Quick answer
Manual S equipment selection is the ACCA standard procedure for choosing heating and cooling equipment whose real capacity, at your home's design conditions, matches the loads from a Manual J calculation within set size limits.
It replaces "pick the next size up" with a check against the maker's performance data, covering sensible, latent and total cooling capacity, heating output and blower airflow.
Quick facts
The key facts about Manual S Equipment Selection, with sources:
- Publisher
- ACCA 1
- Current edition
- ANSI/ACCA 3 Manual S-2023 (3rd Edition); 2nd Edition 2014 1
- Starts from
- Manual J heating load (furnaces, boilers); Manual J cooling load (AC, heat pumps) 2
- Uses
- Manufacturer (OEM) expanded performance data, not AHRI ratings 2
- AHRI rating point
- 80°F dry / 67°F wet bulb indoors, 95°F outdoors 2
Key takeaways
- Manual S picks the equipment; Manual J only says how big the loads are.
- Equipment must meet the load at your design conditions, not at the AHRI test point 2.
- Cooling has to cover both sensible (temperature) and latent (moisture) load.
- Manual S sets upper and lower size limits for each equipment type 2.
- Rounding up to the next size is a warning sign ACCA training calls out 2.
What Manual S is
Manual S is ACCA's guide for selecting and sizing home heating, cooling, dehumidifying and humidifying equipment. It sets size limits so a correctly installed system can deliver comfort 1. It is the second step of the ACCA design chain, after the Manual J load calculation and before Manual D duct design.
Why a separate step? Because the capacity on a brochure is not the capacity in your house. A 3-ton air conditioner delivers more or less than 36,000 BTU/h depending on the outdoor temperature, the indoor air and the airflow. Manual S makes the designer read the maker's data at your conditions 2.
The loads it starts from must come from a sound Manual J, which ACCA describes as the ANSI-recognized standard for equipment sizing loads 5.
If the loads are padded, Manual S will faithfully pick equipment that is too big.
That is why a good bid shows both reports side by side, with the same house data in each.
The Manual S selection path
- Manual J loads → heating load, sensible cooling load, latent cooling load
- Choose equipment type → furnace, boiler, AC, heat pump
- Pull OEM expanded performance data → capacity at local outdoor design and indoor conditions
- Check sensible, latent and total capacity against the loads, within Manual S limits
- Check heating output (and backup heat for heat pumps)
- Check blower CFM at the design external static pressure → hand off to Manual D
How a designer applies Manual S
The ICC-hosted ACCA training lays out three steps 2.
- Start with the sizing values: the Manual J heating load for furnaces and boilers, and the Manual J cooling load for cooling-only units and heat pumps.
- Apply the Manual S sizing rules, which set upper and lower limits on total capacity for each equipment type.
- Use the manufacturer's performance data, read at the actual operating conditions, not the AHRI rating point.
The designer then records equipment type, model, sensible, latent and total capacity, heating output, auxiliary heat, blower CFM and external static pressure 2.
What Manual S checks for each equipment type
| Equipment | Sized to | Also checked |
|---|---|---|
| Furnace | Heating load (output BTU/h, not input) | Blower can move cooling airflow if paired with AC |
| Boiler | Heating load | Emitter output |
| Air conditioner | Cooling load | Sensible and latent split |
| Heat pump | Cooling load, then heating at design temperature | Backup heat; balance point |
| Variable-speed equipment | Cooling load | Low-speed capacity for mild days |
Worked example: input vs output on a real furnace data sheet
Assume a Manual J heating load of 42,000 BTU/h (an assumed load). Here is how two sizes of one real 80% AFUE furnace line compare, from the Goodman GR9S80 specification sheet 4.
| Model | Input (BTU/h) | Natural gas output (BTU/h) | Covers 42,000 BTU/h? | AC the blower can carry at 0.5 in. ESP |
|---|---|---|---|---|
| GR9S80 0403A | 40,000 | 32,000 | No | 3 tons |
| GR9S80 0603A | 60,000 | 48,000 | Yes | 3 tons |
| GR9S80 0804B | 80,000 | 64,000 | Yes, but far above load | 4 tons |
Two lessons. First, size to output, not the bigger input number on the label.
