Variable refrigerant flow (VRF) systems were first introduced to the HVAC community in Japan in 1980 as a highly efficient heating and cooling option in multi-zone comfort applications. It took the U.S. about 25 years to first embrace VRF concepts. Before VRF, engineers opted for the more traditional and less energy-efficient direct expansion (DX) cooling and heat pump (C&HP) options or costlier air- and water-cooled chilled water systems for multiple occupant comfort zones using either 2- or 4-pipe fan coil units.
DX systems were limited in their capabilities and continue to have limitations today. Still, DX systems remain popular because of their first-cost benefit; ability to adapt to C&HP pay-as-you-go phased construction; and their simplistic design, installation, and operation. For HVAC engineers, knowledge of refrigeration cycle theory comes into play with DX design, and, quite often, contractors will bypass a design engineer and go directly to the manufacturer for equipment size and selection options and features. Unless the consultant is in the process refrigeration industry, the design engineer will not need to refer to his or her refrigeration design manual for the majority of HVAC comfort-cooling commercial, institutional, and industrial applications. The same can be said for installing refrigeration technicians trained to install, pressure test, and startup DX systems because refrigeration theory and the associated automatic temperature controls (ATC) for these types of HVAC systems are not complicated. In fact, some HVAC equipment, such as rooftop air-handling units, packaged terminal air conditioner (PTAC) units, etc., comes completely packaged.