C terminal voltage on AC control panel

Question

My smart thermostat shut itself down reportedly (as it ran some time on internal chargeable battery and I browsed through the settings) because the C terminal stopped receiving 24V power supply. That terminal is wired from the C terminal in the control panel of the AC, which also has R, Y, G etc. I used a multimeter to test the AC side terminal by putting one probe on it and the other one on the grounding terminal inside the unit and it measured 0V. When I did the same thing with the R terminal, it measured 24V, which is what it should be.

In order to restore power supply to the thermostat, I tried to hardwire the C terminal in the AC to the R terminal but that burned the internal fuse that the R was connected to, which makes me think that a short occurred, which means the C terminal was actually energized. Another thing I found weird is that the C terminal is supplied by a wire that runs through the blower motor. Why would they wire it like that?

How can I test whether the C wire issue is on the source (AC) or in the thermostat itself. I am not sure where to put the other probe when testing the C wire on the thermostat, like I did on the grounding nut on the AC side.

Answer 1

Background

Voltage is a measurement between two points. It's a misunderstanding, and a common one, that the C wire "supplies 24V" to the thermostat. The 24V supply from your furnace or A/C is between the R and C terminals or wires. The R wire always goes to the thermostat. The other wires (Y, G, W, and so on) cannot serve as the other terminal for a 24V supply to the thermostat because they do other things. You need the C wire so that you have 24V between two wires (R and C) at all times, to power the thermostat.

Your tests so far

All your tests, as you describe them, show normal results.

24V from R to ground, 0V from C to ground is normal. You can't rely on that because sometimes the 24V side is isolated. But if you do see that, it is normal. If you connect C to R, as you did, you are short circuiting the 24V power supply and blowing a fuse is the least you should expect. Normal. The blower motor control circuit (the 24V side of it) should be connected to G and C so yes, there could be a C wire going to it. That's how the blower is controlled.

It sounds like everything you checked at the air handler is normal.

Make sure you actually have a C wire to the stat

What you need to figure out next is why there is not 24V between R and C at the thermostat. Maybe the C wire is broken somewhere? Or you don't really have a C wire going from the air handler to the thermostat? It should be very straight forward: one of the wires connected to "C" in the blower and to the C terminal in the thermostat.

Try to clearly identify a C wire connected correctly at both ends. Post pictures of both ends if you want, clearly showing all the thermostat wires, where they are connected, and also a clear picture of the wiring schematic from the case or door of the unit.

Check if the C wire is broken

If the C wire is correctly connected, you can test if it's broken by borrowing one of the other wires, say R, and doing a continuity test. Disconnect the R and C wires at both ends. At one end, twist them together. At the other end, test continuity between them. If you don't have continuity, one of them is broken. Or you didn't twist them very well. I'll expand on this if necessary. It's rare.