Diagnosis
The high-pitched buzzing or "whine" you hear when the HVAC system is idle is known as coil whine. This occurs when the internal power transformer or capacitors vibrate at a high frequency while converting the 24V AC power from your C-wire into the DC power required to keep the touchscreen and WiFi active. Because the system is off, the lack of ambient noise from the blower or furnace makes this electronic vibration audible.
How to Fix It
While this is typically a hardware characteristic, you should first rule out electrical instability that could exacerbate the noise:
- Inspect Wire Terminals: Remove the thermostat faceplate and ensure all wires—specifically the R and C wires—are seated deeply and tightened securely. A loose connection can create micro-arcing, which mimics a buzzing sound.
- Check Voltage Stability: Ensure your HVAC transformer is providing a steady 24V. Fluctuations in power can cause the internal components to vibrate more intensely.
- Power Cycle: Completely power down the HVAC breaker for 30 seconds and restore it to reset the internal power management circuitry.
If That Didn't Work
If the wiring is secure and the noise persists, the issue is a physical defect in the internal components of that specific unit. Because the noise is caused by the physical vibration of a coil or capacitor, it cannot be fixed via software updates or settings. Your options are:
- Warranty Claim: Contact Emerson support for a replacement. This issue was noted in specific production batches of the ST75, and they typically replace units exhibiting excessive coil whine.
- C-Wire Adapter: In rare cases, adding a power extender kit (if not already using one) can stabilize the current, though this is rarely a permanent fix for hardware whine.
Heads Up
Do not attempt to open the plastic housing of the thermostat to "tighten" internal components. The unit is factory-sealed; opening it will void your warranty and exposes you to live electrical terminals that can cause short circuits if touched with metal tools.