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I'm looking to upgrade the thermostats in my apartment to ones which I can control programmatically over WiFi.

I opened up one of the old thermostats and discovered I have the classic "C-wire" problem.

Digging around behind the wall I discovered two twisted pairs coming out of the same jacket as the other thermostat wires (blue and white, brown and white).

Hoping to identify the C wire I busted out the oscilloscope and measured the voltage from the R wire to each of the four unused wires.

Oscilloscope Trace

In all cases the voltage was 36V pk/pk and 17V RMS. Neither of these values is the 24V I was expecting for the C wire.

Is my measurement methodology incorrect? Are these the expected results? Can I purchase and install smart thermostats using any of these three links as the C wire? If not C wires, what are these?

Unfortunately I do not have access to the furnace end of the wiring.

  • Beware these voltage measurements: they may be high impedance, and the measured voltage may go away when a real load is applied. The asymmetry of the voltage curve implies something strange is going on; few self-respecting AC voltages go from +34V to -1.2V. – Daniel Griscom Jun 13 '16 at 1:55
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I eventually got access to the other end and discovered that Daniel was indeed correct -- these were unwired.

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