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Old thermostat connection

Furnace Circuit Board

In search of help with hooking up a Nest Thermostat E. I have seen several threads seeking solutions for similar issues and have troubleshooted with their proposed solutions with no luck.

I am replacing an older model Honeywell Thermostat which was battery powered (Model: TH4110D1007) with a Nest Thermostat E. The Honeywell did not have a c-wire connected, and used a set of AA batteries for power. The Honeywell had only red, yellow, green, and white wires connected.

After pulling off the Honeywell thermostat I located an unused blue wire behind the wall plate which runs to my Rheem furnace (installed in 2006). I connected the previously unused blue wire to the C terminal at the furnace circuit board and at the thermostat, however it is not powering up the new or old thermostat (without batteries and with c-wire now connected).

I have attached photos of the thermostat hook-up and furnace circuit board. Again for clarification, the blue (c-wire) was previously unused, all I did was hook it up to the furnace and thermostat c terminals. Additionally, the extra red and white wires at the furnace run to my A/C unit.

Seeking any possible solutions or for anyone to tell me what I may have done wrong to not be receiving power.

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  • Do you have a multimeter? – ThreePhaseEel Dec 24 '20 at 16:19
  • I had similar problem. Thermostat refused to work only with AC power from furnace. After installing batteries it works. Probably it designed this way. – user263983 Dec 24 '20 at 17:09
  • I do have a multimeter – JMB Dec 24 '20 at 17:10
  • @JMB -- can you measure the AC voltage between R and C at both the screw terminals on the furnace and the screw terminals on the thermostat base? – ThreePhaseEel Dec 24 '20 at 21:38
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It looks to me as if you have shoved the insulation of the blue wire down under the screw terminal at the furnace end. This could be keeping the internal copper conductor of the wire from even contacting the screw terminal block.

enter image description here

You may have done the very same thing at the thermostat end of the blue wire. Note how in the below picture the adjacent red wire shows the center conductor as a small gap proving that it has been stripped of insulation. Did you forget to even strip the insulation off the blue wire?

enter image description here

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