Thermostat calls for heat but the heating system runs continuously and overshoots the setpoint.

Safety: Turn off power to the heating system immediately if it will not shut off via thermostat.

Steps:

• Confirm on the thermostat display that it is not indicating a heat call (no flame icon or heat status). If the heater still runs, suspect a stuck relay or miswiring.
• Remove the thermostat from the wall. If the heating stops when the thermostat is removed, the thermostat relay is likely welded closed internally; replace the thermostat.
• If heating continues even with the thermostat removed, the heating circuit is miswired or a zone valve/contactor is stuck; inspect wiring at the air handler or boiler and repair or replace the stuck device.
• After hardware issues are resolved, review thermostat control settings (cycle rate, differential, or proportional band) and adjust to reduce overshoot according to Schneider Electric guidelines.