Responded to no heat call for 1-year-old natural gas furnace. Diagnosed limit switch fault code; found limit switch stuck open despite cool system. Observed heat exchanger was in good condition, but discovered Merv 13 filter clogged with white residue from ultrasonic humidifier. Replaced faulty limit sensor and air filter, restoring normal operation. Advised customer about humidifier options and recommended using distilled water if continuing ultrasonic humidifier use.
