Thermostat calls for heat but Bosch boiler or furnace does not start; indicator shows heating demand active.

Safety: Turn off power to the boiler/furnace before accessing internal wiring.

• Confirm call signal: With a multimeter, check for 24 V between W and C at the control board when the thermostat is calling for heat.
• Inspect wiring: Trace the W circuit from thermostat to control board, including any zone controllers or pump relays. Tighten loose connections.
• Check safety lockouts: Look for error codes or lockout indicators on the Bosch boiler/furnace itself. Reset according to the equipment manual if safe to do so.
• Test zone valves/pumps: If using hydronic zones, verify that the zone valve or pump relay associated with the thermostat is energizing when there is a heat call.
• Professional diagnosis: If the control board is not responding to a valid W signal, it may be defective and should be evaluated by a technician.