Energy test guide · verified October 12, 2026
Manual J, S and D: the math behind a right-sized heating and cooling system
Before anyone sizes a new furnace, heat pump or air conditioner, there’s a standard way to work out how much heating and cooling the house needs. ACCA Manual J calculates the loads, Manual S picks equipment to match them, and Manual D designs the ducts. Where the model codes are adopted, they require the sizing to follow Manual S and Manual J, or another approved method.
Three manuals, three jobs
Manual J
Heating and cooling loads
8th Edition (ANSI/ACCA 2 Manual J-2016), the ANSI-recognized method for residential loads. ACCA says codes and most state and local jurisdictions require a Manual J load calculation.
Source: ACCA Manual J ↗ · verified October 12, 2026
Manual S
Choosing and sizing equipment
3rd Edition, Version 1.02 (ANSI/ACCA 3 Manual S-2023, with Addenda A and B 2024), new since 2023. Covers heating, cooling, dehumidification and humidification equipment.
Source: ACCA Manual S ↗ · verified October 12, 2026
Manual D
Duct design
3rd Edition (ANSI/ACCA 1 Manual D-2016), for designing the duct system.
Source: ACCA Manual D ↗ · verified October 12, 2026
When codes require it
As quoted by ACCA from the 2018 model codes:
- IRC M1401.3 and IECC R403.7 / IRC N1103.7 (mandatory): heating and cooling equipment sized per Manual S based on Manual J loads, or other approved methods.
- IRC M1601.1: duct systems built per Manual D, the manufacturer’s instructions or other approved methods.
DOE’s 2021 IECC field-study checklist confirms R403.7 carries into the 2021 IECC. The 2021 IRC text wasn’t opened, so the M1401.3 and M1601.1 wording above is the 2018 version. All of this applies only where a jurisdiction has adopted these codes, and local amendments may change it.
Sources: ACCA 2018 model code references ↗, DOE 2021 IECC field-study forms ↗ · verified October 12, 2026
How to get it done
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Ask for the calculation with the quote
Ask each HVAC contractor for the Manual J load report, the Manual S equipment selection and, if ducts are changing, the Manual D design.
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Compare the equipment to the load
Manual S selects equipment from the Manual J loads, so the quoted capacity should trace back to the calculation.
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Do envelope work first, or tell them about it
If air sealing or insulation is planned, ask for the calculation to reflect the house as it will be, since the loads set the equipment size.
What it costs: no official source publishes a price for a Manual J calculation, so we don’t quote one.
Sources: ACCA Manual J ↗, ACCA Manual S ↗, ACCA Manual D ↗ · verified October 12, 2026
If the quote and the calculation don’t line up
- Ask the contractor to show how the equipment was selected from the loads under Manual S.
- If the ducts are part of the job, ask whether they were designed to Manual D; leaky ducts are a separate problem a duct leakage test measures.
- Get a second quote with its own load calculation if the answers don’t hold up.
Common questions
Is a Manual J required when I replace my furnace, heat pump or AC?
It depends on what your jurisdiction has adopted. As quoted by ACCA from the 2018 codes, IRC M1401.3 and IECC R403.7 (IRC N1103.7) require heating and cooling equipment to be sized per Manual S based on Manual J loads, or by another approved method, and DOE’s 2021 IECC field-study checklist confirms R403.7 carries into the 2021 IECC. These apply only where the codes are adopted, and local amendments may change them. Your local building department can tell you what applies.
Sources: ACCA ↗, DOE BECP ↗ · verified October 12, 2026
What’s the difference between Manual J, S and D?
What’s new in Manual S?
The current Manual S is the 3rd Edition, Version 1.02 (ANSI/ACCA 3 Manual S-2023, with Addenda A and B from 2024). It’s new since 2023 and covers heating, cooling, dehumidification and humidification equipment.
Source: ACCA ↗ · verified October 12, 2026
What does a Manual J calculation cost?
No official source publishes a price, so we don’t quote one. Ask HVAC contractors whether a Manual J report comes with the quote.
Sources
Every source below was opened and checked on October 12, 2026. A date in a source’s name is the “last updated” date shown on that page.
- ACCA: Manual J Residential Load Calculation ↗
- ACCA: Manual S Residential Equipment Selection ↗
- ACCA: Manual D Residential Duct Design ↗
- ACCA: 2018 Model Code References (IRC M1401.3, M1601.1; IECC R403.7) ↗
- DOE BECP: 2021 IECC Single-Family Field Study data collection forms (zip, Sep 2024; checklist item R403.7) ↗
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