Help with Honeywell t-stat wiring, when converting to Nest


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Old 11-10-14, 06:10 PM
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Help with Honeywell t-stat wiring, when converting to Nest

Hello, hoping you can help. I'm trying to convert Honeywell Chronotherm t-stat to Nest, and am confused about the wiring. The boiler is an old honeywell, and I attach three pictures of the upstairs t-stat wiring, and the downstairs t-stat wiring and boiler plate.

My confusion stems from there seemingly being two C wires (green and blue both have 'C' next to them), and yet only one C wire input on my Nest t-stat. And yet downstairs, it would appear that blue is C and green is R, and yet upstairs the white wire goes into R. Very confusing to me!

Ultimately, I'd like to know which wires on my honeywell t-stat should go into which inputs on my Nest?

Thanks
Simon

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Old 11-10-14, 06:54 PM
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Wow, that is a very old Chronotherm.

Rc = green
C = blue
Rh = white
W = red

The old C and C powered the clock.
If your Nest doesn't like the heating circuit swap red and white (T and T on the boiler).
 
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Old 11-11-14, 09:23 AM
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Yes, that worked perfectly - thank you!
 
 

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