how do i clean the white plastic outer case? it's getting yellow and dirty but i'm scared to damage it.

Diagnosis

The discoloration on the Johnson Controls GLAS Smart Thermostat is typically caused by a process called photo-oxidation. The outer bezel is constructed from a high-impact polycarbonate blend. Over time, exposure to UV rays from windows and the accumulation of skin oils (sebum) from frequent touch interactions create a chemical reaction that breaks down the polymer chains, resulting in a yellowish tint. This is not a surface stain but a structural change in the plastic. Furthermore, the "dirty" appearance is often a buildup of electrostatic dust and airborne particulates that cling to the plastic due to the static charge generated by the device's internal electronics. If you use abrasive scrubbers or solvent-based cleaners, you will create micro-scratches in the gloss finish, which will actually trap more dirt and make the yellowing appear more pronounced. You can confirm if the yellowing is surface-level or structural by attempting to wipe a small area with a damp cloth; if the color remains unchanged, the plastic has oxidized.

How to Fix It

To clean the GLAS thermostat without risking electrical shorts or permanent surface etching, follow this precise sequence:

1. Cut power to the HVAC system at the circuit breaker. While the GLAS is designed for touch, liquid seepage into the rear connection pins can cause a short circuit or trigger a "Sensor Error" message on the display.
2. Carefully remove the thermostat head from the wall plate by grasping the sides and pulling firmly but steadily away from the wall.
3. Prepare a cleaning solution using 8 ounces of lukewarm distilled water and exactly one drop of a pH-neutral dish soap (such as Dawn). Avoid using "antibacterial" soaps that contain bleach or strong fragrances, as these can leave a film.
4. Fold a clean, lint-free microfiber cloth into a small square. Dip a corner into the solution and wring it out until it is barely damp. The cloth should never be dripping.
5. Wipe the outer bezel using light, circular motions. Focus on the edges where dust accumulates. Do not apply heavy pressure to the glass touch-interface area.
6. Immediately use a second, dry microfiber cloth to buff away any residual moisture. This prevents water spots and ensures no liquid remains in the crevices where the plastic meets the screen.
7. Allow the unit to air-dry for 10 minutes before snapping it back onto the wall plate and restoring power.

If That Didn't Work

• Isopropyl Alcohol (70% only): If there are stubborn oily smudges that soap won't remove, dampen a cotton swab with 70% Isopropyl Alcohol. Gently dab the specific spot and wipe away immediately. Do not soak the plastic or use 91% or 99% concentrations, as high-percentage alcohol can cause "crazing" (tiny internal cracks) in polycarbonate.
• Magic Eraser (Extreme Caution): For deep-set grime, use a melamine sponge (Magic Eraser) barely dampened with water. Use the absolute lightest touch possible. Because these sponges are micro-abrasives, over-scrubbing will strip the factory gloss and leave a matte, cloudy patch.
• UV-Blocking Film: If the yellowing is continuing rapidly, the root cause is sunlight. Apply a small piece of clear UV-blocking window film to the glass of the nearby window to stop the photo-oxidation process from worsening.
• Bezel Replacement: If the yellowing is structural (oxidized), no amount of cleaning will reverse it. Your only option is to order a replacement outer housing assembly from a Johnson Controls distributor to restore the original white aesthetic.

Heads Up

• Never use glass cleaners containing ammonia (like Windex) or bleach; these chemicals react with the polycarbonate and will accelerate the yellowing process.
• Avoid using paper towels or tissues for cleaning, as the wood fibers in these products act like sandpaper on the GLAS's polished finish.
• Do not spray any cleaning liquid directly onto the thermostat. Always apply the liquid to the cloth first to prevent fluid from entering the vents or the screen seal.
• To prevent future buildup, wipe the device with a dry, electrostatic duster once every two weeks to remove dust before it bonds with skin oils.