Energy modeling software has become an indispensabletool for engineering firms designing commercial buildings,especially when owner objectives involve green or sustainablebuilding. Carrier Corporation’s Hourly Analysis Program(HAP) is one tool that participates in this market. In observanceof the ASHRAE 125th anniversary, this paper intertwinesthe history of HAP with the history of energy modelingtools, specifically how advances in computer technology, engineeringpractice, and calculation methodologies have led toadvancements in tools. This history also demonstrates howASHRAE initiatives to advance state of the art of HVAC engineeringcan flow through software tools to benefit the industry.