Performed 2 system heating maintenance. Found large amounts of black mold growth in upstairs supply plenum. Recommend u/v lighting system to kill mold and bacteria. No filter rack doors, so there is a larger opening to pull in attic air. No surge protection on furnace to stop electrical damage. Main level system flu pipe is single wall and will not pass code until upgraded to double wall. Basement has no form of surge protection. Recommend surge protector and double wall flu installed. Both systems operating normal at this time.