Adding a 24 VAC external transformer: A DIY option for heat-only systems without a common wire
An easy option to add a common, or c-wire, is to install a 24VAC wall plug-in transformer into a nearby outlet.
Compatibility
Note that this is only an option for:
- Heat-only systems
Compatible Sensi models
- Sensi smart thermostat
- Sensi Touch smart thermostat
- Sensi Touch 2 smart thermostat
You cannot install an external transformer on Sensi Lite smart thermostats.
Transformer Information
Transformers with 150-400mA output are the most widely available online and easily provide sufficient power for the thermostat. Please note the wires from the wall plug-in transformer will be visible unless installed behind the wall.
If you purchase a transformer that is used to charge or power electronic devices, you will need to cut off the accessory plug (located at the end of the cord, opposite of the outlet plug) and split the two wires apart. Strip the cord around the two wires to expose a small amount of the copper wiring inside, which will be inserted into terminals on the Sensi thermostat base.
Wiring Instructions
External Transformer
- Insert one wire from the wall plug-in transformer into the “RC” terminal on the thermostat.
- Insert the other wire from the plug-in transformer into the “C” terminal on the thermostat.
Thermostat Wires
- For a heat-only system, you will have already wired your “RH” and “W/E” wires as per the Sensi app instructions.
- “RH” is the 24VAC power for the heating system.
- “W/E” is the first stage for heating.
Thermostat
- Cut the RH/RC jumper wire on the thermostat. See below pictures for where the RH/RC jumper wire is located by Sensi model.
- After wiring everything and cutting the jumper wire, plug in the transformer to a nearby wall outlet, and turn on the power to your HVAC system.
Note: Wire colors from the system may vary
Sensi smart thermostat
The RH/RC jumper wire is located on the back of the thermostat face plate. Do not cut this or use the wall plug-in transformer if you have combined heating and cooling systems or a cool-only system. This option is only for heat-only HVAC systems.
Sensi Touch smart thermostat
The RH/RC jumper wire is located on thermostat sub-base where the wires are inserted. Do not cut this or use the wall plug-in transformer if you have combined heating and cooling systems or a cool-only system. This option is only for heat-only HVAC systems.
Sensi Touch 2 smart thermostat
The RH/RC jumper wire is located on the back of the thermostat face plate. Do not cut this or use the wall plug-in transformer if you have combined heating and cooling systems or a cool-only system. This option is only for heat-only HVAC systems.
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