WiremanWireman · Thermostat wiring and a missing C wire: troubleshooting for electricians · 2023 NEC ·
Thermostat wiring and a missing C wire
What the C wire is, how to test R to C, and the options when there’s none.
Before you start
How much electrical experience do you have?
This sets how much detail each step shows. You can change it any time.
Showing detail
How it works
The thermostat runs on the 24 V AC control transformer in the furnace or air handler. R is the 24 V “hot” and C is the common, the other side of that transformer. Old thermostats only switched R to the other wires and didn’t need power, but smart thermostats need R and C to run all the time. Without C some try to steal power through the heating or cooling wires, which can make the system cycle or the thermostat reboot.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What’s happening?
- Smart thermostat says it needs a C wire or keeps rebootingAt the thermostat, are there any unused wires folded back in the cable bundle?
- Yes, there’s a spareTest: Find the spare at the furnace or air handler
- Spare found, board has CTest: Measure R to C at the thermostatExpect: About 24 to 28 V AC.
- About 24 to 28 V ACC wire connected
- 0 VNo 24 V supply
- The spare isn’t at the furnace (goes to the condenser or is cut)No spare conductor for C
- Spare found, board has CTest: Measure R to C at the thermostatExpect: About 24 to 28 V AC.
- No spare wireNo spare conductor for C
- Yes, there’s a spareTest: Find the spare at the furnace or air handler
- Blank screen, HVAC deadTest: Measure R to C at the thermostatExpect: About 24 to 28 V AC.
- About 24 to 28 V ACC wire connected
- 0 VNo 24 V supply
- HVAC turns on and off by itselfPower stealing without a C wire
- Smart thermostat says it needs a C wire or keeps rebootingAt the thermostat, are there any unused wires folded back in the cable bundle?