Responded to no-heat call where furnace was operating but home temperature was dropping despite thermostat set to 80°F. Diagnosed code 311 (reduced airflow) and discovered one closed return vent. Replaced air filter, opened all supply and return vents, and verified gas pressure was properly adjusted. Home temperature increased approximately 5° before departure. Noted possible system oversizing and recommended potential addition of basement return grill for improved makeup air if issues persist.
