When your home's heating system is turned on at the thermostat but fails to deliver hot air through the supply vents, several potential mechanical restrictions or equipment failures are usually responsible. First, check and replace your indoor air filter, as a severely clogged filter restricts airflow and can cause a furnace to overheat and trip its internal limit switch, shutting down burners while the blower fan keeps running. Second, verify that gas valves are open and ignition systems are firing. Third, check for closed balancing dampers or severe duct leaks. You can troubleshoot by inspecting furnace indicator lights for error flashes. You can verify that normal heating is restored when replacing dirty air filters, resetting limit switches, or repairing ignition faults brings back powerful warm airflow.
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