Heating and cooling # What is ACCA Manual S? Manual S (ANSI/ACCA 3 Manual S) is the ACCA standard that determines residential equipment selection — choosing equipment that matches the calculated load. It is one of the three procedures a correct installation uses, alongside Manual J for the load and Manual J and Manual D. Designation ANSI/ACCA 3 Manual S **Sizing reality. **Correct sizing requires a Manual J load calculation, not a rule of thumb and not a match to the old unit. Oversized equipment short-cycles, controls humidity poorly, and fails sooner. [How a Manual J load calculation works](https://hvacbuddy.shop/sizing/manual-j/). ## What does Manual S actually decide? Manual J says how much heating and cooling the house needs. Manual S is the next step: choosing equipment whose *actual* performance at your design conditions matches that load. That distinction matters because nameplate capacity is measured at standard test conditions, not at your conditions. A nominal three-ton unit does not deliver three tons of cooling on a 100°F day in a hot-dry climate, and Manual S is the procedure for accounting for that rather than assuming it away. ## Why does this go wrong so often? Because the shortcut is easy and invisible. Round the Manual J result up to the next nominal size, and you have skipped Manual S. The result is an oversized system that short-cycles, controls humidity poorly and wears out sooner — and nothing on the invoice shows the step was skipped. Ask which model was selected and why that capacity matches the calculated load. A contractor doing the work properly can answer in a sentence. ## The three standards together What each ACCA standard decides. StandardWhat it decides ANSI/ACCA 2 Manual J - 2016 Residential load calculation — how much heating and cooling the house actually needs.ANSI/ACCA 3 Manual S Residential equipment selection — choosing equipment that matches the calculated load.ANSI/ACCA 1 Manual D Residential duct design — sizing the duct system to deliver the airflow the equipment needs. ## How is the right equipment size decided? Nearly every persistent comfort problem traces back to equipment size, and size is decided before the first box arrives. The standard is the ACCA **Manual J** load calculation (ANSI/ACCA 2 Manual J - 2016), which totals the heat gain and loss of your specific house: orientation, window area and type, insulation levels, air infiltration, internal gains, and your local design temperature. What a Manual J is *not*: square feet per ton, and not the old unit's tonnage. The first ignores everything that makes one house different from another. The second assumes whoever sized the old equipment got it right, and propagates their error for another fifteen years. Oversized equipment reaches the setpoint and shuts off before it has dehumidified, so the house ends up cold and clammy at the same time. Every start is also the hardest moment in a compressor's life, so wear accelerates. And you pay for the surplus capacity twice — once to buy it and again to run it. Ask for the completed Manual J in writing, not the claim that one was done. [What it contains and how to read it](https://hvacbuddy.shop/sizing/manual-j/). ## Which efficiency numbers can you actually compare? If you are comparing efficiency figures, check which generation they belong to first. On January 1, 2023 DOE changed the test procedure and renamed three metrics: SEER became **SEER2**, EER became **EER2**, and HSPF became **HSPF2**. The current procedure measures at a higher external static pressure, much closer to what equipment meets once installed in a real house on real ductwork. DOE is explicit about the consequence: "Although SEER2, HSPF2, and EER2 ratings are similar to ratings using the original metrics, they are not directly comparable. Compliance with the new standards for each model must be determined based on its measured SEER2, HSPF2, or EER2." In practice: a SEER figure from a 2019 article cannot tell you whether a quote reading SEER2 is good, bad or identical. That is why we publish no conversion factor. We do not publish a SEER-to-SEER2 conversion factor. DOE states that ratings on the new metrics are "not directly comparable" to ratings on the original metrics, and that compliance must be determined from the measured SEER2, EER2 or HSPF2 value. Approximate multipliers circulate widely, but they vary by equipment type and configuration, and using one to compare an old article's number against a number on today's quote is how homeowners end up believing they were offered less efficiency than they were. Compare like with like: ask for the SEER2 figure, and check it against the AHRI certificate. The nationwide minimum for a split-system air conditioner is 13.4 SEER2, and for a split-system heat pump 14.3 SEER2 with 7.5 HSPF2. [What each metric measures](https://hvacbuddy.shop/efficiency/) · [minimums by region and state](https://hvacbuddy.shop/efficiency/standards/). ## What should a bid show you? Three bids rarely quote the same job, so comparing headline price compares three different things. The one that was done properly shows its work, and you can check it line by line: - Model numbers for the outdoor unit, indoor coil, and air handler or furnace. - The **AHRI reference number** for that exact combination. A rating applies to a matched system, not to an outdoor unit on its own. - The SEER2 and, on a heat pump, the HSPF2 of that combination. - The Manual J result the sizing came from. - The measured static pressure and any duct work included. - The permit, and who pulls it. - Labour warranty, separate from parts, and whether registration is required. [The full red-flags checklist](https://hvacbuddy.shop/hiring/). ## Why do the ducts matter as much as the equipment? Equipment capacity is rated at a specific airflow. If the ducts cannot move that air, the equipment cannot deliver what its label promises, and the homeowner concludes they were oversold. The number that reveals it is **static pressure**: the resistance the blower works against, measured in inches of water column. It is to a duct system roughly what blood pressure is to a person. A contractor who measures it before quoting a replacement is doing the job; one who does not is assuming your ducts are fine. Duct design has its own ACCA standard, **Manual D**, and it is the step most often skipped on a replacement because the ducts already exist and reusing them costs nothing up front. [What Manual D determines](https://hvacbuddy.shop/ductwork/manual-d/) · [ducts and airflow](https://hvacbuddy.shop/ductwork/). ## How can you verify any of this yourself? Everything regulatory on this site traces to a primary source and carries the date it was read: DOE for efficiency standards, EPA for refrigerants and Section 608, the IRS for tax credits, and ACCA for load calculation. The sources for this page are listed at the end with the date each was consulted. If you find a different figure elsewhere, check two things before deciding which to believe: whether the page says which generation its efficiency metric belongs to, and whether it carries a visible review date. Most of the outdated content in this category fails both tests. Sources (2) - Air Conditioning Contractors of America — [Technical Manuals — Manual J, Manual S, Manual D](https://www.acca.org/standards/technical-manuals) (accessed 2026-08-17) - U.S. Department of Energy — [Central Air Conditioners — Conservation Standards guidance (Final, October 5, 2022)](https://www1.eere.energy.gov/buildings/appliance_standards/pdfs/2023_CAC_Standards_FAQ_10-5-2022_Final.pdf) (accessed 2026-08-17) By [HVACBuddy Editorial Team](https://hvacbuddy.shop/authors/editorial-team/). Last reviewed: 2026-08-18. HVACBuddy publishes independent information. We do not install or service equipment, we do not dispatch contractors, and we do not sell leads. Nothing here is engineering, tax, or legal advice.