Repair or replace defective parts in units and equipment and their controls, including thermostats, automatic switches, fan controls, log switches, damper motors, louvers, relays, filters, controls, belts, compressors, heat exchangers, high limit controls, pressure controls, safety valves, and automatic gas valves. Conduct regular preventive maintenance inspections of refrigeration units, isolated heating and ventilating units, including boilers, gas and electrically operated air-conditioning equipment, and make necessary adjustments or repairs; service and repair evaporative coolers.