Connecting a 24V Transformer to the Nest Thermostat

[BernardJ]

Hi all.

I have a system with 4 wires (R; W; Y and G).
I have no C wire but I don't want to lose the fan control, so I want to connect a 24V transformer to the Nest Thermostat (that have only 6 connectors R,W,Y,G,C and OB).
One of the transformer's wires goes to the C, in the Nest but where do I connect the second wire???

Thanks,
Bernard.

[BernardJ · Accepted solution]

Sportfrog.

I REALLY appreciate your help - thanks A Lot.

I did see that video - it describes 2 wire system, that doesn't use the R on the thermostat.

That got me confused.

Now, that you confirmed the transformer uses the same R on the thermostat, all is clear.

I'll give it a try and see how it goes. 

I will make a note/sticker on the furnace, as you suggested.

Again - Thanks a lot  🙂

Bernard. 

[sporfrog]

There are plenty of articles and videos about doing just that out on the internet. I don't think it is a Nest/Google supported approach, but based on the information available it appears to work. The general idea is to replace the 24v from the furnace transformer with 24v from the external transformer. There could also be potential issues with the furnace as well. Maybe somebody who has actually done it will show up here and share their experience!

From Nest Power Connector:

Warning: Some 3rd party C wire adapters can cause problems or damage your Nest thermostat, even if they claim to be Nest compatible. We recommend using a Nest Power Connector to ensure compatibility and best performance with your thermostat.

The absolute best solution is to wire a C to the furnace or HVAC system. In some cases the cable with the thermostat wires has extra wires not in use - say a blue or black wire. Connect that to C at the furnace and C at the thermostat and done! In other cases running a new thermostat cable with 5 or even better 7 wires is possible. For me I was able to replace the cable fairly easily but that will obviously depend on the situation.

Google also offers a Nest Power Connector to solve low power issues where there is not a C wire.

Good luck. If you proceed with the external transformer share how it goes!

[BernardJ]

Thanks for the quick reply.
There are many  articles and videos and all are not for the Nest with the the 6 connectors :-(.
The ones the connect a 24V transformer, connect one wire to the C connection on the thermostat and the other one to the RC - but the Nest doesn't have the RC. It only has an R connector. 
So, that is my confusion... 😞

[sporfrog]

There is at least one video on YouTube for what you want to do for the "Nest Thermostat". In that video there are only two wires at the thermostat (R, W) but the principles likely are the same. It is basically sharing the R terminal. If you watch the entire video there is a suggestion to have an extra internal fuse for your furnace or HVAC system handy as it could blow. 

I am not a Google employee, just someone who likes Nest products and is trying to help folks get theirs working. I don't think I would personally try this, but the information is there should you want to give it a go. 

Also you may want to put some sort of label on the furnace as a reminder that there is a shared wired and external transformer for the thermostat. In the future, an HVAC technician will expect power at the thermostat to be off when power at the furnace is off.  With an external transformer that would not be the case.

[BernardJ · Accepted solution]

Sportfrog.

I REALLY appreciate your help - thanks A Lot.

I did see that video - it describes 2 wire system, that doesn't use the R on the thermostat.

That got me confused.

Now, that you confirmed the transformer uses the same R on the thermostat, all is clear.

I'll give it a try and see how it goes. 

I will make a note/sticker on the furnace, as you suggested.

Again - Thanks a lot  🙂

Bernard. 

[Patrick_Caezza · Accepted solution]

This is how you should wire it using the Google Nest Power Connector.

 

 

[BernardJ]

Thanks.

[MelbaDT]

Hey folks, 

Appreciate your detailed answers here, sporfrog and Patrick_Caezza. 

BernardJ, thanks for reaching out. I'm just dropping by to ensure everything is covered here. Feel free to let us know if you have more questions. 

Best, 

Melba

[BernardJ]

An update...

After the first transformer (24V 500mA)  "got" the error message, during the setup process, that the C wire doesn't "push" anything and after switching the 2 transformer wires from R to C over and over again, with the same result, I ordered 2 different transformers.
Neither worked and I couldn't finish the setup 😕.

Checked all with a multimeter - didn't read any current (which is weared for itself...).

In any case...
I ended up switching the G wire (at the HVAC unit) into the C position and did the same on the Nest Thermostat, "told" it I don't have a G wire and managed to finish the setup. 

Now everything works, expect for the fan control options...😐

[sporfrog]

Glad you found a solution.

If you are comfortable working inside the furnace, and you want your fan control back, consider the Nest Power Connector (NPC). It connects inside the furnace to the C terminal and sends more power along the wires you do have to keep the thermostat powered.

From the manual:

At the thermostat and HVAC system you would reconnect G wire back to G.

[BernardJ]

I thought the the Nest Power Connector simply does what I did - replacing the G for a C.
You are saying that it works differently so I will keep the fan control of my system = I can run only the fan to circulate the air and such????

[sporfrog]

The Nest Power Connector is designed to provide  power for a Thermostat when there isn't a dedicated C. I haven't used it personally, but yes that it what I am saying.  The wiring diagram I attached is if the HVAC system has a C terminal. You have said you do. 

Read through the Nest Power Connector. I think it will solve your problem and allow you to keep fan control.

[BernardJ]

Yes - There is a C terminal at the HVAC but it has no wire attached to it.
I will give it, that Nest Connector, a try.
Thanks 🙂

[MelbaDT]

Hi, 

I haven't heard from you in a while so I'll be locking this thread if there is no update within the day. Should you have any new issues, updates or just a discussion topic, feel free to start a new thread in the community.
 

Best, 

Melba

[MelbaDT]

Hey folks, 

Appreciate again your responses, sporfrog.

BernardJ, glad you found a fix and yes, give our Nest Power Connector a try so that it will act as your thermostat's common wire. Take a look at this link for more information about it: https://bit.ly/3z34z2e

Best, 

Melba

[MelbaDT]

Hey BernardJ, 

Checking back in just to ensure we've covered everything here. Let us know if you still have some questions. 

Best, 

Melba