37 installation, maintenance, and repair of natural gas mains, meters, storage facilities, and appliances; maintenance of natural gas vehicles, namely, automobiles, trucks and forklifts; conversion of vehicles, namely, automobiles, trucks, and forklifts, from use of gasoline to use of natural gas; on-site maintenance of pipes, valves and ignition systems for industrial, commercial and residential natural gas users; maintenance and repair of heating, water heating and air conditioning systems