Weird furnace wiring

[obsanity]

I'm trying to install Nest thermostat at my aunt's place. The furnace has a weird wiring situation (see picture). It appears that the common wire is connected to the outside compressor (white) and the blue wire is going to the thermostat upstairs. Not exactly sure how cooling can work this way but it does kick in and work. Any idea on how to wire this to Nest thermostat?

 

[Patrick_Caezza]

@obsanity 

Post a picture of the original thermostat wiring.

How the outdoor unit is wired is common for old systems like yours and shouldn't be an issue.

[obsanity]

I can't believe there is no option to post a follow up picture here. Here is a link to photos I took:

https://photos.app.goo.gl/qgNghp7PmGFGMZcC6 

From what I can see on the existing thermostat:

* RC and R are bridged and are connected to the red wire

* W is connected to the white wire

* G is connected to green wire

* Y is connected to blue wire

The blue wire is what seems to be connected to the AC compressor outside which also has a white wire fed into the common connection on the furnace. I'm a bit worried that I will not have the ability to connect the common wire from the furnace to Nest as I know that can create problems. But with this setup, I'm not sure why the Y connection on furnace is not used. 

Thanks for your help

[obsanity]

Forgot to tag you @Patrick_Caezza

[obsanity]

@Patrick_Caezza

 

 

 

 

[Patrick_Caezza]

@obsanity 

Try this wiring.

White - W to W

Red - RC to R (jumper to RH not needed)

Green - G to G

Blue - Y to Y

• Nest thermostats are designed to work without a common ("C") wire in most homes. During installation, a small number of customers may find that the Nest Thermostat does not receive sufficient power. The Nest Power Connector or a C wire may be necessary. See g.co/nest/cwire for details.

[obsanity]

@Patrick_Caezza 

Thanks. I was planning on doing just that but wasn't sure.

But I thought there was an issue with the C wire where some Lennox furnaces have had failure because of Nest hookup?

[Patrick_Caezza]

@obsanity 

Not that I know of.

[obsanity]

@Patrick_Caezza 

So I connected as you suggested and heat works but air only kicks on the fan. The compressor outside does not come on. I switched it back to the old thermostat and the compressor outside turned on. I tried the option in Nest for fan activation between activate and don't activate and nothing makes any difference. Any suggestions?

[obsanity]

Here is the hookup picture:

[Patrick_Caezza]

@obsanity 

It looks like you are going to need to install a C wire, and the easiest way is to use the Google Nest Power Connector.

If there is a C terminal in the furnace you can connect the power connector like this.

If there isn't a C terminal you will need to wire it like this.

[obsanity]

@Patrick_Caezza 

Thanks for replying. Since I do have a C connector in the furnace, could I just run another wire and connect it to Nest C connector and skip the Nest power connector? Even better, couldn't I just use the green wire in place of that and just reconnect that green wire to C connectors on furnace and Nest? I would assume the fan will kick in via the furnace command and I simply won't have the option to turn on the fan by itself with Nest.

[Patrick_Caezza]

@obsanity 

Yes, you can run another wire and connect it to the C terminal on each end. The Google Nest Power Connector was created because running another wire isn't always possible or cost-effective.

Repurposing the green (G) wire as a C wire is an old hack that can work if you don't mind limiting the operation of your HVAC system. I have never recommended or used it as I personally feel it is nothing but a bad hack that lazy techs use.

[obsanity]

Thank you sir. I will likely test it with Green wire to make sure it works and eventually run a new wire to get all the functionality. I'll come back and report if that worked for anyone else reading this.

[obsanity]

So I can confirm, the C wire connection using the green wire does turn on the compressor. However, because G fan connection was removed to do that, the fan does not turn on with the compressor. I imagine there might be jumper settings to kick on the fan on the furnace without Nest command but something tells me with this old type setup, the furnace doesn't actually know the compressor is running.

So I have no choice but to run a new wire so I can have the complete setup. Thanks again for your help @Patrick_Caezza 

[Patrick_Caezza]

@obsanity 

New wire or Power connector. Whichever you find easiest and cheapest.

[Jenelyn_O]

Hi folks,

Thanks for posting in the Google Nest Community forum. I'm sorry for the delay. I hope you get the answer you're looking for. Please let us know if you still have questions in mind, as we'll be willing to assist you further.

I appreciate your help, @Patrick_Caezza.

Best,

Jenelyn

[obsanity]

The solution suggested worked as expected. Just returning here to report. Thank you again for help @Patrick_Caezza 

[Jenelyn_O]

Hey obsanity,

I'm glad to hear that it is working just perfectly fine. Please feel free to create a new thread if you have any questions or concerns, as I'll be locking this thread after 24 hours. 

Cheers,

Jenelyn