HVAC Softwares are needed to design simple and complicated air conditioning systems. There are many areas of design that engineers have to consider when designing parts that constitute a complete working unit. Mechanical engineers will focus on the mechanical design aspect of the unit ranging from plastic parts design to metal that house the unit.
Systems engineers will probably look into the load calculation and duct design aspect of the site where the air conditioner system is supposed to be installed. Test engineers will simulate and do actual capacity testing in the test labs specifically designed for testing purposes.
Electronic engineers will look into the design of the printed circuit board that controls the operations of the HVAC equipment. Electronic design itself is a huge task henc many big companies have dedicated electronic division that design and develop these controllers. Smaller companies may just purchase the electronic control boards from controls companies that sell these controller board.