topic Re: No power at all. in Nest Thermostats

No power at all.

Hhipal — Tue, 12 Oct 2021 23:05:15 GMT

I installed my new learning nest thermostat today. Followed instructions exactly. Simple setup. Only 5 wires. Red, white, yellow, green, blue (common). No power after restarting breaker. No blinking lights or anything. Just black.

checked all connections. All secure.

system is a one year old Bryant a/c with natural gas heat.

Online google support said system is compatible. Recommended nest install pro. But this seems so simple.

Re: No power at all.

Ashepherdson — Tue, 12 Oct 2021 23:15:36 GMT

Do you have a multimeter to test the wires to confirm they are indeed live? Be sure to check as I know my furnace has a secondary breaker. You can plug the nest thermostat into your computer via the usb port as well, just to test the unit itself.

Re: No power at all.

Hhipal — Tue, 12 Oct 2021 23:34:00 GMT

Yes. Did that 28v when checking red and white wire.

Re: No power at all.

Hhipal — Tue, 12 Oct 2021 23:41:41 GMT

Original Honeywell wiring.

Re: No power at all.

Hhipal — Tue, 12 Oct 2021 23:42:43 GMT

Nest wiring.

Re: No power at all.

Ashepherdson — Tue, 12 Oct 2021 23:52:12 GMT

Can you plug the nest thermostat into your computer and see if it powers up?

Re: No power at all.

Hhipal — Wed, 13 Oct 2021 00:13:26 GMT

Next step.

Re: No power at all.

Hhipal — Wed, 13 Oct 2021 00:51:41 GMT

Plugged in via USB and unit powered up. I have blinking green light. I'll try to reinstall tomorrow and see if it powers up.

Re: No power at all.

Hhipal — Wed, 13 Oct 2021 00:52:13 GMT

Plugged in via USB and unit powered up. I have blinking green light. I'll try to reinstall tomorrow and see if it powers up.

Re: No power at all.

Hhipal — Thu, 14 Oct 2021 01:21:05 GMT

I have it working, thank you. I wanted to give you feedback because it may just happen to someone else.

The thermostat's internal battery was not energized by the red power wire. But, it did get charged up charging it up with a USB cable. Once it was sufficiently charged, it detected 5 wires - power, gas burner, A/C, fan, and common. Once it did that, it determined that there was no power on the common wire (blue). I said to myself, self what is that common wire for anyway? I went to the air handler to check it out. I notice that there was a splice of low voltage wires before entering the air handler. At that splice, all of the other colored wires were married up. The blue wire was a not connected, meaning it was doing nothing. I suppose the Hvac tech that put in that Honeywell thermostat connected the blue wire to common, just for grins. As soon as I disconnected it from the Nest thermostat, the thermostat functioned perfectly.

The lesson here was that the thermostat would not power on (to prevent damage to the device due to faulty wiring I presume) because it thought the blue common wire should have been energized. But, after charging with USB, it gave me the E79 error that led me to chase down the blue wire's purpose (none). Simply disconnecting the blue wire resolved the issue. If the installer of the original Honeywell thermostat had not connected that unused blue wire, the Nest thermostat would have gone in with no trouble.

Just thought I would share. Maybe someone else will have a similar experience.

Jaime