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Our old thermostat is working but I want to change to a new one (nest thermostat). I have two exact HVAC unit at home. I change one it works but when I change the second one it doesn't work because it can't detect power.

How do I fix this problem? Where can it possibly go wrong? Do I need to check my furnace?

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With certain versions of the nest thermostat they require the C or common wire for operation. The nest thermostat needs 5 wires to accommodate the extra common wire. Check to see that how many wires the thermostat wire is. You may be lucky and have the extra wire tucked in behind the thermostat.

Another possible issue, if you do have 5 wires hooked up at the thermostat is the C wire may be disconnected at the furnace control board as it was not needed for the old thermostat. If this is the case you would have to open the furnace and add the C wire to the C terminal of the control board.

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This sounds to me like voltage drop over the thermostat wires is too great. I'm making several assumptions here so I may be wrong, but old style thermostats are often simple relays that do nothing more than open or close a given circuit. The mere opening / closing of that circuit is enough to trigger the HVAC, it's not dependent on a given voltage. The NEST thermostats need sufficient power from the thermostat wires to operate, which may not be present if the wire run is long enough from the point of origin. To test this theory, get a voltmeter and test voltages on the wires at your thermostat to see if they're within the indicated operating range of the NEST. If they're not, you'll need to re-run that line with larger gauge wires to reduce the voltage drop.

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    the voltage drop would depend not only on the size of the wire, but the amount of current being demanded. I wouldn't think the power consumption of the Nest would be high enough to cause appreciable voltage drop over any reasonably sized thermostat wire. I suspect a mis-wiring issue instead. – mac Sep 27 '13 at 16:54

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