Sensi ST55 Installation Manual

The Sensi ST55 (model 1F87U-42WF) is a Wi-Fi enabled programmable thermostat for 24V HVAC systems. This manual focuses on installation instructions, including wiring, setup, and configuration. For full operation details, refer to the user manual. Requires C-wire for most systems.

Key Specifications

• Type: Wi-Fi enabled programmable smart thermostat for 24V HVAC systems
• Compatibility:
  • Single/multi-stage conventional (gas/electric), heat pumps, boilers
  • Requires C-wire for heat only, cool only, and heat pump applications
  • Works with Sensi app on iOS/Android; compatible with Alexa, Google Assistant, Apple HomeKit
• Power Source:
  • 24 VAC from HVAC system
  • Backup: 2 AA alkaline batteries
• Electrical Rating:
  • 24V systems; max 1A per terminal
  • Terminals: RH/RC, C, G, Y/Y1, Y2, W/W1, W2, O/B, L, AUX
• Temperature Range: 45°F to 99°F (7°C to 37°C); Humidity: 5% to 90% RH non-condensing
• Operating Conditions:
  • Indoor use only; 40°F to 120°F (4°C to 49°C)
  • Wi-Fi: 2.4GHz network required
• Dimensions: 3.75" H x 4.75" W x 1.25" D; wall-mount design
• Features:
  • 7-day programming via app
  • Geofencing, energy reports, remote control
  • Optional remote sensors
  • Backlit display, intuitive controls
• Thermostat Configuration Options:
  • Models: ST55, 1F87U-42WF
  • UL listed; professional or DIY installation

Preparation

Tools and Package Contents

• Tools: Phillips and/or Flathead Screwdriver, Drill with 3/16 (4 mm) bit, Wire Cutter. You may also need needle-nose pliers.
• Package Contents: Thermostat base, Thermostat faceplate, 2 AA batteries, Mounting screws and anchors, Wire labels.
• Wi-Fi Requirements: Sensi Password (on back of Welcome Guide or thermostat as “Sensi Security Code”), Wi-Fi Network Name/SSID (2.4GHz), Wi-Fi Network Password.

App Requirements

The Sensi app requires: Android (Google Play) version 5.0 or later; iOS (Apple) version 10.0 or later. Note: In the iOS store, search for “iPhone only”.

Installation Instructions

Warning: Turn off power at breaker before starting. DIY or professional installation. Requires C-wire for full functionality.

Turn Off Power

• Turn off the power to your heating and air conditioning system by using your fuse box or the switch next to your furnace.
• Confirm power is OFF: Try to turn on heating or cooling by changing the temperature on your old thermostat. CAUTION.

Remove Old Thermostat

• Remove the cover using a screwdriver or by pushing the pressure latch. Some covers pull off while others need to be released using a screwdriver. Do not remove your wires yet!
• Photograph your wiring: Take a photo of your existing thermostat and old wiring for reference.
• Throw away any jumper wires: Sensi has a jumper wire built into it. Leave all other wires connected.
• Choose wire terminals: Select terminals with wires attached (R, C, X, W, W1, W/E, Y, Y1, G, RH, RC, Y2, L, AUX, B, O/B, O, Other, W2, E).
• Select system type: Gas, Electric, Boiler/Radiant.

Label and Disconnect Wires

• Using the provided wire label stickers, label your wires by removing one wire at a time from the terminal and applying a label sticker.
• After all wires are labeled, remove the old thermostat base from the wall. Secure wires by wrapping around a pencil to prevent falling into the wall.

Install New Base

• Mount the thermostat using the supplied screws, drilling holes and inserting wall anchors if necessary.

Connect Wires

• Insert the wire into the corresponding terminal block, then tighten the screw (e.g., RH, O/B, Y, W/E, C).
• For two-transformer systems (separate RC and RH wires), clip internal RC/RH jumper on back of thermostat.

Attach Cover and Turn On Power

• Push the front display onto the thermostat base. Pull battery tag and attach cover. Tuck wires into the wall.
• Turn the power to your system back on.

Wi-Fi Setup

• On the thermostat, press Menu, then Mode. The Wi-Fi icon will flash, and 00, 11, or 22 will display (indicating Sensi version).
• Follow app instructions: Connect to Sensi network, enter Sensi password, then connect to your Wi-Fi.
• Name your thermostat and register location (use “Locate Me” or manual entry).
• Optional: Input contractor information.

HVAC Configuration

If needed, manually configure via Menu > Advanced Setup. The app should auto-configure during install.

Installing Multiple Thermostats

Repeat the process in the app. Tap + to add another thermostat.

Wiring and Configuration

Caution: Secure connections. C-wire required for certain systems. For invalid configurations, check compatibility at sensi.emerson.com/en-us/compatibility.

Terminal Definitions:

• RH: Power for heating, 24V
• RC: Power for cooling, 24V
• C: Common wire, 24V
• G: Indoor blower (fan)
• Y: First stage cooling/heat pump
• Y2: Second stage cooling or humidifier
• W/E: First stage heat or aux/emergency
• W2: Second stage heat or dehumidifier
• O/B: Reversing valve for heat pump
• L: Heat pump system monitor
• AUX: Auxiliary

Wiring Diagrams:

• Single Stage or Multi-Stage System (No Heat Pump with Two Transformers): Clip RC/RH jumper if separate. Connections: R to RH/RC, C to C, G to G, Y to Y, W to W/E. For multi-stage: Y2, W2.
• Heat Pump Systems: Internal jumper between RC and RH. Connections: R to R, C to C, G to G, Y to Y, O/B to O/B, W to W/E. For multi-stage: Y2, W2.

System Type	Key Connections
Conventional Single Stage	R/RH to R, C to C, G to G, Y to Y, W to W/E
Heat Pump Single Stage	R to R, C to C, G to G, Y to Y, O/B to O/B
Two-Stage Conventional	R to R, C to C, G to G, Y to Y, Y2 to Y2, W to W/E, W2 to W2
Dual Fuel Heat Pump	R to R, C to C, Y to Y, O/B to O/B, W to W/E
Humidifier/Dehumidifier	Y2 or W2 for accessories

Configuration Menu Items Reference:

Display Code	Menu Item	Default	Options
—	Wireless Setup	—	Connect to Wi-Fi
F	Fahrenheit or Celsius	F	F / C
—	Outdoor Equipment Config	—	AC1/AC2/HP1/HP2/AC0
—	Indoor Equipment Config	—	EL1/EL2/GA1/GA2/FAn
—	Reversing Valve Position	—	O / B
2 Fn	Circulating Fan	OFF	OFF/10%-100% (5%)
H AA	Humidification Accessory	OFF	OFF/ON
H SP	Humidification Set Point	OFF	OFF/5%-50% (5%)
dH AA	Dehumidification Accessory	Od	Od/Oc
dH SP	Dehumidification Set Point	OFF	OFF/40%-95% (5%)
—	Wireless Radio	ON	OFF/ON