Second, the blower matters as much as the burner: if the house also needs 4 tons of cooling, the 60,000 BTU/h model's blower is rated for only 3 tons at 0.5 inch of static pressure.
The furnace with the right heat may have the wrong fan. Manual S catches both.
Where Manual S shows up
- In the permit package, beside Manual J and D, where the jurisdiction requires it 3.
- On a good bid, as the model numbers with capacity at design conditions.
- In manufacturer data: expanded performance tables and blower tables in the spec sheet 4.
- On the ACCA design review form, which puts load, equipment and duct checks on one page 2.
Benefits and limits of doing it right
What it does well
- Matches equipment to the house, so cycles are long enough to remove humidity.
- Catches blowers too weak for the cooling airflow.
- Uses real capacity at your design temperature, which matters most for heat pumps.
- Gives a written record for permits and disputes.
Where it falls short
- Needs maker performance data, which can be hard to read.
- Depends entirely on a good Manual J.
- Product steps (2, 2.5, 3 tons) may force a choice near a size limit.
Common mistakes Manual S is meant to stop
ACCA training lists what to watch for: using AHRI rated capacity instead of the maker's engineering data, not interpolating the data to the design conditions, misreading performance tables, rounding up to the next size, and pushing for equipment outside the sizing limits 2.
AHRI ratings are measured at 80°F dry bulb and 67°F wet bulb entering the indoor unit and 95°F outdoors. That is a fair test point for comparing units, but no one keeps a house at 80°F, and not every place has a 95°F design day 2.
Sources: [2]
How Manual S affects cost
Manual S often points to a smaller unit than the old one, which can lower the equipment cost and the running cost. It can also show that a cheaper furnace's blower is too weak, which saves a costly callback.
Lifespan and warranty are not set by Manual S, but oversized units cycle more often, which our maintenance and lifespan research looks at. For bid ranges, use the replacement calculator.
What to ask your contractor
- Ask for the Manual J heating and cooling loads, with sensible and latent split.
- Ask which model they chose and its capacity at your outdoor design temperature.
- For a heat pump, ask for capacity at your winter design temperature and the backup heat size.
- Ask for the blower CFM at the expected external static pressure.
- Ask if the unit is within Manual S limits, and why if it is not.
Keeping the selection valid over time
- Keep filters clean; a clogged filter raises static pressure and cuts the airflow Manual S assumed.
- Do not close many supply vents; it changes airflow.
- Redo the selection after big envelope changes, such as new windows or insulation.
- Keep the selection paperwork with the warranty documents.
Signs equipment was not selected well
- Short run times and clammy air in summer.
- Big temperature swings or loud air at registers.
- Heat pump that leans on strip heat on mild days.
- Furnace that trips its limit switch, a sign of too little airflow for the burner.
- Call a contractor who can measure airflow and static pressure, and check the original selection.
Code status
IRC M1401.3 requires heating and cooling equipment to be sized according to ACCA Manual S based on loads from Manual J or another approved method.
ICC training material cites the 2015 IRC M1401.3 and IECC R403.7 2. Pima County, Arizona, states the same requirement and asks for Manual J, S and D at plan review 3.
Edition. ACCA lists the 3rd Edition, ANSI/ACCA 3 Manual S-2023, with Addenda A and B (2024), and says it helps show compliance with the IRC and the Uniform Mechanical Code 1. Which edition applies depends on the code your jurisdiction adopted.
Manual S vs related terms
| Term | Role | Link to Manual S |
|---|---|---|
| Manual J | Calculates loads | Input to Manual S |
| Manual D | Sizes ducts | Uses the blower CFM and static pressure Manual S confirmed |
| Rated capacity | Output at the AHRI test point | Not used for sizing 2 |
| Tonnage | Nominal size | Real capacity varies by conditions |
| Static pressure | Duct resistance | Sets how much air the blower delivers |
Common misconceptions
- Myth A 3-ton unit delivers 36,000 BTU/h.
