Is a Gen 3 Learning Thermostat compatible with my system?

[cappy2112]

I've got a central heat & A/C system controlled but a White-Rodgers 1F80051 thermostat, which has no C terminal nor wire.

There are 3 AAA batteries in the thermostat, so I'm concerned that I may not be able to use a Google Nest Gen 3 as a replacement.

Since I Can't attach a picture of the thermostat PCB, I'll just name them.

Terminals:

RC. W. RH G Y

The RC & RH are jumpered together.

On the furnace, I have the following terminals

Y. G W R C

 The C goes to the transformer secondary ONLY.

The Y terminal connects to the blue wire which connects to the Y terminal on the thermostat.

The Y terminal also connects to the red wire going to the transformer secondary.

I did go through the compatibility checker, but the results were not conclusive (it may be compatible).

Thanks

[jb76 · Accepted solution]

Hi - it will work but you will likely have power issues with the Nest without a C - Common wire connection. You can purchase a Nest Power Connector to take power from the Rh and W but that will only give you 5 volts to charge the Nest - some have reported continuing issues using these connectors. Other option is run a new thermostat 5-wire bundle that you can connect a dedicated wire to the C (Common) connection on the furnace to keep the Nest above 3.85 V for operation and control (wifi). You can usually get a 50 ft spool of 18/5 gauge wire for $25 at HD but running the wire will be the challenge.

NOTE: Your furnace is compatible but from what you describe there is no C wire going back to the thermostat - just a 24V connection from the transformer to the furnace control board..

J

[jb76 · Accepted solution]

Hi - it will work but you will likely have power issues with the Nest without a C - Common wire connection. You can purchase a Nest Power Connector to take power from the Rh and W but that will only give you 5 volts to charge the Nest - some have reported continuing issues using these connectors. Other option is run a new thermostat 5-wire bundle that you can connect a dedicated wire to the C (Common) connection on the furnace to keep the Nest above 3.85 V for operation and control (wifi). You can usually get a 50 ft spool of 18/5 gauge wire for $25 at HD but running the wire will be the challenge.

NOTE: Your furnace is compatible but from what you describe there is no C wire going back to the thermostat - just a 24V connection from the transformer to the furnace control board..

J

[cappy2112]

Thanks. I'll see if there is an unused wire in that bundle, maybe I'll get lucky.

There is a light switch about 2 feet from that thermostat, so if the extra wire isn't in the bundle, I can just put a 24V transformer next to the light switch and run 2 wires right to the thermostat.

[cappy2112]

Sorry for the delay. I did go to the control board on the furnace and peeled back the insulation as far as I could. The wire to the thermostat is a 4 wire cable only. 😞

[jb76]

That will work - that 24V from the transformer going to ‘C’ on furnace board can be tapped for 24V to Nest - just connect/wrap the exposed C wire you run from Best to that screw/fastener on control board. May be worth it to run a bundle (5wire) from Nest to furnace rather than a thin 18 gauge wire for Common power to Nest…up to you.

J

[cappy2112]

The furnace is on the other side of the house, only (barely) accessible through the attic which is full of blown-in insulation and very hard to see/move around in. The crawlspace under the house is another potential alternative. But the last time I was under there it looked like the furnace area wasn't easily accessible through the crawlspace. That is why I'm thinking of just connecting another 24V transformer

to the light switch that is next to the thermostat. Running a wire from there would be much easier.

[jb76 · Accepted solution]

Hi - Connecting to that transformer would make more sense if doing yourself then as long as you can run that connecting wire back to the C terminal on the Nest baseplate - Nest won't care whether it comes from the furnace or a transformer (spliced into a power line going to the light switch). I did something similar connecting my doorbell to a new transformer that got its power from a nearby outlet (tied the transformer directly in to the electric supply line and then connected 18 gauge from transformer to my doorbell line).

[Markjosephp]

Hello everyone,

I'm dropping by to make sure everything is covered here.

Thanks for sharing your knowledge about the Nest Thermostat installation, @jb76. Big help!

@cappy2112, I appreciate you reaching out! It seems like you've put a lot of thought into your HVAC system and potential thermostat replacement. Based on your description, the absence of a C terminal on your current thermostat might pose a challenge when considering the Nest Thermostats, which typically requires a C wire for power.

Since your current thermostat is powered by batteries and your furnace setup lacks a dedicated C wire, using a separate 24V transformer is a viable solution. Connecting it to the light switch near the thermostat is a clever workaround, especially if running a new wire from the furnace is a difficult task given the layout of your house.

Before proceeding, it's crucial to ensure the compatibility of the Nest Thermostat with your system. The compatibility checker is a good starting point, but if it remains inconclusive, you can share with us your current thermostat's wiring setup and your system's make and model.

Best regards,
Mark

[cappy2112]

Before proceeding, it's crucial to ensure the compatibility of the Nest Thermostat with your system. The compatibility checker is a good starting point, but if it remains inconclusive, you can share with us your current thermostat's wiring setup and your system's make and model.

Best regards,
Mark

---

I started with the compatibility checker, it wasn't conclusive.

I've ordered a Nest Power Connector today to have in case I need it.

