Help with Nest Snow installation

[MikeFitz]

Just bought a Nest Snow.  Old thermostat has red and white wires (plus blue, but it is not connected to anything at the boiler end). Tired connecting red to R term of Nest, and white to W.  Nest said I needed to power connector, so ordered that.  When it came, I opened up the panel at the boiler and the thermostat wires connect to two screw terminals, which are both just generically labeled "T".  No marking for R, W, etc.  Seems like the old thermostat is just closing the circuit on the red and white wires to call for heat, so no power available for the nest to detect.

Anyone have any suggestion to get his to work (or if it can't work)?   

Thanks.

Mike

[CoolingWizard]

You probably have something like a Honeywell, R8185E and that controller for oil burner will have two terminals Marked T. One T outputs 24 V @ 0.2 amps and the other T waits for that 24 V to come back and start the heating process. Now under the cover will be a transformer and that transformer will have a second wire which would be the common side. The LINE side of the transformer is wired to that T connector. This is how you’ll be able to find the common that you can attach the power connector to. But if you have a spare conductor, already in the thermostat cable, such as a blue wire, just attach it to the transformer on underneath the cover on the common side technically, it’s the neutral.

The AC Cooling Wizard

[MikeFitz]

Hi.  Yes, inside the boiler controls there are two terminals marked only "T".  I'll need to go back down tomorrow and look for the transformer.  There were no terminals marked C and any of the other letters that the power connector or Nest unit call out.  Just sending the 24V back down seems to make sense of my guess that the old thermostat just closes the circuit. I do have that spare (not connected at the boiler) blue wire to work with.  So I'm clear, if I can find a common, I can just use that blue wire to send it up to the Nest's "C" terminal and I won't need to use the power connector?

Thanks for your reply.

Mike

[CoolingWizard]

Yes Mike, that is the correct approach. Keep in mind the controller transformer only has 0.2 amps of capacity to work with.

The AC Cooling Wizard

[MikeFitz]

OK. Thanks.  I'll poke at it again tomorrow.  I appreciate the quick replies.

Mike

[MikeFitz]

So, I looked at the boiler controls and the transformer is attached directly to a printed circuit board, so no visible access to those wires.  There are two more screw terminals below where the thermostat leads attach.  My multimeter shows 15+ volts from T1 to the 4th screw terminal and about 6.6 volts from T1 to the 3rd screw terminal.  That same 6.6 volts is present between T1 and T2 when the thermostat is not calling (open circuit). Not sure how to proceed.  I'm attaching pics of the controls and a diagram from the back side of the cover.

[CoolingWizard]

Mike,

on your voltmeter are you using the settings for AC voltage or the setting for DC voltage? You need to be checking using the setting for AC voltage.

[MikeFitz]

I believe the meter is auto-sensing, but I will need to check.  I looked up the Honeywell unit and it appears those other screw terminals are used for something called EnvironCom, so they may not be a source for a common.

Since the Nest has two batteries, does it really need power from the boiler controls, or can the Nest just complete the circuit like the existing thermostat, and the mercury bulb thermostat before that one?

[CoolingWizard]

If your nest uses a pair of batteries then no, you will not need a C wire connected.  
for auto sensing multi meters, you normally have to set it for AC or DC voltage and it’ll auto since that voltage in your particular case you need to be on AC voltage when checking an HVAC systems power

[MikeFitz]

Hi.  Thanks for your continued replies.  I was browsing these boards and stumbled across a post (I think to which you also replied back in March 2022) detailing adding an external 24V transformer into the mix with the nest power connector and decided to order one from Amazon, which should arrive Friday. I will then follow the published instructions for setting that all up.  Some comments from other posts seem to imply that without the correct power coming up to the Nest, that the batteries can drain quickly, and I'd like to avoid that.

Thanks again for the suggestions and useful info.

Mike

[Markjosephp]

Hello folks,

I wanted to ensure that everything was covered here.

Thanks for the inputs, @CoolingWizard.

@MikeFitz, I appreciate your efforts in finding a solution, and I hope you've got the answer that you're looking for. How's the installation? Please let us know if you need more help so we can assist you right away.

Regards,

Mark

[MikeFitz]

Hi Mark.  It is mostly working.  I still get some random complaints that the Nest is not getting proper power, but they clear by walking through the menus and running the power test.  I think this happens after the Nest has actually called for heat, but that is not happening due to the Nest being on my oil-fired boiler, which is secondary to my mini-splits that recently returned to service.  I do have a third conductor in the cable to the nest that is currently unused and I think if I tie that to the other side of the 24V transformer and the C point on the Nest, that may resolve the issue, but I just haven't gotten around to testing.

Thanks.

Mike

[CoolingWizard]

Mike, the Nest 3rd generation uses the W wire to reach the COMMON side of the transformer through the furnace control system. When your furnace is activated the power incoming on the R wires to sent to the Furnace on the W wire and therefor the COMMON power is not available.  

If you feel comfortable about working with your equipment, you can indeed attach the unused conductor to the transformer and to the C terminal on the nest back plate. Make sure to disconnect the AC power to the furnace before making any changes to the wiring. 

Ken, The AC Cooling Wizard

[Markjosephp]

Hi there,

Thanks for the continued support, @CoolingWizard.

@MikeFitz, I'm checking back in — how's it going? Have you managed to install a C-wire as suggested by Ken?

Regards,
Mark

[Jake]

Hey there,

I wanted to check in and see if you are still in need of any help. Please let me know, as I would be happy to assist, and answer any questions you may have.

Best regards,
Jake

[EmersonB]

Hello everyone,

@MikeFitz, just one quick final check in here since activity has slowed down. We'll be locking the thread in the next 24 hours, but if you still need help, we would be happy to keep it open. If there's more we can do, just let us know.

