From Nest Gen 3 to Nest Generation 4 Wiring, setup, and configuration questions

[jsk414]

Hello all,

Sorry for the long read but I have concerns that my system isn't set up properly and hasn't been for years. 

My HVAC system is as follows: Professionally installed in 2016 with a 3rd gen nest learning thermostat (A0013). The cooling system is a two-stage central A/C. My heating system is a dual fuel system that has a electric heat pump (on the outdoor A/C system) and a indoor natural gas furnace. 

I just upgraded from a Nest Learning Thermostat 3rd Gen (A0013) to the newest 4th gen Nest. Before I began my old nest showed the following system details in the NEST app (I guess this is no longer used and is now google home). Unfortunately I forgot to click the "wiring" tab in the nest app before I removed that 3rd gen thermostat, so I wasn't able to grab a screenshot of what info was listed there, but at least I remembered to grab this:

Heat source 1   Electric

Heat source 2  Electric

Alt. heat source 1   Gas

Alt. heat source 2   Gas

Heat type 1    Forced air

Heat type 2    Forced air

Alt. heat type 1   Forced air

Alt. heat type 2  Forced air

Heat pump orientation  O

I just figured I could easily copy what the previous installer had done wiring as the terminals for the 3rd gen nest and 4th gen are the same. However when I took the old 3rd gen nest off I was a little confused by the way it was connected and wired. 

The existing wiring for the gen 3 nest was as follows:

Y1 (yellow)

Y2 (blue)

G (green)

OB (orange)

W1 (white)

C (black)

*asterisk (brown)

Rh (red)

When I installed the new 4th gen nest I copied that wiring exactly as my system always seemed to perform properly. When I was setting up the new 4th gen nest it asked me what type of system I had, I told it dual fuel. It asked me when I wanted the alt heat to run, and I told it under 34 degrees outside temp (I was told that is when heat pumps lose their efficiency and natural gas is better?). It also asked me what the Auxiliary wire (I am assuming that is the asterisk wire) was: humidifier, dehumidifier, or emergency heat. I told it emergency heat because I was assuming that is what I would label my secondary heat source and I do not believe there is a humidifier or dehumidifier on this system (that would be another piece of equipment, wouldn't it?). After set up my System details show the following in the GOOGLE HOME app (bye bye nest app): 

Heat Pump orientation: O

Dual Fuel: 34

Heat Source: Electric

Alt. Heat Source: Gas

Emergency Heat Source: Gas

Heat type: Forced Air

Alt. Heat type: Forced Air

Emergency Heat type: Forced Air

The system appears to be running fine. It has been cold so the gas furnace is clearly turning on when the heat is called for. I temporarily messed with the auxiliary heat setting to force my heat pump to be utilized and it did, I then switched it back to auxiliary (gas) heat under 34 and it went back to using my furnace. My concerns are as follows:

1) Why was my brown wire always connected to *(asterisk) port on the 3rd gen nest and I copied to the 4th gen? I am assuming that is my gas furnace. Shouldn't that have been in W2? However it has been running correctly.

2) Why did the set up of the 4th gen nest in the google home app want to know what the *(asterisk) port was between a humidifier/dehumidifier/Emergency heat and when I told it was for emergency heat thinking that meant auxiliary heat it still seems to function? Should I somehow tell it I don't have emergency heat, and if so what is that wire being used for?

3) I noticed there is a setting in the google home app on the nest thermostat to utilize emergency heat, so I toggled that on, thinking that would force my furnace on but no heat runs when I toggle on emergency heat. The app thinking the house is heating but my furnace and my heat pump are both not actually running. The blower fan does though. 

4) Under the 3rd gen nest in the old NEST app it showed I had a heat source 1 and 2 and a alt heat source 1 and 2. However for the 4th gen nest in the new GOOGLE HOME app it just lists one heat source, one alt. heat source, and now lists and emergency heat source? 

5) Is switching from heat pump to natural gas furnace under 34 degree outside temperature a good set point?

6) This is probably my most important concern. Yes my system seems to be running how it should for the years since my 3rd gen nest was professionally installed however could it have been wired and/or configured in a way to run either more energy efficient or run better to prolong the life of the HVAC?

7) Is there any just setting I need to change in the google home app? 

THANK YOU!

[jsk414]

bumping this up, could really use some guidance. Thank you. 

[luciaaa]

Hi, @jsk414!

Thank you for reaching out to the community. I understand you have some questions about your Nest Learning thermostat (4th gen) installation. I really appreciate you sharing all the detailed information about your previous installation and the current one. I’d be happy to help you with your concerns. 

