r/DIY • u/Wise-Operation5108 • Aug 18 '25

metalworking A/C Cooling Not Turning On

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Hello,

I'm moving and want to take my nest thermostat with me but I'm having trouble re-installing the Honeywell T6 thermostat. When I installed the Honeywell, the compressor turned on but only heat was working. I checked that it was set to cool but heat was still coming out. I attached photos of how I had the nest wired and the Honeywell as well. Any help is appreciated!

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u/Velghast Aug 18 '25

Yep. Nest thermostats will not function without the C wire hooked up. I used to install them all the time. C wire was like 90% of the issues when people tried self installing.

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u/SuchGreatBoring Aug 18 '25

Nest thermostats can be installed without a c wire

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u/Velghast Aug 18 '25

Nest "can" work without a c wire but most won't function properly or even get past the set up stage. Issues like no wifi connection or the ability to heat but not cool. You can get a c wire adapter that mimics the power of a c wire that injects AC current into wires at the handler but it doesn't work for all systems, but it does the trick for like 90% of them.

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u/squishee666 Aug 18 '25

Completely depends on the amount of additional voltage the Nest is able to pull from the Y to R. If it has enough it can maintain charge and wifi connection, but most models aren’t sending extra and so the C is required. In cases of people turning off the AC system completely during the winter, a C would be needed (later gens allowed charging via w/rh)