I appreciate the help, CoolingWizard, Mark and Jake.
    
Best,
Emerson

[MikeFitz]

1. Still not working as it should.  I tried adding a C-wire connection with a spare conductor, running it to one side of the 24V transformer. The boiler would fire, but the power connector power test won't run.  It gives the warning about the test running down the battery, but when you click to continue, it flashes something on the screen too quickly to read and throws an M25 error. I removed the C-wire for now.

[CoolingWizard]

Mike on your boiler there are two connections, labeled “T”.  All your boiler is looking for is the first tee to be connected to the second T. When that happens the boiler will light. Now, if you have an air conditioner, there may be a way to get this all to work better. You would need to connect the T with Rh, and the second T to W. Then add a 24VAC transformer and supply Rc with one side. I was hoping that using a Volt Meter, set to AC Voltage, you could find Common checking Red wire on and screw 1 or 2 below it.  If you I’d, put the blue wire on the one that showed 24 V AC +/- 4  volts. Then put blue wire on C of the Nest Backplate 

Ken, The AC Cooling Wizard

[MikeFitz]

There is no air conditioner.  It is a very old oil-fired boiler.  Boiler has the two "T" terminals.  What I need is to power the Nest with the 24-volt transformer I bought and then connect the two "T" terminals to two terminals on the Nest that will close the circuit when heat is called for.  The Nest only has terminals marked Y, C and W on the left side, and G, R and OB* on the right side. Currently, I have the R and W terminals connected to the the left-hand  R and W/Y terminals of the Nest  power connector.  One side of the 24-volt transformer is also connected to the R terminal of the Nest.  The other side of the transformer is connected to the "C" wire of the power connector.  The right-hand "R" wire from the power connector goes to one of the T terminals on the boiler and the W/Y on the right side goes to the other T terminal on the boiler. This seems to work, with the exception that when the oil burner is routinely calling for heat, the Nest throws random errors about power not being correct and to install a power connector, even though the Nest sees the connector installed.  That's where I'm stuck. Not having any insight into the connectors on the Nest as to which terminals can supply power to the Nest and which terminals provide a short (close the connection between the two "T" terminals, makes this difficult to find a solution.  Same goes for the power connector.  No schematics of how that unit works.

[CoolingWizard]

Did you test the T’s with a volt meter and the 2 and 3 three screws? I was hoping that one of those would be the common side of the transformer for the EnviraCOM. Terminal screws two and three should have 24 V AC on them. And one of those is going to be the common side of the transformer.
The model thermostat you have requires 24VAC on R and C.  If you can do that, then it will provide you with power for the nest and the AA batteries will be a back up.

Ken, The AC Cooling Wizard

[MikeFitz]

Yes, I metered the terminals.  When not calling for heat, you see 30+ volts AC.  When calling for heat, you see zero, as the thermostat closes the loop and you get a short. Since the R and C terminals of the Nest need to have the 24-volt power feed. Which other two terminals can I use to provide the short back to the boiler controls when calling for Heat?   W appears to be the heat terminal, and I have the R and W wires connected, but without the power connector, that doesn't work.  With the power connector between the Nest and the boiler controls, it at least mostly works, if you can live with the random errors.

[CoolingWizard]

The R and W connect to the T and T terminals.  One of the T screws sends out 24VAC to the thermostat and the W sends it back to the other T terminal.  If you short the two T screws together it will start the boiler.  One of the T terminals is also listed as 1. That is the data terminal for the EnviraCOM and terminals 2 and 3 are the 24VAC. And one of those should be the Common side of the transformer.  

Ken, The AC Cooling Wizard

[MikeFitz]

I appreciate all the info, but what I really need is a recommendation for how to make this thing work, either with or without the Nest power connector.  If I connect the T terminals to the R and W terminals of the Nest, the Nest gets power until it calls for heat and then appears to run off battery until heat is not longer called for.  In my thinking, that is because the R and W get shorted to send the incoming 24 volts on the R terminal back out to the boiler via the W terminal.  At least, that is what the behavior indicates when I don't provide my own 24volts and/or the power connector.

[CoolingWizard]

Use a  Volt Meter, place in in AC Volts and place one lead on the T red wire, and the other to terminal 2 and then 3. See if you find 24 Volts AC. If you do, that will be the Common you are looking for.  It has been a while but if memory recalls, #3 should be the  common.

Ken, The AC Cooling Wizard

[Jenelyn_O]

Hi there,

Checking in — were you able to try the steps provided by CoolingWizard on how to use a Voltmeter and proceed to the troubleshooting? Please let us know how it goes. 

I appreciate your help, @CoolingWizard, Mark, Emerson, and Jake. 

Best,

Jenelyn

[Jenelyn_O]

Hi MikeFitz,

I wanted to follow up if you still need help. Please let me know if you’re still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.

Thanks,
Jenelyn

[Jenelyn_O]

Hello there,

We haven't heard from you in a while so we'll be locking this thread if there is no update within 24 hours. If you need assistance or want to discuss topics, feel free to start a new thread.

Regards,
Jenelyn

[MikeFitz]

I plan to dig out the multimeter and follow the earlier recommendations to see if I can find a C wire and connect it.  I will need to uninstall the power connector to start with a clean slate.

[Jenelyn_O]

Hey MikeFitz,

That sounds like a plan! Please keep us posted. For your safety, make sure to turn off your breaker before touching any wires.

Best,
Jenelyn

[Jenelyn_O]

Hi there,

It's me again! I wanted to follow up if you were able to proceed following the previous recommendation. Please let us know if you still need help. 

Regards,
Jenelyn

[Markjosephp]

Hi MikeFitz,

It has been a few days since your last reply, and we're going to lock the thread within 24 hours. If you have any questions, please feel free to create a new post.

Regards,

Mark