1

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.

Improve this question
1
  • 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
1

I eventually got access to the other end and discovered that Daniel was indeed correct -- these were unwired.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.