Why each standards body exists, what problem it uniquely solves, how they interact on a real project, and the design decisions each one drives — not memorization, engineering judgment.
Earlier modules treated ASHRAE ventilation rates, load-calculation methods, and equipment efficiency numbers as fixed inputs. This module goes underneath that: why ASHRAE 55, 62.1, 90.1, and 15 are what they are, why mechanical work in the United States answers to two competing model codes (the IMC and the UMC) rather than one, and how SMACNA and ACCA function as voluntary industry standards that codes and energy codes pull in by reference to avoid re-specifying construction and procedural detail from scratch.
It also covers how energy codes (IECC and ASHRAE 90.1) layer on top of the base mechanical code, how AHRI certification turns a manufacturer's cut-sheet claim into a verified fact, then walks a single worked example — a rooftop unit replacement — where all five layers apply at once, and closes with how to resolve conflicts when they disagree.