Back in the early 1980s I worked as the national service manager for a startup Florida-based company that began manufacturing a new duct-mounted variable-air volume (VAV) system for residential and light-commercial applications.
Of course, VAV systems were nothing new by then because they had been applied in upscale commercial systems for years to provide individual temperature control in offices and other smaller spaces. There have been pneumatically-controlled, electrically-controlled and even balloon-driven systems that worked off pressure in the ducts. However, such systems, though surely the most comfortable, always have been expensive, their operation always has been a mystery to service techs, and there are problems with getting them to switch over properly between heating and cooling modes.