Changing from a Honeywell THM5320R to a Nest Learning Thermostat (3rd Gen)

[swhitney617]

Hi all, I'd appreciate some guidance on moving from a Honeywell THM5320R to a Nest Learning Thermostat (3rd Gen). I tried switching today, but wasn't quite able to figure out the wiring.

Thanks so much in advance for any help!

A little bit about my setup:

HVAC Units

I currently have an upstairs furnace connected to an AC unit outside. The wiring from the furnace and the AC unit both go into a panel mounted on the furnace itself, which then connects (I think via Bluetooth) to a thermostat in a different room.

Old Wiring

There are two sets of wires going into the current thermostat unit attached to the furnace (images below):

Bundle 1 (seems to be coming from outside from the AC unit):

1. Red1 connecting to C

2. White1 connecting to Y

Bundle 2 (seems to connect to the furnace):

1. Blue connecting to C

2. Red2 connecting to R

3. White2 connecting to W

4. Green connecting to G

5. Yellow connecting to Y

New Wiring

I've tried a few different configurations on the Nest with limited success. I'm a little confused, primarily because in my old configuration, there are two wires going into C (blue and red) and two going into Y (white and yellow). I've done a fair amount of googling, and haven't been able to figure out why that is.

Attempt 1:

1. Blue connecting to C

2. Green connecting to G

3. Red1 connecting to Rc

4. Red2 connecting to Rh

5. White1 connecting to Y1

6. White2 connecting to W1

7. Yellow connecting to Y2

Result of attempt 1: The thermostat UI told me that it couldn't detect power in Rc or Rh

Attempt 2 (swapped the red wires):

1. Blue connecting to C

2. Green connecting to G

3. Red1 connecting to Rh

4. Red2 connecting to Rc

5. White1 connecting to Y1

6. White2 connecting to W1

7. Yellow connecting to Y2

Result of attempt 2: I was able to get the furnace fan to run, but the AC unit outside wasn't running, and the app said that the AC was running in cooling stage 2

Attempt 3 (swapped White 1 and Yellow):

1. Blue connecting to C

2. Green connecting to G

3. Red1 connecting to Rh

4. Red2 connecting to Rc

5. White1 connecting to Y2

6. White2 connecting to W1

7. Yellow connecting to Y1

Result of attempt 3: The furnace fan was still able to run, but the AC unit outside still wasn't running. I think the app said it was just cooling this time (and didn't say the 'stage 2' part).

Troubleshooting

I've done all of the basic troubleshooting, including making sure all of the wire connections are tight, making sure the app is set to Cool mode and the target temp is below the current temp, etc.

Reverting back

While I'm figuring this out, I decided to switch back to my old thermostat, and put all of the wires back in their original configurations. Unfortunately, the AC unit still isn't running, so now I'm worried I messed something up when trying to install the Nest thermostat

Old thermostat images

(before I messed with any wiring)

https://imgur.com/a/Kdq0huS

[alejandom]

Hi @swhitney617,

Thanks for reaching out to the community about your Nest thermostat installation. I understand you've already tried some troubleshooting steps, and I appreciate your effort!

Based on your description of the wiring, here's a possible configuration for your Nest Learning Thermostat (3rd Gen):

• Blue wire to C (Nest combines these terminals)
• Green wire to G
• Red1 wire (from outside AC) to Y (Nest combines these terminals)
• Red2 wire (from furnace) to Rh
• White1 wire (from outside AC) to Y (Nest combines these terminals, likely with Red1)
• White2 wire (from furnace) to W

Here are some additional steps that might help:

• Double-check the wiring: Make sure all the wires are securely fastened to the correct Nest terminals according to the configuration above.
• Power cycle your HVAC system: Turn off the power to your HVAC system at the breaker box for a few minutes, then turn it back on and wait for the system to fully restart.
• Verify Nest app settings: Ensure your Nest app is set to "Cool" mode and the target temperature is lower than the current room temperature.
• Check for a C wire: The Nest might require a separate "C" wire for continuous power. If your current setup doesn't have a dedicated C wire (just the jumper wires), you might need to consult a qualified electrician to add one, or explore alternative Nest power options (if available for your model).

Regards,

Mario.