bcc100 says 'call for heat' but my furnace isn't firing. old thermostat worked fine.

Diagnosis

The "Call for Heat" message on the Bosch BCC100 indicates that the thermostat's internal logic has triggered a request for warmth, but the physical 24VAC signal is not reaching the furnace's control board. Since your previous thermostat functioned correctly, the issue is almost certainly not with the furnace itself, but with how the BCC100 is interfacing with the 24V system. This typically happens because the BCC100 is a "smart" controller that requires a precise equipment profile to activate its internal relays; if the software configuration doesn't match the physical hardware, the relay for the W terminal remains open even when the screen says "Call for Heat." Another common culprit is a loose connection at the RC or RH terminals. The BCC100 requires a steady 24VAC power supply to energize the switching relays. If the power wire is barely making contact or if there is a voltage drop, the display will have enough power to show the status, but the relay won't have enough "oomph" to close the circuit to the W wire. You can confirm this by using a multimeter to check for 24VAC between the R and W terminals at the furnace control board while the thermostat is calling for heat; if you see 24V at the board but the furnace isn't firing, the issue is the furnace. If you see 0V, the BCC100 is failing to send the signal.

How to Fix It

Follow these precise steps to synchronize the software configuration with your physical wiring:

1. Remove the thermostat faceplate from the wall bracket to ensure no wires have slipped out of the terminals during installation. Ensure the copper wires are stripped to exactly 1/4 inch and are clamped tightly.
2. Press the Menu button on the main screen and navigate to Installation.
3. Select Equipment Setup. You must verify the "Heat Source" is set correctly. For a standard furnace, select Gas/Oil. If you select "Heat Pump" by mistake, the BCC100 will look for an O/B wire and may ignore the W terminal entirely.
4. Navigate to Wiring within the Installation menu. Look at the virtual wiring diagram on the screen. Ensure that the W terminal is toggled to "Active." If the system thinks you have a multi-stage system (W1/W2) but you only have one wire, it may be trying to activate a second stage that doesn't exist.
5. Check the C-Wire setting. The BCC100 requires a common wire for stable operation. If you are using a power extender kit or a workaround, ensure the "C-wire" option is enabled in the settings to prevent intermittent relay failure.
6. Save the settings and return to the home screen. Set the temperature 5 degrees above current room temperature and listen for a physical "click" from the relay inside the BCC100 unit.

If That Didn't Work

• Perform a Hard Power Cycle: Turn off the breaker to your furnace for 60 seconds. This resets the furnace control board and clears any "lockout" modes that may have been triggered by rapid cycling during the thermostat installation.
• Swap the R-Wire: If you have both an RC and an RH wire, ensure the jumper is removed if the BCC100 is providing its own power, or that the correct wire is in the RH terminal. The BCC100 is sensitive to which power leg is used for heating vs. cooling.
• Bypass Test: Carefully remove the W and R wires from the BCC100 base and touch them together (jump them) using a small piece of insulated wire. If the furnace fires immediately, the BCC100 hardware is defective or the internal relay is stuck.
• Check the Fuse: Inspect the 3-amp or 5-amp purple/orange blade fuse on the furnace control board. If a wire touched the chassis during the BCC100 installation, the fuse may have blown, meaning the thermostat is calling for heat, but there is no power to send.

Heads Up

• C-Wire Necessity: Never attempt to run the BCC100 without a dedicated C-wire; using "power stealing" methods can lead to erratic relay behavior and "ghost" calls for heat.
• Voltage Warning: This device is rated for 24VAC only. Connecting it to a 120V or 240V line-voltage system will instantly fry the logic board and void your warranty.
• Firmware Updates: Once you get the system firing, immediately connect the device to Wi-Fi and check for firmware updates. Bosch frequently releases patches that fix relay timing issues.
• Tighten Terminals: Use a small precision screwdriver to ensure the terminal screws are snug; a "loose-fit" wire can cause the thermostat to show a call for heat while the actual electrical connection remains broken.