Maintenance and repair 13 Air conditioning 9 Heat pumps 8 Refrigeration and refrigerating machinery 8 Gas 4 Heating 4 Heating and cooking 4 Refrigerators 4 Ventilation 4 Bearings (Machinery) 2 Bolted joints 2 Bolts and nuts 2 Energy conservation 2 Engines and power systems 2 Gas appliances 2 Hoisting machinery 2 Industrial and technical education 2 Ladders 2 Lubrication and lubricants 2 Machine-shop practice 2 Measuring instruments 2 Metalwork 2 Metalwork machinery and safety 2 Rigging 2 Scaffolding 2 Teaching methods and strategies 2 Tools 2