I had a busy holiday season and installed a new mesh router system, this was more critical than getting the thermostat installed.

I'll probably try the thermostat this coming weekend.

My thermostat is a White-Rodgers Model 1F80051, the furnace is made by Lennox (will get the Model # if I install the power connector. See wiring diagram below.

[Markjosephp]

Hi cappy2112,

Your White-Rodgers thermostat and Lennox furnace combo bring their own set of considerations, but with the wiring diagram and the Nest Thermostat Compatibility Checker, you're on the right track. Please check out the Nest Power Connector article to guide you with its installation.

If you run into any hiccups or have questions during the installation process, feel free to reach out for assistance.

Best,
Mark

[cappy2112]

The compatibility checker isn't conclusive- "it may work"

I hesitate on replacing the thermostat now because the battery needs the C wire in order to charge.

The Nest Power connector is also inconclusive, as it doesn't always work when the C wire is missing.

I'm looking at ways of running a 2 wire cable from the thermostat to the furnace through the attic. It's a relatively short run, but the obstacles of getting over to the wall where the furnace is are quite difficult.

[Markjosephp]

Hi there,

I'm sorry for the late reply. I understand your hesitation in replacing the thermostat without a conclusive compatibility check, especially considering the need for a C wire to charge the thermostat. It's wise to explore alternative solutions. Running a 2-wire cable from the thermostat to the furnace through the attic seems like a viable option, although the obstacles you mentioned may pose some challenges.

If navigating those obstacles proves to be difficult, you might want to consider consulting with a professional HVAC technician. They can assess the feasibility of the wiring installation and provide guidance on the best approach. Additionally, they may have experience with similar installations and can offer valuable insights to ensure a reliable and effective solution. You can still install the Nest Thermostat by following the wiring setup from your old thermostat without the jumper wire.

Thanks,

Mark

[cappy2112]

I did buy one of the Nest power adapters, but after reading up on them they are often unreliable (as in not adequately powering the thermostat) depending on the circumstances.

I've run a 2 wire cable from the furnace across the attic, but not down the wall to where the thermostat will be. I need to go up once more to find the place to run the 2 wire cable down the wall. We have other construction going on which has been taking precedence over the thermostat.

[cappy2112]

I do have a question though- The 3rd Gen specs show that the thermostat can upload AND download up to 50MB in each direction, EACH WEEK!!.

Here is the exact quote from the specs: "50 MB/week upload and 50 MB/week download".

What can the thermostat possibly be downloading that it could consume 50MB each week?

The same for the upload bandwidth.

I would be willing to bet that the firmware image is much less than 50MB, so what data does it need to download EACH WEEK? ????

[Markjosephp]

Hi cappy2112,

I appreciate that you've taken the initiative to address the power supply for your thermostat. As for your question about the 50MB upload and download specifications, Typically, the Nest thermostat doesn't download or upload large amounts of data weekly. The 50MB limit mentioned in the specs is likely more of an upper threshold, and actual usage may vary. The thermostat mainly uses this bandwidth to communicate with the Nest servers, check for software updates, and send and receive configuration data.

Some of the data exchange could include weather updates, energy reports, and system status. Additionally, the thermostat may send data to Nest servers for analysis, to improve features, or for troubleshooting purposes. 

Feel free to ask if you have any more questions in mind.

Regards,
Mark

[cappy2112]

I've successfully run a 2-wire cable from the furnace to the thermostat, through the attic.
However, I'm puzzled as to why I don't see 24VAC on the screw terminals at the furnace.
I've connected the meter to the R & C wires on there terminal strip at the furnace, but don't read any voltage (I've tried both AC & DC just to be safe). I've also verified that the multimeter is working by putting the probes into An AC outlet and read 120VAC.

I've found a wiring diagram and schematic of my furnace on the inside of the front panel cover, so I'm attaching that as well. Hopefully someone here understands that and can help.

[cappy2112]

[cappy2112]

I'm happy to say that my Nest Gen 3 learning thermostat is installed and working (at least the heat works, I' haven't tested the A/C yet). The battery voltage shows 3.935, so the ground wire that I ran through the attic is working.

I decided not to put a 24VAC transformer in the attic and just run that wire down the wall to the thermostat because the summer temperatures in the attic might cause the transformer to overheat and eventually fail.

You can ignore the schematic above, I've figured out the last bit of confusion.

Before finding the schematic, I made an assumption about a 2 wire cable that runs from the terminal strip in the furnace. The 2 wire cable connected to the terminal strip goes to the A/C compressor.

There is a transformer on the wall next to the furnace, and a 2-wire cable (with the same color wires as those in the 2-wire cable mentioned in the previous sentence). I assumed that the 24VAC was coming from this transformer, but I was wrong. Looking at the schematic above, I was able to find the real 24VAC transformer inside the furnace, and measured 26VAC with a meter. It turns out that the transformer on the wall near the furnace is probably for the doorbell. 😉

I had a friend help me get the wire form the attic down the wall to the thermostat, which went very well.

I'm done going into the attic for a long time.

The thermostat is connected to the IOT wifi network, and is showing "78" for the wifi signal level. Assuming that 100 is the highest, this is great.

Thanks for your help getting this working.