topic Unsure about thermostat compatibility! in Nest Thermostats

Unsure about thermostat compatibility!

KloudAce44 — Tue, 23 Aug 2022 01:09:59 GMT

So I did the compatibility check, I noticed a lone brown wire (as seen in pic) which I’ve read could be an HVAC safety heater trigger of some sort (paraphrasing here) but still not sure if I can move forward with getting a Nest thermostat. I also noticed from my Y port was another brown wire connecting Y and W. And that the G port had a blue wire. So I’m a bit confused here. Any help?

Re: Unsure about thermostat compatibility!

CoolingWizard — Tue, 23 Aug 2022 15:00:40 GMT

First, here is some help, the color of the wire makes no difference. The important thing is that if say BROWN wire is attached to the G terminal in the thermostat, the same BROWN wire must be attached to the G terminal in the HVAC equipment cabinet. Now, most HVAC thermostat cables have the colors that correspond to the terminal letters. Y is for cooling and use a yellow wire. In your thermostat some technician mixed this up a bit. But if the old thermostat is working, the Nest Thermostat will work too. Let’s talk about the terminals and the HVAC industry standards.

The Rh and Rc are input power for the thermostat. Rc is for cooling, and Rh is for heating. When the heating and cooling system are the same equipment, there is a jumper wire between Rh and Rc. You see this on your thermostat. On systems that are separate, say boiler for heat and an AC compressor for cooling, the input power for the Rh will come from the Boiler and not be jumper combined with Rc. Your thermostat is using a Red wire for input power connected to Rh with red jumper wire between Rh and Rc.

G terminal is for the indoor Blower Fan. In AUTO, the Blower Fan turns on when the thermostat calls for either Heating or Cooling. When the switch is moved to ON, the Blower Fan will turn on independent of a call for Heating or Cooling. Your thermostat is using a Blue wire on G terminal.

Y terminal is used to call for Cooling. When thermostat determines it needs to cool, it connects Rc to Y, and that causes the AC compressor and Blower Fan to turn on. Your thermostat is using a Yellow wire on Y terminal.

W terminal is used to call for Heat. When the thermostat calls for Heat it connects Rh to W. Your thermostat used a Brown wire number from Y terminal to the W terminal. (Frankly, this makes no sense.) there is a Brown wire not connected in the background.

O and B, these terminals are for a heat pumps. It controls the 4-way reversing valve of the heat pump. This is how a heat pump switches from Heating mode to Cooling mode and Vice versa. Why two terminals? Some heat pumps are B models, and some are O models. In older thermostats you had to connect it to either O or B depending on what model you had, Newer thermostats are software controlled and the O and B terminals are typically combined on a O/B terminal. Your thermostat is using a White wire on the O terminal.

To better assist you, we need more information. Do you indeed have a Heat Pump system? I believe you have a Heat Pump. Otherwise, a call for Heat (W terminal) will energize the Y terminal which turns on the AC compressor. And if you do not have a heat pump, you system will be trying heat and cool at the same time.

My understanding of your system.

You have an O model Heat Pump, it defaults to Heating Mode:

Red Thermostat cable conductor is incoming AC Power. (Rc and Rh)
Yellow Thermostat cable conductor initializes the AC compressor.

White Thermostat cable conductor is used to energize the Heat Pump reversing valve. To get the system cooling, when Y is energized, O must also be energized. Your heat pump defaults to Heating. When your thermostat calls for Heat, it will energize energize the Y without the O.
The brown wire jumper between Y and W will not be needed.

The Blue thermostat cable conductor is connected to the G.

The only missing wire conductor is the C or common. The HVAC control system uses 24 Volts AC power. This 24 VAC power is supplied to the thermostat and connected to Rc. Since it is AC power, we need the voltage return path, that is the C. Smart thermostats are basically computers and need the 24VAC power. If not, they need batteries. To make this work, you need R and C.

This is a lot of information. Let’s stop here and let you respond.

Ken, The AC Cooling Wizard.

Re: Unsure about thermostat compatibility!

KloudAce44 — Tue, 23 Aug 2022 16:50:03 GMT

@CoolingWizard

Im not too sure if I have a heat pump. Did some research to figure it out. I ran the Heater and the system kicked on the fan started rotating on top of the system. Also kicked on when Cool was turned on. Hope that helps? Also we have no gas here it’s all electric.

Re: Unsure about thermostat compatibility!

CoolingWizard — Tue, 23 Aug 2022 17:16:55 GMT

If you can reach your outdoor unit, send me picture of the manufacturer label showing the model number and serial number and I can look the model up.

Ken

Re: Unsure about thermostat compatibility!

Jeran — Mon, 29 Aug 2022 19:42:26 GMT

Hey there KloudAce44,

I'm just checking up on this thread, did you have any more questions or need any additional help?

Thank you for helping out on this thread, CoolingWizard!

Best regards,
Jeran

Re: Unsure about thermostat compatibility!

