Multiple c wires

[Robbiesam]

I have a natural gas furnace (converted from oil years ago) with an old honeywell mercury thermostat that I was hoping to replace with a nest. The old thermostat has wires labeled R, W, and 2 wires labeled C.

Using a multimeter when I connect the R and orange C wires it reads 24V (same as R and W) so I interpreted that as C wire and connected nest with those 3 wires but Nest says it does not detect power in c wire. 

Interestingly when I test the other C wire with R it measures 0 but with W it also measures 24 V. I also tried that in nest as C wire in case but same no power error.

I cant seem to find anything online to explain the 2 C wire setup or how I can correctly setup to nest. The furnace connections are a mess and I cant make any sense or locate a labeled board. 

I was able to setup nest with only 2 wires but would love to have C wire function. Any help is greatly appreciated.

[Patrick_Caezza · Accepted solution]

I found a manual on the thermostat and it looks like when both C wires are being used, they are for an external 24VAC transformer. So if you measure voltage between them you should have 24Vac.

It might be for supplying power to the thermostat only.

Note -  I am correct and the two C terminals are for powering the thermostat timer. See page 7 of the linked manual

Chronotherm Fuel Saver Thermostat and Subbase 

With all of that in mind, you can try and set it up as a dual transformer system.

If you try this wiring, You do so on your own and I am not responsible for any negative results.

Orange - C to Rc

Green - C to C

Blue - R to Rh

Red - W to W1

[MelbaDT]

Hey folks, 

Thanks again for your suggestion, Patrick_Caezza. 

Robbiesam, I'm jumping in to check if everything is covered here. If you're not comfortable with Patrick's suggestion above, please contact a local HVAC technician or a local pro. 

Best, 

Melba

[Robbiesam]

Thank so much Patrick. I tried this and I believe it is now working properly.

Really appreciate the help!

[Ryan_G]

Hey Robbiesam,

Thanks for the update and it's great to hear that it is up and running now. We'd love to know how it goes for the next couple of days and let us know how it goes by replying to this thread.

Thanks for always helping us, Patrick_Caezza.

Best regards,

Ryan

[Ryan_G]

Hey Robbiesam,

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

[MelbaDT]

Hi, 

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

Best, 

Melba