- Reality Only at the rating point; at your design conditions it may be more or less 2.
- Myth Furnaces are sized by input BTU.
- Reality Size to output; an 80% furnace rated 60,000 input puts out 48,000 4.
- Myth If unsure, go one size up.
- Reality ACCA training lists rounding up as a mistake 2.
Climate and equipment selection
Selection is climate-specific because capacity is read at your outdoor design temperature. In hot, dry Arizona, sensible capacity dominates.
In humid Florida or Georgia, the latent split matters more, so a unit that cools well but dries poorly can fail Manual S. In cold Ohio, heat pump capacity at the winter design temperature decides whether backup heat is needed.
When this matters to you
Manual S matters any time someone proposes a size. If two bids differ by a ton, the one with a Manual J and S behind it is the one to trust.
Next step: compare bids in the quote comparison tool and check the airflow side on our blower motor page.
Questions about Manual S Equipment Selection
What is ACCA Manual S?
Manual S is ACCA's standard for choosing home heating and cooling equipment. It takes the loads from a Manual J calculation and checks the maker's performance data at your design conditions to pick a model within set size limits.
What is the difference between Manual J and Manual S?
Manual J calculates how much heat the house gains and loses. Manual S chooses equipment that meets those loads. You cannot do Manual S without a Manual J first.
Is Manual S required?
The International Residential Code, section M1401.3, requires equipment sized by Manual S based on Manual J loads. Enforcement varies. Some jurisdictions, such as Pima County in Arizona, require the calculations at plan review.
Why not use the AHRI rating to size equipment?
AHRI ratings are measured at fixed lab conditions: 80°F dry and 67°F wet bulb indoors and 95°F outdoors. Your home and climate differ, so Manual S uses the maker's expanded data at your actual design conditions.
Should a furnace be sized by input or output?
By output. An 80% AFUE furnace with a 60,000 BTU/h input delivers about 48,000 BTU/h of heat, as the Goodman GR9S80 data sheet shows. Compare output to the Manual J heating load.
What edition of Manual S is current?
ACCA lists the 3rd Edition, ANSI/ACCA 3 Manual S-2023, with Addenda A and B from 2024. The 2nd Edition from 2014 is still sold. The edition your code requires depends on what your state or city adopted.
How does Manual S handle heat pumps?
A heat pump is sized from the cooling load, then checked for heating capacity at the winter design temperature. Backup heat covers any gap. In cold climates this check often decides the model.
Can a contractor go bigger than Manual S allows?
Manual S sets upper limits for each equipment type, and ACCA training lists pushing past them as a mistake. If a bid exceeds them, ask for the reason in writing.
Does Manual S cover dehumidifiers and humidifiers?
Yes. ACCA describes Manual S as a guide for selecting and sizing heating, cooling, dehumidifying and humidifying equipment. In humid climates the latent capacity check often decides whether a cooling unit alone can hold indoor humidity in range.
Sources
- ACCA, Manual S Residential Equipment Selection (ANSI/ACCA 3 Manual S-2023), retrieved .
- ICC / ACCA, An Overview on ACCA's Residential HVAC System Design Process (PMG educational program), retrieved .
- Pima County, Arizona, HVAC Sizing Requirements, retrieved .
- Goodman, GR9S80/GD9S80 specification sheet SS-GR9S80_GD9S80-R32, retrieved .
- ACCA, Manual J Residential Load Calculation (8th Edition), retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Procedure and AHRI rating conditions from the ICC-hosted ACCA training document; edition from ACCA's Manual S page; furnace input, output and blower ratings copied from the Goodman GR9S80 specification sheet, all read 10 October 2026. The 42,000 BTU/h load is assumed.
Suggest a correction. We fix errors and say what changed.