can I test thermostat with only C and R wires connected ?

[jge1]

I just got a Nest Thermostat E, and wanted to have a look at the settings before I install it.   So I simply connected a 24vac source (a wall wart) to the R and C terminals.   It let me go through a bunch of the setup stuff (internet connection, location, etc) but at some point gave me an E194 code, "no power wire detected, R wire required".

Is there something wrong with the thermostat, or is it simply confused because there is nothing connected to any of the other terminals ?

[jge1 · Accepted solution]

I am a dope. The instructions for the thermostat state clearly that it is not compatible with stranded wires. I assumed that was just a simple way of ruling out incompatible systems, since 24vac thermostats are invariably wired with solid 18awg wire. On my bench hookup, I used the stranded wires coming out of the 24vac wall wart. Evidently they weren't connecting properly (into the spring terminals in the Nest thermostat base), hence the error message about no power. I knew I was on to something when I inserted solid wires and the spring levers did not return to their original position, the way they did with the stranded wires. It still seems odd, since the stranded wires were firmly gripped - they resisted pulling out.

Post Your AnswerDiscard

[MelbaDT]

Hey jge1, 

Thanks for reaching out and sorry for the delayed response. 

We also appreciate that  you've taken the time to keep the thread updated on what you've done to resolve your own concern. Give yourself a pat on the back!

Feel free to let us know if you have further questions. 

Cheers, 

Melba

[Ryan_G]

Hey folks,

I just wanted to follow up and see if you're still in need of help. Please let us know as we would be happy to answer any questions you may have. 

Best regards,

Ryan

[jge1]

Thanks, I am satisfied on answers to this question.   But no one has addressed the followup question I asked about connecting the 'W' wire: "connecting the W wire to a dual-fuel heat pump".

[Ryan_G]

Hi jge1,

Thanks for the reply.

For a duel-fuel heat pump, you can connect the W wire to your Nest Thermostat as long as you've selected the right equipment (heating type and source) from the settings.

I hope it helps.

Regards,

Ryan

[jge1]

That does help, Ryan.   But the other question is, there is a signal coming from the compressor that the compressor energizes if it goes into defrost mode.   I gather you're supposed to connect that to the W terminal on the furnace/air-handler as well (so the furnace knows to turn on during the defrost cycle).   But wouldn't there be an potential electrical conflict between the signal from the thermostat, and the signal from the compressor, both hooked to the same terminal on the furnace/air-handler ?

[Ryan_G]

Hi jge1,

Thanks for the reply.

For a dual-fuel or dual-transformers system, there are wires connected to the Rc and Rh and the Nest Thermostat will be able to determine and manage them at the same time. As long as you've selected the right wiring assignment from the equipment settings.

Regards,

Ryan

[Ryan_G]

Hey folks,

I just wanted to follow up and see if you're still in need of help. Please let us know as we would be happy to answer any questions you may have. 

Best regards,
Ryan
 

[Ryan_G]

Hey there,

Thanks for being here in the Google Nest Community. 

I'd like to bump in here and make sure that you’ve seen our response.
Please let us know if you have any other questions or concerns as I will be locking this thread in 24 hours.

Have a great day.

Best regards,
Ryan