 53A by taking a wire nut off. .  Take the blue wire that was coming from the thermostat and connected to the W*Y on he left side with two wires next to the R. use the Wango connector provided with the Power connector.  Separate the red thermostat wire from the red 35A wire. Connect the red thermostat wire with a wango and the R wire from the left side of the power connector.  Now let us work on the right side of the power connector.  Take the W*Y from the group of three wires on the power connector, and connect to the yellow wire 53A with a wire nut.  Take the C wire on the right side and connect it with a wire nut to the Blue 36B wire.  Finally the R power connector wire and wire nut it to the red 35A wire.

I hope this helps. 

Ken

[Honu143]

This is great, thanks so much. I'll let you know how it goes. Fingers crossed.

Will the Blue 36B wire have 36B marked on it? Also, is it likely the blue wire that's not attached to anything (with just a nut on it) in the low voltage box and pictured below.

[CoolingWizard]

I am pretty sure it is, the wire will the number 36B printed on it. If it was bundled with the others, strong chance it is indeed the common wire. 

Ken, The AC Cooling Wizard

[Honu143]

Okay, I just wanted to check before I got started. Thanks, Ken!

[Honu143]

All done and the Nest is working—and without error messages. Thanks so much for the help!!! I’m so grateful 🙏🏻

[EmersonB]

Hello folks,

@Honu143 we're glad that it's working now and thanks for getting back to us. Please let me know if you have other questions or concerns as I will be locking this after 24 hours. Feel free to start a new thread and we'll be happy to help.

I appreciate the help, CoolingWizard and Houptee.

Regards,
Emerson