The Sensi ST55 ($129) is an affordable smart thermostat packed with useful features including support for geofencing, voice control via Amazon Alexa, Apple Siri, and Google Assistant, and support for the HomeKit and SmartThings home automation platforms. It’s easy to install and program, and it offers usage reports, local weather reports, and smart alerts. That said, it doesn’t integrate with quite as many third-party devices as our Editors’ Choice winner for affordable smart thermostats, the identically priced Nest Thermostat.
Functionality Over Fashion
The ST55 eschews the stylish design that you get with its $169 sibling, the Sensi Touch Smart Thermostat ST75, and the sleek, puck-shaped Nest Thermostat. It’s a rather plain-looking device that sports a matte white finish and measures 3.7 by 5.8 by 1.0 inches (HWD).
In the center is a 1.7-by-2.7-inch (HW) monochrome LCD and below that are Mode, Fan, and Schedule buttons. Off to the right are up and down arrows and a Menu button, and there’s a light button on the top edge that illuminates the LCD. Use the Menu and arrow buttons to access thermostat settings where you can configure Wi-Fi, select a temperature mode (Celsius or Fahrenheit), configure cooling and heat pump settings, adjust fan speed, and configure humidification set points.
The ST55 can be used with most single- and two-stage HVAC systems, but requires a C (common) wire. It works with electric, gas, and oil furnaces, central air conditioning, boilers, humidifiers and dehumidifiers, and systems with heat pumps. It doesn’t work with electric baseboard heating.
Included in the box are the thermostat, a mounting base with wire terminals and mounting hardware, wire labels, an installation guide, and a welcome guide. The mounting base accommodates two AA batteries (included) that will continue to keep the device running in the event of a power outage.
Despite its smallish size, the LCD offers a wealth of information. Along the top of the screen are Wi-Fi and battery level indicators and the current time. Below that is the current set temperature and the mode (heating, cooling), and in the center of the screen is the current room temperature. The indicators at the bottom edge of the screen correspond to the buttons below them (Mode, Fan, Schedule) and display the status of each.
The ST55 uses a 2.4GHz Wi-Fi radio to connect to your home network. It can be controlled using the mobile app or with Alexa, Google, and Siri voice commands (the latter of which you don’t get with Nest). It also integrates seamlessly with HomeKit, SmartThings, and Wink home automation systems, and supports geofencing that sets the thermostat back three degrees when you leave the thee-mile radius from your home. However, it doesn’t support IFTTT applets that would allow it to work with numerous third-party smart home devices.
The thermostat uses the same Android and iOS mobile app as the ST75. The home screen displays the current temperature and humidity, the local weather, and the current schedule. At the bottom of the screen are buttons for Mode (heat, cool, off), Fan (on, auto), Schedule, and Usage. The Schedule button takes you to a screen where you can create heating and cooling schedules and enable geofencing, and the Usage button opens a screen where you can view daily and monthly usage reports.
Tap the gear icon in the upper right corner to open a settings screen where you can enter temperature set-point thresholds, edit account information, change passwords, and set up email and push notifications for maintenance reminders. Here you can also access online help, sign up for energy-saving programs, and change the thermostat’s configuration settings.
Easy to Install, Easy to Use
Installing the ST55 is fairly easy, but if you’re not comfortable working with low-voltage wires, you might want to enlist the help of someone who knows what they’re doing or hire a pro. You can choose to follow the on-screen installation instructions in the app, or skip them if you’ve already installed the device. The instructions walk you through each step of the installation and can be very helpful if you’ve never installed a thermostat before.
To start, I turned off power to my current thermostat at the circuit breaker box and proceeded to uninstall it, but first I took a picture of my existing wiring. With the old thermostat removed, I connected my wires to the ST55 mounting plate terminals and then attached the mounting plate to the wall. I snapped the thermostat onto the mounting plate and it immediately powered up. Next, I opened the app, tapped the plus icon in the upper left corner of the home screen, and selected the Sensi Smart thermostat.
I skipped the installation instructions and followed the on-screen directions to start the Wi-Fi radio by pressing the Menu button on the device and then pressing the Mode button. I verified that the thermostat screen was displaying the number 22, then pressed the Schedule button. I verified that the screen now displayed 00 and used my phone’s Wi-Fi settings to connect to the thermostat. I entered the eight-digit security number (found on the included security card), gave the device a name, and the installation was complete.
The ST55 performed well in testing. It responded quickly to app and local commands to raise and lower heating levels, and it followed my heating schedules without issue. Alexa and Siri voice commands to change temperature set points and relay the current temperature worked as expected, as did the geofencing feature. Programming the thermostat was easy thanks to the dedicated Schedule button and the user-friendly mobile app.
A Strong Alternative to Nest
With the Sensi Smart Thermostat ST55. you get a wealth of features for a reasonable price. Granted, it’s not as pretty as the Nest Thermostat, but it works with multiple home automation systems and can be controlled using Alexa, Google, and Siri voice commands. It’s also easy to install and program and worked without a hitch in testing. If you don’t need native HomeKit or Siri compatibility, however, the Nest Thermostat offers a more stylish design and greater third-party device support for the same price, earning it our Editors’ Choice award.
