ACCA Manuals J, S, D, and T are the residential playbook for sizing and delivering air. They are not trivia for engineers. They are how you avoid a loud, short-cycling, uneven system that still carries a 10-year parts warranty and a 2-year comfort complaint.
Square footage is one input. It is not the calculation.
Manual J — how much heating and cooling the house needs
Manual J is a load calculation. It accounts for insulation, windows, orientation, infiltration, occupants, appliances, and design temperatures for your city. Glendora is not the same load as a house of the same size in the marine layer, and a 1955 ranch with original single-pane is not the same as a 2018 rebuild.
The output is BTUh for heating and cooling — the target the equipment has to hit. If your contractor never walked the house or asked about window upgrades, they did not do a J. They guessed.
Manual S — picking equipment that actually hits that load
Manual S is equipment selection. You do not grab 'a 3-ton' because the last unit said 3 tons. You pick a matched system whose expanded performance data hits the Manual J load at your design conditions, with a sensible heat ratio that fits the climate.
This is where oversizing gets caught. A 3.5-ton unit might have been the lazy choice. Manual S may show a 3-ton variable or a 2.5-ton two-stage is the one that fits. It is also where indoor coils get matched so you do not pair a high-SEER2 condenser with a leftover coil from 2009.
Manual D — the duct system that can move that air
Manual D sizes ducts for the CFM each room needs at a friction rate the blower can live with. This is how you stay near 0.5 inch static instead of 0.9.
Replacing the outdoor unit and leaving 30-year-old undersized flex is how brand-new equipment 'doesn't work.' The equipment is fine. The pipes for the air are not.
Manual T — registers, grilles, and throw
Manual T is the unglamorous last mile: supply and return grilles that deliver the air without sounding like a leaf blower or dumping all the cold air at the ceiling in a 20-foot great room. Wrong grille size or a return stuffed behind a sofa will make a perfect J/S/D job feel like a bad one.
What 'we did a load calc' sometimes means
Some software will spit out a number if you type square footage and zip code and accept every default. That is a costume. A real J uses your windows, your insulation, your attic, and your orientation. We should be able to show you the room-by-room CFM, not just a tonnage.
| Manual | Question it answers | If you skip it |
|---|---|---|
| J | What load does this house have? | Wrong tonnage |
| S | Which unit hits that load? | Mismatched coil, poor humidity |
| D | Can the ducts deliver the CFM? | Noise, high static, dead rooms |
| T | Will the air actually mix in the room? | Drafts, hot/cold spots |
You can ask for the file
A contractor who did the work can show you the load report and the duct design. A contractor who did not will change the subject to brand or financing.
Every install we warrant starts with these calculations. If your last quote was '2,000 feet, so 4 tons,' get a second one.