Boiler’s thermostat calls for heat but boiler does not respond; no error code displayed.

Safety first: Turn off power before working on low-voltage wiring.

• Step 1 – Verify thermostat operation: Ensure the thermostat is set to HEAT and above room temperature. Replace batteries if applicable.
• Step 2 – Check boiler terminals: At the boiler control, measure for 24V across R and C. Then check for 24V between R and W (or T-T) when there is a call for heat. If no voltage change, the thermostat or wiring is suspect.
• Step 3 – Jumper test: With power OFF, temporarily install a jumper between R and W at the boiler. Restore power; if the boiler starts, the issue is in the thermostat or field wiring.
• Step 4 – Inspect zone controls: If a zone control panel is used, verify that it is powered and that its boiler demand output is functioning.
• Step 5 – Repair or replace: Replace damaged thermostat cable, faulty thermostat, or miswired connections as needed.

Note: Label all wires before disconnecting to avoid miswiring.