why does my bcc100 make a loud click every time the heat turns on or off? it's new.

Diagnosis

The audible clicking sound you are hearing from your Bosch BCC100 Connected Control Thermostat is the physical engagement and disengagement of the internal electromechanical relay. Unlike high-end "silent" thermostats that use solid-state switching (TRIACs), the BCC100 utilizes a physical armature that snaps into place to complete the 24V AC circuit between the R (power) and W (heat) terminals. When the thermostat determines the room temperature has dropped below your set point, it sends an electrical pulse to the relay coil, which magnetically pulls a metal contact closed. This "snap" is the click you hear. When the target temperature is reached, the coil is deactivated, and a spring forces the contact open, creating a second click. Because the BCC100 is designed to be compatible with a wide range of HVAC systems—including older high-voltage-relay furnaces—it uses these heavy-duty mechanical switches to ensure a clean, reliable signal is sent to your system's control board without electrical leakage. You can confirm this is normal by placing your ear near the unit during a temperature change; the click will occur exactly one second before your furnace blower or burners ignite.

How to Fix It

Since this is a fundamental design characteristic of the BCC100 hardware, there is no software setting or configuration change to "silence" the relay. However, you can minimize the frequency of these clicks and the perceived volume by optimizing your system settings:

1. Adjust the Temperature Swing (Hysteresis): Access the installer settings to increase the temperature differential. If the swing is set too narrow (e.g., 0.5°F), the thermostat will "short cycle," clicking on and off every few minutes. Increasing this to 1.5°F or 2.0°F reduces the number of times the relay fires.
2. Verify Wall Mounting: Ensure the thermostat is seated perfectly flush against the wall plate. If the unit is slightly loose or vibrating against a hollow drywall section, the wall can act as a sounding board, amplifying the mechanical click. Tighten the mounting screws firmly but do not strip the plastic.
3. Check for Drafts: Ensure there is no air leaking from the hole in the wall where the wires emerge. A cold draft hitting the internal sensor can cause the relay to trigger more frequently than necessary, leading to constant clicking. Seal the wire hole with a small amount of plumbers putty or foam insulation.

If That Didn't Work

If the clicking is accompanied by other symptoms, it may indicate a hardware fault or installation error rather than normal operation:

• Rapid-Fire Clicking (Chattering): If the relay clicks rapidly (several times per second) without the heat actually turning on, you likely have a voltage drop or a failing C-wire connection. Check that the 24V power supply from the furnace transformer is stable. Use a multimeter to verify you have a steady 24-28V AC between the R and C terminals.
• Buzzing or Humming: If the click is followed by a continuous 60Hz hum, the relay coil may be stuck or vibrating. This often happens if the thermostat is receiving "dirty" power or if there is an electromagnetic interference issue from nearby high-voltage lines. Try relocating the thermostat away from large appliances.
• No Heat Despite Clicking: If you hear the click but the furnace does not ignite, the relay is functioning, but the signal isn't reaching the furnace. Check the W-wire for a loose connection or a break in the line. Use a jumper wire to momentarily bridge R and W at the furnace board to see if the system fires independently of the thermostat.

Heads Up

• Avoid Third-Party Covers: Do not place tight-fitting decorative covers over the BCC100; while they might muffle the sound, they can trap heat around the sensor, causing inaccurate readings and more frequent relay cycling.
• C-Wire Importance: Always ensure a dedicated Common (C) wire is connected. Relying on "power stealing" (if applicable in some configurations) can cause the relay to behave erratically or click unexpectedly.
• Seasonal Maintenance: Check your HVAC air filters every 30-90 days. A clogged filter causes the system to overheat, which can trigger a safety limit switch that cuts power to the thermostat, causing the relay to click off prematurely.