With everything smart thermostats can do (scheduling, voice control, remote access, and remote temperature sensing), it’s obvious they’re a major upgrade over standard models. But, with the upgrade comes the need for constant 24-volt power with a common wire. When a homeowner couldn’t figure out how to wire his new smart thermostat, he called home technology expert Ross Trethewey for help.
Wire Requirements for Smart Thermostats
Smart thermostats have different requirements than standard models. These units require a 5-wire setup, with the red wire providing power, the white wire controlling heating, the yellow wire controlling cooling, the green wire controlling the fan, and the blue wire attached to the common.
The common wire — often blue — is the most critical wire for smart thermostat installations. As explained in a This Old House segment, the common wire provides a 24-volt power supply that lets the smart thermostat’s onboard computer actually function. Without it, the thermostat is stuck running on batteries, which drain quickly and can cause connectivity issues.
Common Wiring Challenges
Knowing that a five-wire setup is required, we can now identify some common challenges, pun intended. Older thermostats might’ve used fewer wires, as there wasn’t a need for a common wire. Also, those with dual transformers have two sets of wires running to the thermostat—one for the air handler and one for the heating unit. In this case, there are often extra wires in the box, and depending on the circumstance, one can be used for the common wire.
In both of these cases, you might need to start troubleshooting. Here’s what to do:
How to Troubleshoot Smart Thermostat Wiring
- Start by removing the thermostat from the wall and inspecting the wiring. Thermostats typically pull straight off their base, but some might need to be unscrewed.
- Identify your wiring situations. If you have five wires, each attached to a terminal and one of those terminals is the common (marked “C”), you’ll be able to swap your thermostat over without any issues.
- If there are fewer wires:
a. Inspect the wires and their positioning. If there is a wire controlling the fan’s “auto/on” function, consider using this wire as the common. Take a picture with your smartphone.
b. Find the air handler and shut off the power.
c. Remove the unit’s cover to access the wiring. Check the unit to ensure there is no power with the voltage tester.
d. Inspect the wiring diagram on the inside of the unit to determine which wire should control the fan. If it is the same color as the wire in the thermostat, you may use this wire as the common. Remove the wire from the terminal and label it “Common.” Do not reconnect it yet, and leave the power off.
e. At the thermostat, remove the wire from the fan control and label it as “Common.”
f. Install the new smart thermostat base and connect all of the wires to their appropriate terminals, including the new common wire.
g. Return to the air handler and connect the new common wire to the common terminal.
h. Replace the cover and turn the power on.
i. Program your smart thermostat. - If there are more wires:
a. Inspect the extra wires and take a picture.
b. Find the air handler and shut off the power.
c. Remove the unit’s cover to access the wiring. Check the unit to ensure there is no power with the voltage tester.
d. Inspect the wiring inside the air handler. If there are extra wires that match the extra wires in the thermostat, you might be able to use one of these wires as the common.
e. Return to the thermostat. Use a wire nut to connect two of the extra wires.
f. Return to the air handler. With the multimeter set to the continuity function, place the test leads on the two wires you connected at the thermostat.
g. If the multimeter beeps, that means there is continuity and one of these wires can be used as the common. If it doesn’t, you’ll need to call a professional.
h. Identify which wire you want to use as the common. Label it as “Common.”
i. Return to the thermostat’s location. Disconnect the wires and label the new common wire.
j. Install the new smart thermostat base and connect all of the wires to their appropriate terminals, including the new common wire.
k. Return to the air handler and connect the new common wire to the common terminal.
l. Replace the cover and turn the power on.
m. Program your smart thermostat.
When checking continuity on spare wires, set your multimeter to the continuity function and touch the probes together first — you should hear a beep confirming a closed loop. Then place the probes across the two spare wires you bundled together at the air handler. If the display reads “OL” (open loop) and you hear no beep, that means the wire is spliced somewhere between the thermostat and the air handler, and the spare conductors were never connected at the splice point. In that case, you won’t be able to use those extra wires as your common.
If the continuity test fails and you can’t use the spare wires, you have a few options. Running a new thermostat wire from the thermostat location to the air handler is one solution, but it can be challenging — especially in multi-story homes where you need to fish wire through multiple wall cavities. A more practical approach is to repurpose an existing wire, such as the fan wire, and reassign it as the common. On a typical seven-conductor, 18-gauge thermostat cable, the fan’s “auto/on” function wire can often be sacrificed for this purpose since many smart thermostats can control the fan through the system’s programming.
What If Neither Step Helps?
If you still don’t have enough wires, or you can’t find an extra wire with continuity, you’ll need to call for professional help.
Resources
To install a smart thermostat, you’ll need electrical tape, hand-held drill, pliers, screwdriver, and extra twist-on connectors.
Use a multimeter to check for wiring continuity.
Use foil tape to reseal the face of the air handler—which was manufactured by Ecobee.
Thermostat Placement Matters: If your smart thermostat installation involves relocating the unit, keep a few rules in mind: avoid mounting it on an outside wall, where cold air infiltration through the wall can throw off readings. Don’t place it where direct sunlight could fool or overheat the sensor. A central interior location, mounted about 60 inches off the floor, is ideal.