1. The brown wire was likely connected to the star connector because it corresponds to emergency heat. It should not be connected to the W2 terminal, as that terminal is not for emergency heat.
2. While the terms "auxiliary heat" and "emergency heat" are sometimes used interchangeably, there's a key difference. Auxiliary heat is a secondary heat source (often electric resistance heat) that supplements the primary heat source (such as a heat pump) when it can't keep up, especially in cold weather. Emergency heat is a separate heating system used only when the primary heat source fails entirely. It's designed for short-term emergency use.
3. If your heat pump's emergency heat feature isn't working after installation, or if there's no "Emergency Heat" option on your Nest thermostat:
• Your system might not have emergency heat. See Emergency Heat for heat pump systems for steps on how to check.
• The thermostat wires might not be in the correct terminals. Double-check your wiring diagram and ensure the wires are in the correct terminals on your Nest thermostat.
• Your Nest thermostat might not be set up correctly. The correct settings depend on your Nest thermostat model. See Troubleshoot Emergency Heat for instructions.
4. The Google Home app interface functions differently from the Nest app. As you add heat sources in the Google Home app, they may appear as confirmed once detected by the thermostat.
5. If you live in a climate where temperatures rarely drop below 34°F (1°C), a heat pump might be sufficient most of the time. Modern heat pumps can operate efficiently at lower temperatures, but their efficiency decreases significantly below freezing.
6. There are lots of ways to help improve energy savings and reduce your utility bills. You can refer to this article to check the available options: How to improve energy savings with your Nest thermostat. 
7. To determine the correct wiring configuration and compare it to the settings you shared, please provide a picture of your current wiring.

Don’t hesitate to reply if you have any questions.

Best regards, 

Ana

[jsk414]

Hi Ana @luciaaa ,

Thank you so much for your response. I have attached a picture of my wiring below . Most of your responses made a lot of sense to me except for point 2. The auxiliary vs emergency heat. I have a dual fuel setup. I have a heat pump and a completely separate gas furnace. The idea was to run the heat pump until about freezing because its more efficient and cheaper to run (although it is a high efficiency gas furnace as well). The system was put in not to be auxiliary as you describe where the secondary is there to help the heat pump keep up and both run concurrently but it run as a either or setup. If below 34 outside then the furnace should only run, if above 34 then heat pump only runs. This sounds more like how you describe emergency heat, but my furnace is designed to run in not emergency short term situations but could very well be a primary source for most homes, no system "failure" needed... if that makes sense. 

Wiring diagram

[luciaaa]

Hi, @jsk414!

Thank you for your response and for letting me know about your concerns regarding your heating system, specifically whether you have emergency heat or auxiliary heat. I'm happy to assist you further. 

Based on the picture provided, it appears you have emergency heat since there are no wires connected to the auxiliary heat terminals. However, to ensure the wiring is correct and confirm whether you have emergency heat instead of auxiliary heat, I kindly suggest contacting a Nest Pro technician or an HVAC technician of your choice to confirm this information. 

Feel free to reply if you have any questions. 

Best regards, 

Ana

[jsk414]

Thank you @luciaaa 

I will look to find a pro. But if you could, can we circle back to the description of emergency heat? Is it possible that the "emergency heat" isn't for an "emergency" when "one unit fails" as you described. Just a truly secondary source? 

[luciaaa]

Hi, @jsk414!

Thank you for your prompt response and for letting me know you will contact a technician. I’m happy to help you further.

Regarding your question about heating systems: the emergency heat is not a secondary heat source. There's a distinction between auxiliary heat and emergency heat, especially in systems with heat pumps.

• Auxiliary Heat: This is a secondary heat source that supplements the heat pump when it can't keep up with the heating demand (e.g., during very cold weather). It's designed for regular use as needed and is often automatically engaged by smart thermostats like Nest. It's not a backup for a broken heat pump in the sense of a complete replacement. It works in conjunction with the heat pump.
• Emergency Heat: This is a separate, usually electric, heating element intended for emergency situations when the heat pump is completely malfunctioning. It's not meant for regular use because it's significantly less efficient and much more expensive to run. You should only use it for short periods until the heat pump is repaired.

Feel free to reply if you have any questions.

Best regards,

Ana

[jsk414]

So I am just confused by those definitions.

Both of my heating systems (high efficiency heat pump and high efficiency gas furnace) are meant to be primary systems. The gas furnace is set up in the * wiring and the nest set up asked me what it was and I said E heat. However my system is recognized as dual fuel and my heat pump is running when above 34 degrees and my gas furnace is running when under the 34 degrees. 

[luciaaa]

Hi there!

Thank you for your reply. I'm happy to help you further with your concern. Based on the details provided in previous posts, it seems your system has a heat pump as its primary heat source and an emergency heat option. If you would like to know if you have a secondary heat source, the best option would be to hire a Nest Pro technician or a technician of your choice. They can determine the correct wiring configuration and the type of heating system you have.

Please don't hesitate to reply if you have any questions.

Best regards, 

Ana