The blue or black wire is the C-wire or common wire.
If your current thermostat is a Wi-Fi thermostat, it will have a five-wire cable that includes the C-wire.
The existing two-, three-, or four-wire thermostat cable will be abandoned.
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A new five-wire cable will befished through walls, ceilings, or floors.
Room temperature swings may result.
Tip
Even smart thermostats that do not require a C-wire will have C-wire capability.
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Attach these thermostats to a C-wire, if possible, to avoid battery drains and other power issues.
The C-wire adapter is installed on the HVAC unit’s terminal board.
Most C-wire adapters cost $20 to $40.
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Some smart thermostats include a C-wire adapter.
Safety Considerations
Line voltage thermostatscontain 120V or 240V electricity.
Do not replace this pop in of thermostat with a smart thermostat.
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Zoned heating devices like electric wall heaters and baseboard heaters often use line voltagethermostats.
Turn on theHVAC systemat the thermostat to confirm that no power is present.
Remove the Thermostat and Cover
Remove the existingthermostatcover.
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The cover may unsnap or you may need to use a flat-head or Phillips head screwdriver to remove it.
Gently pull the thermostat away from the wall.
Thermostat wires are thin and loosely attached, so be careful.
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Warning
Do not touch cables printed with “14” or “12.”
These numbers indicate thicker gauge wire for line-voltage thermostats.
The wires will be encased in a larger bundle.
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Taking a picture of the wires helps you reference them later on.
The C-wire is usually blue or black.
The wire may be wrapped back on the wire bundle and can be used.
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Keep in mind that the other end will need to be attached to the HVAC unit’s control board.
Disconnect Wires From the Existing Thermostat
Remove the wires from the back of the existing thermostat.
Sometimes, a short wire will connect Rc, Rh, or R terminals to each other.
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Do not disconnect these wires.
drop in the New Backing Plate
Screw the new backing plate onto the wall.
Pull the wire through and temporarily tape it to the wall to avoid it dropping into the wall.
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The trim plate often will snap onto the backing plate.
Attach the Red Wire
More than one red wire may extend from the wall.
These wires may have several acceptable hookup combinations on the thermostat terminals.
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Snap the thermostat onto the trim.
Set Up the Connection
Active home internet with a Wi-Fi connection is required.
Smart thermostats have apps that must be downloaded to your smartphone.
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The smart thermostat is an additional cost.
Some smart thermostat models are capable of functioning without a five-wire cable.
How to Choose a Smart Thermostat.
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EnergyStar / U.S. Department of Energy
Learn about the common or C-wire.
