Manual S is the ACCA standard for choosing heating and cooling equipment once you know the load. It takes the heating and cooling loads from a Manual J, checks real manufacturer performance data at the home's design temperatures, and confirms the selected equipment meets the load without oversizing beyond set limits. The current version is ANSI/ACCA 3 Manual S - 2023.
This guide is in outline form. The full version will walk through each step with worked examples.
In this guide
What Manual S does and does not do
- Why a load in BTU/h is not yet an equipment choice.
- The difference between nominal tons, AHRI rated capacity and capacity at your design conditions.
- Where Manual S sits between Manual J and Manual D.
Reading expanded performance data
- How to find total, sensible and latent capacity at the cooling design temperature and indoor wet bulb.
- Reading heat pump heating capacity at the heating design temperature.
- Using manufacturer software or the NEEP product data when expanded tables are not published.
Sizing limits for cooling, heating and heat pumps
- How Manual S sets acceptable ranges above the load for air conditioners, furnaces and heat pumps.
- Heating-dominant vs cooling-dominant homes and which load drives the selection.
- Rebate thresholds on top of Manual S: for example, NYS Clean Heat's 120% of building heating load and 115% of building cooling load justification triggers.
Heat pumps in cold climates
- Capacity at design temperature, balance points and backup heat.
- Full-load vs partial-load designs and how to document each.
- More detail in cold climate heat pump sizing.
Latent capacity and humidity
- Checking that sensible capacity covers the sensible load and latent capacity covers the latent load.
- Why an oversized system can meet the total load and still miss on moisture.
Documenting the selection
- What a Manual S worksheet or report should show.
- What rebate reviewers look for, including NEEP spec sheets and manufacturer data.
Try it before the full guide is live
Run an estimate in the heat pump sizing calculator or the AC sizing calculator to see how load and capacity relate. Free tools give estimates only, not a Manual S selection.
Tradewright selects real equipment against a room-by-room Manual J using manufacturer data at your design temperature. See equipment selection.
Questions
What is ACCA Manual S?
Manual S is the ACCA standard for selecting and sizing residential heating, cooling, dehumidification and humidification equipment. It takes the loads from a Manual J and checks manufacturer performance data at the local design conditions to choose equipment that meets the load without excessive oversizing. The current version is ANSI/ACCA 3 Manual S - 2023.
Can I size equipment from nominal tons?
No. Manual S works from the manufacturer's expanded performance data at your actual design conditions. Nominal tons and AHRI rated capacity are measured at test conditions that may not match your site. NYS Clean Heat says directly that sizing shall not be based on nominal size or AHRI rated capacities.
How does Manual S treat heat pumps differently?
A heat pump's heating capacity drops as the outdoor temperature falls, so you must check its capacity at the heating design temperature, not just its cooling size. You then decide whether the heat pump will carry the full heating load or use backup heat below a balance point, and document that choice.
What happens if the selected equipment is oversized?
Manual S sets limits on how far capacity may exceed the load. Rebate programs may add their own. NYS Clean Heat requires a sizing justification letter when heating capacity exceeds 120% of the building heating load in a heating-dominated project, or when cooling capacity exceeds 115% of the building cooling load in a cooling-dominated one.
Does Manual S come before or after Manual J?
After. Manual J produces the loads, Manual S selects equipment to meet them, and Manual D designs ducts to deliver the selected equipment's airflow to each room.
Sources
- ACCA: Manual S, 3rd edition (ANSI/ACCA 3 Manual S - 2023) (accessed 2026-10-16)
- NYS Clean Heat Heat Pump Program Manual, Version 3 (September 1, 2026), Section 3.2 System Sizing (accessed 2026-10-16)
- NEEP: Guide to Sizing and Selecting Air-Source Heat Pumps in Cold Climates (accessed 2026-10-16)