Responded to no-heat call for 8-year-old American Standard furnace that was not maintaining set temperature. Diagnostic testing revealed furnace was tripping on high limit protection due to severely clogged filter that had not been changed since May of previous year. Removed and replaced air filter, verified proper operation through first and second stage heat cycles without tripping. Inspected heat exchanger - condition good. Instructed customer on 2-3 month filter replacement schedule to prevent future issues.
