HVAC System Design Calculator: Manual J Load Calculation Software (J-S-T-D)
All four ACCA steps in one workflow - load, equipment, registers, and ducts
Free Manual J load calculation software that runs the complete ACCA sequence in one place: Manual J load calculation, Manual S equipment check, Manual T register sizing, and Manual D duct design. Each result flows automatically into the next step, so you never re-enter data. Works on any mobile browser and is built for use on job sites.
Complete HVAC Design Workflow
Step J - Manual J Load Calculation
Calculate your building's heating and cooling loads.
The ACCA Manual J-S-T-D Design Sequence
These four steps are not independent checklists. Each one requires the output of the step before it. Running them out of order - or skipping any - produces a system that cannot perform correctly.
Load calculation comes first - it sets the sizing target
Without Manual J, you have no number to size to. A Manual J calculation that is 20% off produces a system that is 20% off in every subsequent step.
Manual S uses the J loads directly to set equipment limits
ACCA Manual S sizing limits are expressed as percentages of the Manual J load. The CFM output of Manual S is the input for Manual T and Manual D.
Manual T uses the CFM from Manual S to check each register
Face velocity, throw distance, and return grille sizing all depend on how much air the system is moving. That airflow comes from Manual S equipment selection.
Manual D uses CFM and static pressure to size every duct run
Friction rate - the core of Manual D duct sizing - requires knowing the total system CFM and the available static pressure after component losses.
Prefer individual calculators? Manual J · Manual S · Manual T · Manual D
Worked example: a 2,000 sq ft home in a mixed-humid climate
Enter 2,000 sq ft, 9 ft ceilings, a mixed-humid climate zone, average insulation and air tightness, 8 windows, 3 occupants, and the default design temperatures (95°F summer, 15°F winter, 72°F indoor). Run each step and the numbers chain like this:
- Manual J: heating load 31,700 BTU/hr (2.5 ton), cooling load 42,100 BTU/hr (3.5 ton), sensible heat ratio 73%.
- Manual S: the cooling window is 95 to 115 percent of the load, so 39,995 to 48,415 BTU/hr. Enter an OEM cooling capacity of 42,000 BTU/hr (a nominal 3.5-ton unit) and heating output of 40,000 BTU/hr - both PASS. The target airflow works out to 1,330 CFM.
- Manual T: that 1,330 CFM sizes the return grilles; a 350 CFM largest room lands on a 10 inch register at roughly 720 FPM with about 13.6 ft of cooling throw - enough for a 16 ft room.
- Manual D: with 0.70 in. w.c. equipment ESP and default coil, filter, and duct-run losses, available static pressure is 0.23 in. w.c. over a 180 ft total effective length - a friction rate of 0.128 in. w.c. per 100 ft, comfortably inside the 0.06 to 0.18 wedge.
Change any input and every downstream number updates - that is the point of running Manual J, S, T, and D as one connected workflow instead of four disconnected programs.
From load calc to a scheduled, invoiced job
You've designed the system - now win and run the job. Field Promax HVAC software helps HVAC contractors schedule jobs, dispatch technicians, manage work orders, and invoice customers from one platform. Trusted by 100,000+ contractors across the USA and Canada.
Book my free demoFree demo · no credit card required.