Performed single system heating maintenance. Checked amps, filter, cleanliness, electrical, and safeties. System is heating to manufacture specifications. Suggestions and concerns 1. Insulation levels are poor. Customer would like to bring it up to R38. 2. High limit switch failed testing and needs replacement 3. Duct work is poorly designed. Recommend replacement 4. Growth inside of the supply plenum. Recommend adding Apco-x