bosch thermostat keeps restarting on its own every few hours, screen goes black then boots up.

Diagnosis

The spontaneous rebooting of the Bosch BCC100 typically indicates a "brownout" condition where the 24VAC power supply is dipping below the operational threshold for a fraction of a second. Because the BCC100 is a connected device with a constant Wi-Fi draw, it is far more sensitive to voltage fluctuations than a traditional mercury or non-connected digital thermostat. When the voltage drops, the internal capacitor drains, causing the screen to go black and the system to trigger a full boot sequence. This is most commonly caused by a loose connection at the R (Power) or C (Common) terminals, or a failing 24V transformer in your HVAC air handler. However, a frequent hidden cause is a "tripped" limit switch in the furnace. If the furnace is overheating due to a dirty filter, the limit switch will momentarily cut power to the 24V circuit to protect the heat exchanger, causing the thermostat to lose power and restart. You can confirm this by checking if the restarts correlate with the furnace running for a long duration or by measuring the voltage at the R and C terminals with a multimeter; it should read a steady 24V-28V AC without dipping.

How to Fix It

1. Shut off the power to your HVAC system at the main breaker panel to avoid shorting the control board.
2. Gently pull the BCC100 display off the wall plate. Inspect the pins on the back of the unit for any bends or debris.
3. Examine the wiring terminals. Specifically, look at the R (Red) and C (Common) wires. If the wire is stripped too far, bare copper may be touching the adjacent terminal; if it is not stripped enough, the terminal clamp may be gripping the plastic insulation rather than the wire.
4. Loosen the terminal screws, pull the wires out, trim them cleanly, and strip exactly 1/4 inch of insulation.
5. Re-insert the wires and tighten the screws firmly. Tug on each wire gently to ensure a "mechanical lock" is achieved.
6. Check your HVAC air filter. If it is grey or clogged, replace it immediately. A restricted airflow causes the furnace to overheat, triggering the safety limit switch that cuts power to the thermostat.
7. Snap the BCC100 display back onto the wall plate, ensuring it clicks firmly into place to maintain a solid connection between the pins and the plate.
8. Restore power at the breaker and monitor the device for 4 hours.

If That Didn't Work

• Install a 24V C-Wire Transformer: If your system lacks a dedicated common wire or the existing transformer is failing, install a 24V 40VA plug-in transformer. Connect the transformer wires directly to the R and C terminals to provide a stable, isolated power source independent of the furnace's control board.
• Inspect the Furnace Control Board: Open the furnace cabinet and look for burnt capacitors or "bulging" components on the integrated circuit board. A failing capacitor on the board can cause voltage ripples that trigger the BCC100 to reboot. If you see discoloration around the 24V output terminals, the board likely needs replacement.
• Verify Wi-Fi Signal Strength: In rare cases, a critical failure in the Wi-Fi radio during a signal "handshake" can cause a software crash and reboot. Move your router closer or install a Wi-Fi extender to ensure the BCC100 has a signal strength of at least -65dBm.
• Check for Wire Chafing: Inspect the wiring run from the thermostat to the furnace. If the wire is pinched by a staple or rubbed raw against a metal stud, it can create an intermittent short-to-ground, causing the 24V fuse on the furnace board to partially fail or the system to reset.

Heads Up

• Voltage Warning: Never apply 120V power to the BCC100 terminals; doing so will instantly fry the logic board and void the warranty. Only 24VAC is compatible.
• Filter Maintenance: Change your HVAC filters every 30-90 days. This is the number one preventative measure to stop "phantom" thermostat restarts caused by overheating limit switches.
• Firmware Updates: Ensure your Bosch Home app is updated. Occasionally, Bosch releases firmware patches that optimize power management and stability for the BCC100.
• Tightness Check: Periodically check the terminal screws every six months, as thermal expansion and contraction can occasionally loosen the grip on the copper wiring.