An HVAC load calculation is the math that matches a system to the building: Manual J produces the heating and cooling loads, Manual S turns those loads into an equipment selection, and Manual D sizes the ductwork.
Where the code requires one, the load calculation comes first, and the equipment selection is made from its numbers.
Here's what each manual decides and when the math is required.
What Manual J, S and D are
Manual J is the load-calculation step.
ACCA describes Manual J 8th Edition — published as ANSI/ACCA 2 Manual J-2016 — as the national ANSI-recognized standard for producing the equipment-sizing loads for single-family detached homes, small multi-unit structures, condominiums, townhouses and manufactured homes.
The method behind it is CLF/CLTD, and the output can be a block load for the whole building or a room-by-room load table.
Manual S is the equipment-selection step.
ACCA's Manual S — Residential Equipment Selection, now in its 3rd edition as ANSI/ACCA 3 Manual S-2023 — covers selecting and sizing residential heating, cooling, dehumidification and humidification equipment using the calculated loads and the manufacturer's performance data, with size limits by climate.
The 3rd edition added expanded size tolerances for variable-capacity heat pumps.
Manual D closes the chain with air.
It is ACCA's residential duct design standard, built from normative sections that carry required engineering calculations, mandatory duct system features, worksheets, and a list of prohibited practices.
One unit from this paperwork is worth knowing cold: the ton.
NIST defines a ton of refrigeration as 12,000 Btu per hour — the conversion that applies when a sizing question needs loads and capacity in the same units.
| Manual | Edition | What it decides |
|---|---|---|
| Manual J | ANSI/ACCA 2 Manual J-2016 (8th Edition) | The heating and cooling loads — a block load or a room-by-room load table |
| Manual S | ANSI/ACCA 3 Manual S-2023 (3rd edition) | Equipment selection — heating, cooling, dehumidification and humidification equipment sized from the calculated loads and the manufacturer's performance data |
| Manual D | ACCA's residential duct design standard | The duct system — required engineering calculations, mandatory features, prohibited practices |
How much over the load a selection may land is a question the manual itself answers, and the answer has moved over time.
A 2012 DOE Building America guideline — citing the 1995 edition of Manual S — summarised it as recommending furnace size within 140% of the peak heating load and cooling capacity no more than 15% above the total cooling load, with heat pumps up to 25% over the heating load in heating-dominated climates.
That is a summary of the edition then in force, not today's limits: the current ones are set by the 2023 third edition, and ACCA publishes the manual.
When a selection needs defending, read the current manual — not a remembered percentage.
Who actually runs the load calculation?
The honest answer is that the sources this page rests on don't tie the calculation to a job title.
The ACCA standards pages and the IRC section we checked describe calculations and selections — what Manual J, Manual S and Manual D produce — not who produces them.
So a shop where the designer runs it, one where a comfort advisor runs it, and one where the lead tech runs it are all arrangements our sources neither confirm nor forbid.
What those sources do establish is the sequence, and where a tech's hands land on it.
Upstream: the loads come first, because Manual S selects equipment from the calculated loads — by the time a unit is specified, the load numbers already exist.
Downstream, verification carries the contractor's name: the airflow check on ACCA's installation-verification protocol, ANSI/ACCA 9 QIvp, is contractor-recorded, and it has to land within 15% of design airflow.
For an apprentice that points at the right first skill: reading the report, not authoring it.
A block load covers the whole building; a room-by-room table breaks the load out by room; the selection shows which equipment was matched to those numbers using the manufacturer's data.
Being able to read all three — and to spot when the box on the truck doesn't line up with the report — is a skill worth building early, and it is one entry in the wider set our HVAC technician skills guide maps.
If you're still working toward your first solo call, our guide for HVAC apprentices covers what those years involve.
Ask to see the load calc — and the selection — on your next install
Why sizing matters for callbacks
Capacity gets decided at the selection step: Manual S matches equipment to the calculated loads, so the capacity a building ends up with traces back to that selection.
If the selection was wrong, the system starts service at the wrong capacity.
The cost of getting installation decisions wrong has at least one well-documented number behind it: ACCA cites a NIST study finding that not following the requirements of its Quality Installation specification, ANSI/ACCA 5 QI, can increase annual energy consumption by 30% or more.
The loads and the selection are decided before installation begins — which is why the paperwork stage is where a sizing mistake either gets caught or gets installed.
Sizing also sets numbers the finished system is checked against.
Under ANSI/ACCA 9 QIvp, recorded airflow has to be within 15% of design airflow.
So when a comfort complaint turns into a callback, the design documents are the natural place to start: the room-by-room load table says what each room was calculated to need, and comparing that against what the system actually delivers helps you see whether the mismatch was designed in or installed in.
That diagnostic habit is part of the job of a working HVAC technician — inherited installations included.
The tools and software load calculations run on
This page stays out of the software-review business, but the shape of the work is fixed by the standards themselves, so you know what any tool's output should look like.
Manual J produces a block load or a room-by-room load table.
Manual S works from the manufacturer's performance data for the equipment under consideration, applying its size limits by climate.
Manual D ships with worksheets for the duct calculations.
Those are the artifacts to look for in whatever tool produced the report.
One caution belongs here for new techs.
The shortcuts that circulate on the truck — square feet per ton, CFM per ton — are common approximations that assume standard air.
They are not sizing methods: sizing is what Manual J and Manual S exist for.
Use a shortcut, if at all, as a sanity check after the calculation — never instead of it.
Where code requires a load calculation (and how to verify it)
The clearest hook is the model residential code.
Section M1401.3 of the 2024 International Residential Code requires heating and cooling equipment and appliances to be sized in accordance with ACCA Manual S — or another approved sizing methodology — based on building loads calculated in accordance with ACCA Manual J, or another approved calculation methodology, with limited exceptions for multistage and variable-refrigerant-flow equipment.
ACCA says a load calculation performed per Manual J is required by national building codes and most state and local jurisdictions.
The fine print matters, though: the IRC is a model code, so its requirements apply where a state or city has adopted it, and the edition in force varies by place.
Whether the requirement is enforced on a given job is decided locally, by the jurisdiction issuing the permit.
Mind the scope as well: Manual J's own scope is residential — the single-family detached homes, small multi-unit structures, condominiums, townhouses and manufactured homes ACCA names.
What governs commercial load calculations sits outside the residential manuals covered here, so a commercial job is its own lookup with the building department — not a scaled-up Manual J assumption.
Career information, not legal advice: whether a load calculation is required on your job is set by the code your state or local jurisdiction enforces — confirm with the local building department before you skip it.
Verify the requirement where the job is permitted