Lite Smart Thermostat ST25 Series

Sensi Lite Smart Thermostat 1F76U-22WFB & ST25 Series

The Sensi Lite Smart Thermostat is a Wi-Fi-enabled thermostat that is installed and configured through the Sensi app. This guide applies to the ST25 series and 1F76U-22WFB series and walks through the full installation process, from preparing the existing HVAC wiring to mounting the thermostat and connecting it to a 2.4GHz Wi-Fi network.      

The thermostat uses two AAA batteries and can be configured for supported conventional heating and cooling systems, heat pumps, electric or gas furnaces, fan-only systems, selected multi-stage configurations and three-wire hydronic zone systems. The installation should be completed through the Sensi app so the correct wire terminals and HVAC settings can be identified for the system being connected.      

What Is Needed Before Installation

The official installation guide recommends gathering the required network information, tools and thermostat hardware before starting.   

The thermostat is compatible only with a 2.4GHz Wi-Fi network for the setup procedure described in the guide.      

Downloading the Sensi App

The Sensi Lite is installed and configured through the Sensi app.

Before beginning the physical thermostat installation:

1. Download the Sensi app to a smartphone or tablet.
2. Open the app.
3. Create a Sensi account.
4. Use a valid email address.
5. Create and remember a password.

The thermostat becomes linked to that email address after registration. The account is also required for remote access and the features available through Sensi.   

Add the Sensi Lite Thermostat to the App

After signing into the app:

1. Tap the plus (+) sign if there are no registered thermostats.
2. Choose the thermostat model.
3. Select Sensi Lite Smart Thermostat.

The Sensi Lite option applies to:

• ST25 series
• 1F76U-22WFB series

The model number can be checked on the back of the thermostat cover.   

New Installation vs. Currently Mounted

The Sensi app asks whether the thermostat is already mounted.

Not Mounted

Choose Not Mounted when performing a DIY installation.

This path walks through:

• Identifying the wires on the old thermostat
• Labeling them for Sensi
• Connecting the wires
• Configuring the thermostat
• Connecting to Wi-Fi

The guide specifically recommends this option for users who did not have the thermostat professionally installed.   

Is Currently Mounted

Choose Is Currently Mounted when the Sensi Lite has already been installed on the wall.

This option asks the user to identify the connected wires, checks the HVAC settings and then handles the Wi-Fi connection.

The guide recommends this path when a professional technician has already installed the thermostat.   

Turn Off HVAC Power Before Wiring

Before touching any thermostat wires, turn off power to the heating and air-conditioning system.   

This step comes before removing or disconnecting the existing thermostat wiring.

Remove the Existing Thermostat

Remove the Old Thermostat Cover

Depending on the existing thermostat, its cover may:

• Pull directly off
• Release through a pressure latch
• Require a screwdriver   

Photograph the Existing Wiring

Before disconnecting any wires, take a clear picture of the existing thermostat and its wiring.

The Sensi app prompts for this photo, and the image is saved to the phone’s camera roll or photo gallery. It can be useful later if the installation requires troubleshooting or technical support.   

Remove Old Jumper Wires

Any jumper wires from the previous thermostat should be removed.

The Sensi Lite has a built-in jumper, so jumper wires from the old thermostat are not required.   

Identify the Existing Wire Terminals

In the Sensi app, select every terminal on the old thermostat that currently has a wire attached.

The app checks the selected terminals and determines whether the wiring arrangement is a valid configuration.

If the app reports an invalid configuration, the official guide directs users to Sensi’s compatibility resources.   

Select the Furnace Type

The installation process also asks what type of HVAC system is being connected.

The correct furnace or indoor-equipment type should be selected in the app before the wiring is moved to the new Sensi base.   

Label the Thermostat Wires

Before disconnecting the wires from the existing thermostat:

1. Use the provided wire-label stickers.
2. Label each wire according to its existing terminal.
3. Label one wire at a time.
4. Disconnect the wires only after they have been identified and labeled.   

This helps make sure each wire is moved to the correct terminal on the Sensi Lite thermostat.

Remove the Old Thermostat Base

Once every wire has been labeled, disconnect the wires and remove the old thermostat base from the wall.   

Keep the labeled wires accessible so they can be connected to the new thermostat base.

Install the Sensi Lite Thermostat Base

Use the mounting screws included with the thermostat to secure the new Sensi base to the wall.   

