Thermostat calls for heat but furnace does not start

Safety first: Turn off power to the furnace at the breaker before touching wiring or removing panels.

Steps to troubleshoot:

• 1. Confirm call for heat: Ensure the thermostat is in Heat mode and the setpoint is above room temperature. Look for 'Heat On' indicator.
• 2. Inspect W wiring at thermostat: Remove the thermostat and verify the W (or W1) wire is securely connected.
• 3. Inspect W wiring at furnace: At the furnace control board, confirm the same conductor is attached to the W terminal.
• 4. Jumper test (advanced): With power on and extreme caution, briefly jumper R to W at the furnace control board. If the furnace starts, the issue is between the thermostat and furnace or in the thermostat itself.
• 5. Reset furnace: Turn furnace power off for 30 seconds, then back on to clear lockouts.

When to call a professional: If the furnace does not start even when R and W are jumpered, or if it locks out repeatedly, an HVAC technician should inspect the furnace controls and safety devices.