A proper understanding of the functional basis of energy use in residences has been crucial in spurring energy conservation in such building stock. In commercial buildings, however, such understanding is lacking, partly because there are numerous possible HVAC system types and control strategies, and partly because the detailed algorithms available to simulate energy use in various types of HVAC systems provide little insight into identifying ways of saving energy in existing buildings. The objective of this paper is to derive closed-form steady-state functional relations for air-side cooling and heating thermal energy use for four of the most widespread HVAC system types, namely terminal reheat and dual-duct, both under constant air volume and under variable air volume operation. Expressions are derived for hourly energy use as a function of climatic variables, building characteristics, and system parameters. The effects of economizer cycle and deck reset schedules are also treated. The expressions derived could be utilized to develop functionally accurate regression models of monitored energy use for retrofit savings determination and to ascertain whether the HVAC system is operating properly, as well as for parameter estimation from either short-term or long-term monitoring and for sensitivity analyses of how various physical and operating parameters affect energy use.