These days the HVAC requirements of large buildings have become quite complex and require professional skilled technicians in this area. In small residential or commercial applications the air condi¬tioning contractor or electrician is usually responsible for controls along with other responsibilities.in large buildings the specification for the HVAC system usually has a separate section for controls and, they have some technicians specializing only in control systems install and check out the controls. The HVAC engineer describes how a system should be con¬trolled. The controls designer selects the equipment that will accomplish the intent of the HVAC engineer and then diagrams it for installation. The controls draftsman formalizes the plan of the designer and develops the control drawings used to communicate with the personnel in the field. The HVAC technician will install the controls in accordance with the manufacturer's instructions and the controls drawings. The technician will start up the sys¬tem and insure that the final control system does what the engineer originally specified. The technician also maintains and troubleshoots systems once they are in operation to insure that they work properly over the life of the equipment. The technician who succeeds in the controls field is one well knowledgeable in all types of HVAC systems. The controls technician must be able to identify the components of the HVAC system and know how they are intended to interact. The HVAC systems have a control variables. A residential application may require a minimum temperature of 72°F, using heating only. If it requires a minimum of 72°F and a maximum of 78°F, the HVAC system must be capable of both heating and cooling. Also 50% relative humidity would require an additional humidification or dehumidification capability.