After the base is firmly installed:

1. Match each labeled thermostat wire with its corresponding Sensi terminal.
2. Insert the wire into the correct terminal.
3. Tighten the terminal screw.
4. Check that each connection is secure.   

Sensi Lite Wiring Terminals

The wiring diagrams on page 10 show the thermostat terminals used for supported conventional and heat-pump installations.

The thermostat terminal strip includes:

The page 10 diagrams show the same thermostat terminal layout for both conventional systems and heat-pump systems, but the function of O/B and W/E changes according to the system configuration.      

Conventional HVAC Wiring

For conventional systems without a heat pump, the wiring diagram identifies:

• R — 24VAC power
• Y — first-stage cooling
• G — indoor blower/fan
• W/E — first-stage heat
• C — 24V common

The configurable O/B terminal can also be used for second-stage heat, second-stage cooling or a three-wire zone connection according to the installer configuration.   

Single-Stage Conventional Configuration

The diagram lists single-stage system configurations as:

• AC
• GA
• EL   

Multi-Stage Conventional Configuration

The guide supports a configuration for:

• 2H — second-stage heating
• 2C — second-stage cooling

The diagram states that the system can be configured for multi-stage heating or multi-stage cooling, but not both.   

Heat Pump Wiring

For heat-pump systems, page 10 shows:

• R — 24VAC power
• O/B — heat-pump reversing valve
• Y — first-stage heating and cooling compressor
• G — indoor blower/fan
• W/E — first-stage auxiliary/emergency heat, identified as second-stage heat
• C — 24V common   

The diagram states that multi-stage operation is not available for heat-pump configurations.   

Common Wire Requirements

The wiring diagrams include an important note that a common connection is required on heat-only, cool-only or heat-pump systems.      

Scan the Thermostat QR Code

After the thermostat base has been installed and wired, locate the QR code:

• On the back of the thermostat, or
• On the Welcome/Get Started insert included in the box.   

Scan the QR code through the setup process.

The Sensi app requires access to Bluetooth for the thermostat to connect properly during this step.   

If the QR Code Will Not Scan

Select Use PIN in the app and manually enter the code printed beneath the QR code.   

Install the AAA Batteries

Install the two provided AAA batteries in the thermostat before attaching the cover.

After the batteries are installed, push the thermostat cover onto the mounted base.   

Restore HVAC Power

Once the base is wired, batteries are installed and the thermostat cover is attached, turn the power to the heating and air-conditioning system back on.   

Connect the Sensi Lite Thermostat to Wi-Fi

Enter Wi-Fi Setup Mode

On the thermostat:

1. Press and hold the Mode button for 4 seconds.
2. Wait for the setup and Wi-Fi icons to appear.   

Select the Home Wi-Fi Network

In the Sensi app:

1. Select the home Wi-Fi network.
2. Enter the network password.
3. Continue through the setup prompts.   

2.4GHz Wi-Fi Requirement

The Sensi Lite thermostat is compatible with 2.4GHz Wi-Fi networks for this setup.

If the network does not appear in the app, the guide says that it may be:

• Hidden, or
• Incompatible with Sensi.   

Name the Thermostat

After Wi-Fi connection, the app lets the user choose a predefined thermostat name or enter a custom name.

Press Next after selecting the preferred name.   

Register the Sensi Thermostat

After a successful connection, the Sensi app prompts for thermostat registration.

The registration process offers:

• Locate Me, or
• Manual entry of street address, city, state, ZIP code, country and time zone.   

After registration, select Start Using Sensi to open the main app screen.   

HVAC Configuration Menu

The installation guide includes a configuration reference for checking the HVAC setup when the system has been configured manually or troubleshooting is needed.   

Indoor Equipment Settings

The configuration table defines:

• GA — single-stage gas system
• FN — fan-only system
• EL — single-stage electric system   

O and B Reversing Valve Settings

For heat-pump applications:

• O energizes the reversing valve during cooling and covers most applications.
• B energizes the reversing valve during heating.

The guide specifically mentions Rheem and Ruud among manufacturers that may use the B terminal behavior.   

2H, 2C and 6 Settings

The configuration reference defines:

• 2C — activates second-stage cooling.
• 2H — activates second-stage heating.
• 6 — used for three-wire zone hydronic systems.   

