Specification

A c-wire (also referred to as a common wire) is a low voltage power wire that many
Wi-Fi thermostats require for their features to work. A lot of homes don’t have a
c-wire because older thermostats didn’t require one. The Sensi Wi-Fi Programmable
Thermostat is one of the few Wi-Fi options that does not require a c-wire in most
cases, making installation quick and easy. However, if you have a C-Wire, it is
recommended that you connect it.
What is a C-Wire?
Sensi was designed with low power consumption in mind, so it works in the majority of combined heating and cooling system homes
even without a c-wire. With Sensi, two AA batteries (included) provide backup power that can last for months in most cases.
Refer to the table to check compatibility and see if a c-wire is required for your system:
Do I Need A C-Wire To Use Sensi?
How Do I Know If I Have A C-Wire?
To determine if you have a c-wire, remove your old thermostat's cover plate.
You should see several wire terminals labeled with letters. If a wire is connected
to the "C" terminal, you have a c-wire. Some manufacturers use the letters “B”
or “X” for a common connection. If you see a “B” or “X” the thermostat
manufacturer can help you determine if it’s a common connection. If you
have a C-Wire, it is recommended that you connect it.
K
Rc
RYC
O/B
GAL
K
Rc
W2Y2
1. Extra, unused wire - Check to see if there is an extra, unused wire behind the old thermostat. If there is an extra wire, it can be
attached to the “C” terminal on the system, and the “C” terminal on the thermostat.
2. A new wire – Run a new wire from the heating or cooling system to the thermostat. Attach the wire to the “C” terminal on the system
and the “C” terminal on the thermostat. Thermostat wire is 18 gauge solid copper wire available at most hardware stores.
3. Repurpose the fan connection (“G” wire) - On heat-only or cool-only systems that have a fan connection (“G” wire) it can usually be
repurposed as a c-wire (common). When repurposing “G” as a c-wire, the “Fan On” thermostat fan on function will no longer operate
but heating or cooling will operate normally.
• For cooling systems (thermostats with “R” or “RC”, “Y” and “G” connections) attach the “G” wire to the “C” terminal on the system
and the “C” terminal on the thermostat. Add a jumper between the “G” and “Y” terminals on the system to turn on the fan and the
compressor at the same time.
• For electric heat systems (thermostats with “R” or “RH”, “W” and “G” connections) attach the “G” wire to the “C” terminal on the
system and the “C” terminal on the thermostat. Add a jumper between the “G” and “W” terminals on the system to turn on the fan
and electric heat at the same time.
• For gas heat (thermostats with “R” or “RH”,”W” and “G” connections) attach the “G” wire to the “C” terminal on the system and the
“C” terminal on the thermostat. Gas heating systems have a fan control to operate the blower after the heating starts so no jumper
should be required between the “G” and “W” terminals on the system.
Can I Add A C-Wire To My System To Use the Sensi thermostat?
If you do not have a c-wire, there are several options to add one. If you would like to add a c-wire yourself we
recommend that you contact a professional contractor to discuss the following options:
Smart Comfort.
Smarter Savings.
Only from Emerson.
Heat Pump
Line Voltage or Electric Baseboard Heating
Millivolt Heating
Proprietary or Communicating
System Type
Is Sensi Thermostat Compatible?
Heat-only
YES*
Cool-only
YES*
YES*
No
No
No
*Requires a c-wire
Sensi thermostat is designed to work on the
following heating and cooling systems
Gas, Oil, Electric, Forced Hot Air Systems
Heat Pumps (Air Source or Geothermal, Electric or Dual
Fuel Gas / Electric*)
Hot Water Boilers
Radiant Heat Systems
Up to 4 stages of Heating, and 2 stages of Cooling
Heat-Only, Cool-Only and Heat Pump systems require a
c-wire (common wire) connection to work with Sensi.
* The Sensi thermostat provides automatic dual fuel
control without an outdoor sensor. A fossil fuel kit
may be added if user selectable balance point settings
are desired.
Heating (up to four stages) and Cooling (up to two stages)
YES