my glas is making a quiet but annoying high-pitched buzzing or humming sound from the unit itself.

Diagnosis

The high-pitched buzzing or humming emanating from the Johnson Controls GLAS Smart Thermostat is typically a phenomenon known as coil whine or electromagnetic resonance. This occurs when the internal inductors or capacitors on the thermostat's power conversion board vibrate at a frequency audible to the human ear. Because the GLAS unit converts the incoming 24VAC power from your HVAC transformer into a steady DC voltage for its high-resolution display and Wi-Fi radio, any instability in the input voltage can cause these components to vibrate. This issue is often exacerbated by "dirty" power or voltage fluctuations coming from the HVAC control board. If your transformer is undersized or if there is significant electrical noise on the C-wire (Common wire), the internal power regulator works harder to stabilize the current, resulting in a persistent 60Hz hum or a higher-frequency electronic squeal. You can confirm this is a power-related issue if the noise changes in pitch or intensity when the screen brightness increases or when the Wi-Fi module is actively transmitting data.

How to Fix It

To eliminate the resonance and reset the internal power regulators, follow these precise steps to perform a hard electrical purge and voltage stabilization:

1. Locate your main electrical service panel and identify the circuit breaker labeled for your HVAC system, Furnace, or Air Handler.
2. Switch the breaker to the "OFF" position. Do not simply restart the thermostat via the software menu, as this does not discharge the capacitors on the power board.
3. Wait exactly 90 seconds. This duration ensures that the internal capacitors within the GLAS unit and the HVAC transformer have fully discharged their stored energy.
4. Return to the breaker panel and switch the power back to "ON."
5. Once the unit boots, navigate to the Settings > Display menu and reduce the screen brightness to 70%. Lowering the current draw from the internal power converter often reduces the physical vibration of the coils, silencing the whine.
6. Check the wiring at the wall plate. Ensure the 24V RC and C wires are stripped to exactly 1/4 inch and are clamped tightly. A loose connection on the C-wire can create micro-arcing or voltage drops that trigger the humming sound.

If That Didn't Work

• Install a 24VAC Isolation Transformer: If the hum persists, the issue is likely electrical noise from your furnace's control board. Install a dedicated 40VA 24VAC transformer between the furnace and the thermostat. This isolates the GLAS unit from the "noisy" power of the HVAC system, providing a clean sine wave that typically eliminates coil whine.
• Verify C-Wire Voltage: Use a multimeter to measure the voltage between the R and C terminals. It should be between 24V and 28V AC. If the voltage is dipping below 22V, the internal regulator will struggle, causing the high-pitched buzzing. If voltage is low, you may need to upgrade your HVAC transformer from a 20VA to a 40VA model.
• Check for Interference: Ensure the thermostat is not mounted directly above a high-voltage line or adjacent to a large electronic appliance. Electromagnetic interference (EMI) from nearby 120V wiring in the wall can induce a hum in the GLAS unit's sensitive internal circuitry.
• Firmware Update: Check the GLAS app for any pending firmware updates. In some hardware revisions, Johnson Controls released power management updates that optimize how the Wi-Fi chip handles power, which can reduce the load on the inductors and lower the audible noise.

Heads Up

• Never attempt to open the GLAS thermostat casing to dampen the coils; doing so will void your warranty and likely damage the fragile ribbon cables connecting the display to the logic board.
• Avoid using "C-wire adapters" or "power stealing" kits with the GLAS model if possible, as these often introduce the exact voltage fluctuations that cause audible humming.
• If the buzzing is accompanied by a burning smell or the screen flickering, shut off power immediately; this indicates a failing capacitor rather than simple coil whine and requires a full unit replacement.
• Always use a dedicated Common (C) wire for this model to ensure the most stable power delivery and the quietest operation.