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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8389
Experience:  Furnace, A/C and heat pump specialist.
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I just installed the Nest Tstat in new house I just purchased.

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I just installed the Nest Tstat in new house I just purchased. The HVAC system is 8 year old Amana. I have 5 wires Red, white, yellow, green, blue. The system is working fine but I did not hook up the blue wire. Is this the C - common wire? When I did hook it up to the C terminal the system says there is no power to system and will not turn on fan. What say you? Do I hook it to something else or is the Nest messing it up? Red - Rc, green- G, yellow - Y1, white-W1, blue?


Hello. My name is XXXXX XXXXX I'll do my best to help. Troubleshooting often requires meters and test equipment. Let's get started and see what we can do.

Lets open the furnace and look at the control board. Tell me what the five wires each connect to on the furnace board.We will go from here.

Customer: replied 3 years ago.
Ok so yellow to Y, white to W, red to R, green to G then the blue is not connected to anything. There is a separate set of Red and white. The red to the Y and the white to the C. That is it.
Ok, connect that blue to C in the furnace and C on the nest. That C is Common.The Y and C there are sending 24 volts to the outdoor unit.
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Customer: replied 3 years ago.
Ok so I connected both and the system is now working. Why would it shut it off if it was not connected at the HVAC? and why did the installer not connect in first place? It was obviously done on purpose.

The C (Common) wire is optional on many thermostats but mandatory with the NEST.

Some thermostats using AA batteries do not need a common as the batteries are replaceable and provide power source,. However the NEST requires a 24 volt AC power supply. That is obtained through the R wire and the C wire.

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