CoolingWizard — Mon, 29 Aug 2022 19:53:07 GMT

The fact that your outdoor unit turned on when it was in heat or cool mode makes me believe that chances are you have a heat pump. There are a lot of different manufactures out there who make aftermarket thermostats. Now this one might require that the W be connected to the Y to make sure the outdoor compressor comes on. When there is a call for heat on W, it sends at current over to Y and that causes the compressor to turn on. For heating that’s all that would be needed if it’s a heat pump with an O model. If it requires cooling the thermostat must also connect W, to O to initialize the reversing valve to put it in cooling mode. How we can check this, is with your old thermostat remove the brown wire between Y and W. If youse then put the thermostat front back on and tell it to turn on the heat, go see if the outdoor unit is running. If it is indeed running, then your thermostat knows about heat pumps, and knows that all it needs to do is energize Y for heating. Then set your system to cool and make it turn on, and go see if the outdoor unit is running, and that indeed cool air is coming into the house. If this indeed happens, which I’m sure it does, the thermostat initialized the O for cooling in addition to the Y.

Re: Unsure about thermostat compatibility!

KloudAce44 — Thu, 01 Sep 2022 02:20:45 GMT

@Jeran @CoolingWizard Sorry for the late response. Been busy. It’s on the roof I don’t have access to it as I don’t have a ladder. I live in a condo. I’ll do that test when I can and let you know asap.

Re: Unsure about thermostat compatibility!

Jake — Thu, 01 Sep 2022 18:32:57 GMT

Hey KloudAce44,

I wanted to follow up with you. Please let us know once you have found out more details, as we want to ensure you are good to go.

Thank yo so much CoolingWizard for the helpful replies.

Best regards,

Jake

Re: Unsure about thermostat compatibility!

Jeran — Mon, 05 Sep 2022 15:42:37 GMT

Hey thereKloudAce44,

Did you have any more questions or did you need some additional time?

Thank you,
Jeran

Re: Unsure about thermostat compatibility!

Jake — Tue, 06 Sep 2022 16:57:47 GMT

Hey KloudAce44,

I wanted to check in with you, and ensure everything is good to go. Please let me know if you have any questions, as I will be locking the thread in 24 hours.

Best regards,

Jake

Re: Unsure about thermostat compatibility!

CoolingWizard — Tue, 06 Sep 2022 17:43:03 GMT

Ok, I now understand the wiring of this thermostat. Whom ever installed did not fully understand how to wire it.

You do indeed have a heat pump system. When this thermostat calls for heat, it energizes W, and W is cross-connected with Y. Y turns on the compressor.

When cooling is called for, the thermostat energizes O and Y. O switches the heat pump 4-way valve to cooling mode.

When setting up your nest learning thermostat 3rd Generation, do not input the brown wire jumper. You will input Rc(red wire), O (white wire), G (blue wire), and Y (yellow wire). You will also ignore and not need the red jumper between Rh and Rc.

Re: Unsure about thermostat compatibility!

Jake — Tue, 06 Sep 2022 17:51:42 GMT

Hey KloudAce44,

I wanted to check in and see if you managed to see CoolingWizard's post. Please let me know if you have any questions from here. I would be happy to assist.

Thanks so much CoolingWizard for the helpful replies.

Best regards,
Jake

Re: Unsure about thermostat compatibility!

Jake — Wed, 07 Sep 2022 18:08:40 GMT

Hey KloudAce44,

I want to ensure you are good to go. Please let me know if you are still having trouble, as I will be locking the thread in 24 hours due to inactivity.

Best regards,
Jake

Re: Unsure about thermostat compatibility!

KloudAce44 — Wed, 07 Sep 2022 18:24:13 GMT

What about constant (Normally Blue Wire to C) For the Thermostat to be powered?

Re: Unsure about thermostat compatibility!

KloudAce44 — Wed, 07 Sep 2022 18:24:36 GMT

Yes. Sorry I'm pretty busy.

Re: Unsure about thermostat compatibility!

KloudAce44 — Wed, 07 Sep 2022 18:25:18 GMT

Also haven't had time to do the process you have shared with me.

Re: Unsure about thermostat compatibility!

CoolingWizard — Wed, 07 Sep 2022 18:27:58 GMT

The Nest E and Nest learning thermostat 3rd generation can attain the return path using Y, or W. If you have a fee wire that can be connected to the common side of the transformer, that would be the preferred approach.

Re: Unsure about thermostat compatibility!

KloudAce44 — Wed, 07 Sep 2022 18:33:20 GMT

So After connecting everything and have a free wire (not the free brown wire that is in the photo?) I can disconnect and reconnect to the common side on the transformer and connect that wire to the C in the thermostat for constant power? Just making sure I got it right so no issues occur.

Re: Unsure about thermostat compatibility!

KloudAce44 — Wed, 07 Sep 2022 18:39:22 GMT

I've read that the brown wire can be connect to the thermostat if there is a emergency back up heating system in my overall heat pump. is that correct?

Re: Unsure about thermostat compatibility!

CoolingWizard — Wed, 07 Sep 2022 20:18:53 GMT

yes indeed you can do this. Keep this in mind, some air handler‘s have a control board with matching letters and screw terminals R,G,Y,W,C, some do not.

if there is a control board and it has a C terminal screw attach your spare wire to that C terminal screw.

if there is no control board, you will need to locate the transformer and find the step down side of the transformer. The red wire will be attached to one of the two wires on the step down side, and the second wire will be attached to the common elements in the air handler. You will locate that and attach your spare wire with that side.

Ken