Contractors and builders often hear "Manual J," "Manual S," and "Manual D" used interchangeably, but they're three distinct reports, each covering a separate stage of HVAC design. For anyone newer to the terminology — or explaining it to a client or GC — it helps to think of them as three sequential questions: how much conditioning does this building need, what equipment provides it, and how does that equipment's output actually reach each room.
Manual J is the load calculation — the report that determines a building's actual heating and cooling demand in BTUs, based on its envelope, orientation, occupancy, and local climate. It's produced room by room and then totaled for the whole building, and it's the document almost every other HVAC decision, and most permit reviews, start from.
Manual S takes the Manual J numbers and matches them to specific equipment, using the manufacturer's actual performance data rather than a rounded nominal size. Because equipment capacity shifts with outdoor temperature and operating conditions, this step confirms what a given unit really delivers at the project's design conditions — not just what's printed on the label.
Manual D designs the duct system that delivers the selected equipment's airflow to each room in the amounts Manual J calls for. It sizes trunk lines, branch runs, and registers, and checks that the whole system stays within an acceptable static pressure range for the equipment. Without it, even correctly sized equipment can underperform if the ductwork can't move air where it needs to go.
Each report depends on the one before it — Manual S needs Manual J's load figures, and Manual D needs both the load and the selected equipment's airflow characteristics. Skipping straight to equipment selection or duct installation without the earlier steps means working from assumptions rather than calculated values, which is where oversizing, undersizing, and airflow imbalances typically originate.
On a typical new construction or full replacement job, the sequence runs in order: Manual J establishes the load, Manual S selects equipment matched to that load at real operating conditions, and Manual D sizes the ductwork to deliver it. Contractors who run all three — rather than treating Manual J as sufficient on its own — tend to see fewer comfort complaints and callbacks after the system is installed.
Which of the three reports is required for permits?
Manual J is the most consistently required across jurisdictions. Manual S and D are increasingly requested for new construction and full system replacements, though requirements vary locally.
Can these three reports be done out of order?
Not accurately — Manual S depends on Manual J's output, and Manual D depends on both. Doing them out of sequence means working from estimates instead of calculated figures.
Is it normal for one company to handle all three reports together?
Yes — because the reports are sequential and interdependent, having them prepared together (or as a bundle) helps ensure consistency across all three.
Psychro.Pro prepares Manual J, S, and D reports individually or as a discounted bundle, built in the correct sequence so each report reflects accurate output from the one before it.Learn more.