Troubleshooting the Sensi Lite Thermostat

Heating or Cooling Is Not Turning On

If the HVAC system does not start:

1. Confirm that the HVAC system has power.
2. Check the HVAC control board for an error code.
3. Reset the breakers.
4. Make sure furnace or air-handler access panels are completely closed and secured.
5. Check the thermostat wire connections.
6. Confirm that the thermostat was wired according to the Sensi app.   

If the HVAC control board reports an error or the equipment has no power, the guide recommends contacting an HVAC professional.   

Test With the Previous Thermostat

The guide also suggests reinstalling the original thermostat as a diagnostic step.

If the heating or cooling system still fails to start with the original thermostat, the issue may require professional HVAC service.   

Thermostat Screen Is Not Turning On

If the Sensi Lite screen remains off after wiring, the installation guide says the batteries may not be installed correctly.   

Check the two AAA batteries and make sure they are installed correctly before continuing the Wi-Fi setup.

Sensi Lite Will Not Connect to Wi-Fi

The official troubleshooting steps recommend:

1. Update the Sensi app to the latest available version.
2. Reboot the smartphone.
3. Reboot the router by unplugging it for a few minutes and reconnecting it.
4. Make sure Bluetooth is enabled in the phone settings.
5. Allow Bluetooth permission for the Sensi app.
6. Make sure Location access is enabled in the phone settings.
7. Allow Location permission for the Sensi app.
8. Try another smartphone or tablet if necessary.
9. Sign into the second device using the same Sensi account email and password.   

Also verify that the Wi-Fi network is 2.4GHz.   

Reconnecting Sensi After Changing the Router or Wi-Fi Password

If the router is replaced or the Wi-Fi password changes, the thermostat must be reconnected to Wi-Fi.

The guide notes that the reconnection procedure depends on the smart device being used and directs owners to the Sensi support instructions for the appropriate process.   

Accessing the Thermostat From Multiple Devices

Any device signed into the same Sensi account can control thermostats registered to that account.

The guide states that there is no limit to the number of devices that can access the thermostat.   

Sharing the Sensi account email address and password with another person gives that person access to the thermostats registered to the account, so account credentials should be shared carefully.   

Forgot Sensi Account Password

If the account password is forgotten:

1. Open the Sensi app login screen.
2. Select Forgot Password.
3. A temporary password is sent to the account’s registered email address.   

Installing Multiple Sensi Lite Thermostats

Additional thermostats can be added to the same account through the Sensi app.

After the first thermostat has been installed:

1. Open the app’s main screen.
2. Press the + sign.
3. Follow the same installation process for the next thermostat.   

Account Settings

The Sensi app’s Account area can be used to change account information and email preferences.

According to the guide:

• iOS: Tap the person icon in the upper-right corner.
• Android: Tap the three dots in the upper-right corner.   

Customer Support

For installation or setup problems, the official guide provides:

Sensi Support Website: sensihelp.com
Technical Support Phone: (888) 605-7131   

The support site includes installation and feature troubleshooting resources for Sensi thermostats.

Frequently Asked Questions

Does the Sensi Lite thermostat require the Sensi app for installation?

Yes. The official installation guide states that the Sensi app is required to correctly install and configure the thermostat. The app is also used to select the wiring arrangement, connect to Wi-Fi and register the thermostat.   

Does the Sensi Lite work with 5GHz Wi-Fi?

The installation guide specifies 2.4GHz Wi-Fi for the Sensi Lite. If the network does not appear during setup, the guide says it may be hidden or incompatible.   

How do you put the Sensi Lite thermostat into Wi-Fi setup mode?

Press and hold the thermostat’s Mode button for four seconds. The setup and Wi-Fi icons appear on the thermostat display.   

What batteries does the Sensi Lite thermostat use?

The installation guide instructs the user to install the two provided AAA batteries before attaching the thermostat cover to the base.   

Why is the Sensi Lite thermostat screen blank after installation?

The guide states that improperly installed batteries may cause the screen not to turn on after wiring. Check the two AAA batteries and their installation before continuing.   

Why is the heating or cooling system not turning on after installing Sensi Lite?

Check HVAC power, breakers, furnace or air-handler access panels, thermostat wire connections and the wiring configuration in the Sensi app. If the HVAC control board has an error or the system also fails with the original thermostat, the guide recommends professional HVAC service.   

How can another phone control the Sensi Lite thermostat?

Install the Sensi app on the other phone or tablet and sign in using the same Sensi account credentials. The guide states that there is no limit to the number of devices that can access thermostats registered to that account.