Under  direction,  operates  and  makes  minor  repairs  to stationary steam boilers or steam generators and auxiliary boiler equipment  serving: (1) a building of public assembly and  having the  safety valve set at fifteen pounds per square inch gauge  or less  and having 499 square feet of heating surface or (2) having safety  valves  set higher than fifteen pounds  per  square  inch gauge  and  rated  over six boiler horsepower but  not  over  500 boiler  horsepower or (3) having safety valves  set  higher  than fifteen  pounds per square inch gauge and rated over  500  boiler horsepower but not over 1,000 boiler horsepower on a shift  under the  general supervision of a properly licensed engineer  or  (4) having  safety valves set higher than 15 pounds per  square  inch gauge and r