Installing Your Johnson Controls GLAS Smart Thermostat

What You'll Need

• Johnson Controls GLAS Smart Thermostat
• GLAS app (iOS or Android)
• 2.4GHz Wi-Fi network
• Compatible 24V HVAC system
• Phillips head screwdriver
• Wire labels or tape

Before You Start

The Johnson Controls GLAS is one of the most visually distinctive smart thermostats on the market, featuring a translucent OLED touchscreen that displays information over a see-through glass panel. Behind the looks, it is a functional smart thermostat that works with most 24V HVAC systems including conventional furnaces, heat pumps, and systems with up to 2 stages of heating and 2 stages of cooling. It also includes built-in air quality sensors for VOC and CO2 monitoring.

C-Wire Requirement

The GLAS requires a C-wire. The transparent OLED display and air quality sensors draw more power than a typical thermostat, so there is no battery backup or alternative power option. If your existing thermostat cable does not include a C-wire, you will need to run a new cable or install an add-a-wire adapter before mounting the GLAS. Check your current wiring carefully -- you need at least 5 conductors (R, C, G, W, Y minimum).

Step-by-Step Installation

1. Turn off HVAC power at the breaker. Confirm the system is off before disconnecting any wires.
2. Remove the old thermostat. Pull off the faceplate, photograph the wiring, and label each wire with the terminal letter.
3. Remove the old wall plate. The GLAS has a unique mounting footprint, so you may need to patch old screw holes.
4. Install the GLAS backplate using the included screws. Thread wires through the center. Use the level to ensure proper alignment -- the transparent display makes any tilt very noticeable.
5. Connect wires to the matching terminals. The GLAS has standard terminals: R, C, W1, W2, Y1, Y2, G, O/B. Insert each wire and tighten the screw terminal firmly.
6. Carefully attach the GLAS display to the backplate. The translucent screen is fragile, so handle it gently and press it on evenly.
7. Restore power at the breaker.
8. Follow the touchscreen setup:
   • Select your system type and confirm detected wiring
   • Connect to your 2.4GHz Wi-Fi network
   • Download the GLAS app and create an account
   • Register the thermostat and configure preferences

HVAC Compatibility Notes

The GLAS supports up to 2 stages of heating and 2 stages of cooling for conventional systems, and heat pump operation with auxiliary heat. It has an O/B terminal for heat pump reversing valves. The thermostat does not have a dedicated accessory terminal for humidifier or dehumidifier control. If your system includes humidity equipment, you will need to control it separately. For heat pump installations, configure the reversing valve type (O or B) during setup.

Smart Home Integration

The GLAS originally launched with Microsoft Cortana support and has since added Amazon Alexa and Google Assistant compatibility. Cortana integration is still available but less commonly used. Apple HomeKit is not supported. The GLAS runs a version of Windows IoT Core, which gives it a different software foundation than most smart thermostats. The GLAS app provides remote access, scheduling, and air quality monitoring. Integration with broader smart home platforms is somewhat limited compared to ecobee or Nest, though Alexa and Google routines allow basic automation.

Energy Saving Features

• Air quality monitoring displays real-time VOC and CO2 equivalent levels on the transparent screen, helping you understand indoor air conditions
• Scheduling with custom temperature programs for each day of the week
• Geofencing through the GLAS app adjusts the temperature when you leave and return home
• Humidity monitoring displays the current indoor humidity level, helping you make informed comfort decisions
• UV index display provides outdoor UV data from weather services, a unique feature among thermostats
• Energy usage data shows HVAC runtime patterns in the app

Troubleshooting

• Display visibility: The transparent OLED screen can be hard to read in certain lighting conditions, especially with direct sunlight behind it. If readability is an issue, consider the thermostat's wall location -- a wall without a window directly behind you when viewing the screen will give the best visibility.
• Air quality sensor calibration: The VOC and CO2 sensors need 24 to 48 hours of continuous operation to fully calibrate. During this period, readings may fluctuate significantly. After calibration, if readings still seem unusual, check that the thermostat is not near cleaning products, paint, or other strong chemical sources that could trigger high VOC readings.
• No power or blank screen: The GLAS requires a C-wire. If the display does not turn on after installation, check the C-wire connection at both the thermostat and the furnace control board. A loose or missing C-wire is the most common cause of a dead display.
• Won't connect to Wi-Fi: The GLAS supports 2.4GHz Wi-Fi only. If your router broadcasts combined bands, try temporarily separating them. The GLAS uses Windows IoT Core, which can occasionally be slow during the initial Wi-Fi handshake. Give it a minute after entering credentials before assuming it failed.
• HVAC not responding: Run the system test from the thermostat's settings menu to check each component. Verify that the system type matches your equipment (conventional vs. heat pump). For heat pumps, confirm the O/B wire configuration matches your system's reversing valve type.
• Factory reset: Navigate to Settings on the touchscreen and select Factory Reset. This clears all configuration, Wi-Fi settings, and schedules. You will need to run the full setup process again.