Nest 3rd gen thermostat low battery

Toledorick

Hello, I am having some recent trouble with both my 3rd Gen Thermostat. It all started with one at first. That wasn't powering on due to not getting power via the Rh wire has it has for the past 2 yrs. I only have a furnace with 2 wires, white (W) and red (Rh). This has been the wires used for the past 2 yrs to control the thermostats. I have 2 separate zone. First floor and 2nd floor and the 2nd floor was the one not powering up for over a day.

On 1/14/24, I lost power to both thermostats.

I then contacted hvac / oil furnace tech and found out that my 24v transformer had gone bad. A new was installed. I was able to see that both thermostat turned on and manually was able to turn my furnace and call form heat with both thermostat.

However, I'm still getting no power to either thermostats, it shows low battery and won't connect to the internet. But I can manually control the heat. 

Now after installing a new transformer I'm getting low battery on both. I can manually operate them but won't connect. 

I attached some photos of my new transformer, from the thermostat and my current furnace setup. I have an oil furnace. Nothing fancy. And I was thinking of buy the nest power supply adapter. I just don't know how to run the cables in the adapter and would I need 2 separate adapters since I have 2 separate zones? 

What could be wrong?

Thanks in advance. 

Rick 

Jamie Hall

A bit puzzled. You have a white and red from the transformer. You have R and W connected on the Nest. You have no C connection, apparently. What is connected to the zone valves? R on the Nest should go to the transformer. Then W to the zone valve it controls. Then from the zone valve there should be a wire back to the other terminal of the transformer. That may work, though Nests are known to not work reliably, if at all, if they don't have a C wire...

EBEBRATT-Ed

You need to have your technician come back and install a separate transformer and relay to hook up the Nest thermostats.

The gadgets they sell don't work. Nests have a battery that needs to be charged. With a 2-wire set up they only charge on the off cycle and some boilers/furnaces do not have a large enough transformer.

If you want them to work bite the bullet and see the attached wiring diagram. @EdTheHeaterMan has a post on site called "how do I connect this wire" look for that as well

EdTheHeaterMan

The NEST thermostats can be operated without the use of an isolation relay according the the Taco Tech Support folks. They reached out to NEST to get the best solution for using Nset thermostats with Taco products. Here is what they have come up with



When I used this diagram to install the first generation NEST thermostats to the Zone Valves that you have on your Crown Boiler, I would get an Error 85 message. I discovered that the Taco 570 series zone valves actually disconnect power from the thermostat and transformer many times during the call for heat. It is built into the zone valve heat motor in the actuator. I believe that NEST has addressed that problem with software updates that get installed via the internet, but I have solved that using the isolation relay that @EBEBRATT-Ed indicated in a previous comment. Here is my FIX using an isolation relay.



IF you look closely, both diagrams have a wire connected from the C on the NEST to the C on the zone valve transformer.

Your 2 wire installation that has worked fine for 2 years may work again but you may have to charge the batteries using a USB charger to get the batteries in the NEST thermostats back to at least 80% charged. Here is what most likely happened.
1. You had a zone valve transformer failure that required a replacement transformer.
2. That caused the NEST batteries to use up most of the stored power letting the batteries get LOW.
3. You had the Transformer replaced during the cold season when the off cycles are short
4. That short off cycle does not leave time for the NEST batteries to recharge
5. So you are always playing catchup and never getting enough charge to operate the WiFi section of the thermostat.

If charging the batteries via the USB cord, while the thermostat is disconnected from the system, does not work, then you may need to add another wire to the thermostat so you can connect the C terminal on the NEST to the C terminal on the Transformer. That C wire WILL SOLVE your issue. Running an new thermostat wire with 3 or 4 conductors may not be as hard as you think. Electricians do it all the time.

Toledorick

Here are some close up photos of both my taco and the wiring. Also my 1st floor nest thermostat finally connected to my app after 2 days (meaning today (1/16/24) and I am able to control it as I was capable for the past 2yrs. I haven't done anything new. I haven't ran a 3rd cable or the c cable. But the 2nd floor nest thermostat it still telling me low battery and won't connect. So running a new wire from the 2nd floor to the basement sounds like a huge task and one that I don't want
So I ordered the nest power adapter. Hoping to figure out this cable and just added it to the 2nd floor valve, so the 2nd floor nest thermostat could get enough power to connect to wifi. 
to break walls for. And if I'm guessing correctly, being That this house was build on 1986, I bet the current thermostat wire is staple to the beams. 

Toledorick

Toledorick

@EdTheHeaterMan your respond is amazing as well as the other. After reading multiple times and looking at the diagram. I noticed you have 2 transformers. Am I right by saying that? Where would I connect the 2nd transformer and what would it power it up? And when it comes to the relay. Are there an specific one? Name? So I can search it on either home depot or Lowes and pick it up. Or a plumbing supply store.

Thanks. Again 

EdTheHeaterMan

First I would try to recharge the NEST battery using the USB cord, unless your thermostat does not come equipped with that charging port.
Next I would use the first Taco Diagram where there is no extra relay. Since you have the 3rd gen thermostat, you should be ok with just adding a third wire to the NEST thermostat. That would connect from C on the NEST to the C on the transformer (hopefully where the white wire is)

Toledorick

@EdTheHeaterMan, if I have to run a whole 14/3 or 18/3 wire from the thermostat in the 2nd floor to the basement. I may come across some trouble. I trying, just to test, pulling the current thermostat cable out from the second floor up, and I was able to pull about 6 inches and then I felt resistance and couldn't pull anymore. So if I were to pull the cable down to the basement it appears that it will get stuck some where. And if that so, how would I still add the c wire from the basement to the thermostat? Without running to this problem?

I also forgot to add that 3 or 4 days ago when I noticed the issue with the thermostat. I plugged it to a micro USB to charge and it never charge after couple of hrs of been plugged. 

EdTheHeaterMan

You do not have to put the thermostt wire in the same place that you would put it if the walls were not covered with sheetrock. The wire can actually go up into the attic then come down to the basement next ot a plumbing chase or chimney chase or even down the outside wall with a piece of molding covering it, or in a conduit.

Try to think outside the box when it comes to running a wire. I have staples a thermostat wire to the molding of a door frame inside a closet where you can't see it unless you stand inside the closet. then follow the closet below it into the next floor, then finally thru the closet on the bottom floor to get into the basement.

Wires do not need to be inside the walls to work, you can hide them in closets and between floor boards and run them around floor molding to keep them hidden.

EdTheHeaterMan

Toledorick said:

I also forgot to add that 3 or 4 days ago when I noticed the issue with the thermostat. I plugged it to a micro USB to charge and it never charge after couple of hrs of been plugged. 

Then the thermostat battery is dead and it is time to get a replacement thermostat. Those batteries are not replaceable.

Toledorick

EdTheHeaterMan said:

I also forgot to add that 3 or 4 days ago when I noticed the issue with the thermostat. I plugged it to a micro USB to charge and it never charge after couple of hrs of been plugged. 

Then the thermostat battery is dead and it is time to get a replacement thermostat. Those batteries are not replaceable.

I'll try to charge it again today and see if I get any charge. If not then, I bet this whole time the thermostat had been the problem. I got lucky working with this unit without the c wire for. 2yrs. So hopefully I can get lucky again.