Manual J, Manual S and Manual D explained

Manual J, Manual S and Manual D are three design standards published by the Air Conditioning Contractors of America (ACCA). Used together, they answer three questions in order: how much heating and cooling does this house need, which equipment meets that need, and what ducts will deliver the air.

Manual	What it does	Current edition listed by ACCA
Manual J	Calculates the home's heating and cooling loads	ANSI/ACCA 2 Manual J, 2016 (8th Edition)[Source 1]
Manual S	Selects and sizes the equipment to match those loads	ANSI/ACCA 3 Manual S, 2023 (3rd Edition), with 2024 addenda[Source 2]
Manual D	Designs the duct system	ANSI/ACCA 1 Manual D, 2016 (3rd Edition)[Source 3]

Manual J: the load calculation

ACCA describes Manual J as "the national ANSI-recognized standard for producing HVAC equipment sizing loads for single-family detached homes, small multi-unit structures, condominiums, townhouses, and manufactured homes."^{[Source 1]} A Manual J calculation works room by room from the actual house: wall, ceiling and floor insulation, window area and type, orientation, air leakage, duct location, occupants, and the local design temperatures. The result is a heating load and a cooling load in Btu per hour at those design conditions.

Manual S: choosing the equipment

ACCA calls Manual S "a comprehensive guide for selecting and sizing residential heating, cooling, dehumidification, and humidification equipment."^{[Source 2]} It works from the manufacturer's performance data at the home's design conditions rather than the nominal size. ACCA says the 2023 edition added "expanded heat pump sizing tolerances to support electrification."^{[Source 2]}

Published summaries of the allowed sizing range differ, partly because they describe different editions:

• The DOE Building America Solution Center says cooling capacity at design conditions "should be between 95% and 115% of the design total heat gain (or 95% to 125% for heat pumps in IECC climate zones 4-8)."^{[Source 7]}
• The DOE Solution Center's cold-climate page says the 2023 edition allows heating-focused sizing for cold-climate heat pumps, including sizing to cover a set percentage of the design heating load.^{[Source 8]}
• NEEP's cold-climate sizing guide suggests matching capacity at the design temperature to between the full heating load and 115% of it, without auxiliary heat, or designing for 75% to 85% of the load with supplemental heat for the remainder.^{[Source 9]}

The current edition's exact limits are in the manual itself, which ACCA sells. Ask the contractor which Manual S edition and which sizing option they used.

Manual D: the ducts

Manual D, "Residential Duct Systems," sizes the supply and return ducts so each room gets the airflow the Manual J calculation calls for, at a static pressure the blower can handle.^{[Source 3]} When a heat pump replaces a furnace and air conditioner on existing ducts, the ducts may not have been designed for the new airflow; see Static pressure.

Why rules of thumb mis-size

A common shortcut is a fixed number of square feet per ton. The DOE Building America Solution Center states: "'Rules of thumb' are too often used to size comfort systems, which results in excessively oversized systems. This leads to increased cost, wasted energy, and too-frequent on and off cycling."^{[Source 4]} A DOE contractor guide says contractors "sometimes oversize central air conditioners because they use rules of thumb rather than performing load calculations," and that the contractor should use Manual J and Manual S.^{[Source 5]}

On ACCA's blog, building scientist Allison Bailes compared the usual rules of 400, 500 or 600 square feet per ton with 40 Manual J calculations he had done. The lowest result was 624 square feet per ton, and "the majority... are above 1,000 sf/ton."^{[Source 6]} That sample is one practitioner's, not a national study, but it shows how far the shortcut can be from a calculated load.

Square footage leaves out what drives the load: insulation, windows, air leakage, orientation and climate. Two houses of the same size can need very different equipment.

Oversizing causes short run times and frequent cycling (Heat pump short cycling) and, in cooling, poorer moisture removal. Undersizing a heat pump means more reliance on backup heat in cold weather. See Thermal and economic balance points and Heat pump backup heat and lockout temperatures.

Is a Manual J required?

Many residential codes require it. The International Residential Code section M1401.3, as summarized by Pima County, Arizona, requires heating and cooling equipment to be sized with ACCA Manual S based on loads calculated with Manual J or another approved method.^{[Source 11]} A DOE and New Buildings Institute permitting checklist for heat pumps includes "ASHP is sized using ACCA Manual S based on loads determined using ACCA Manual J," citing the energy code.^{[Source 10]} Whether your jurisdiction enforces this, and which code edition it adopted, varies. ACCA says Manual J is "required by national building codes and most state and local jurisdictions."^{[Source 1]}

What to ask for

• A copy of the Manual J report for your home, with the design temperatures used.
• The Manual S selection showing the chosen equipment's capacity at those design temperatures.
• For new or modified ducts, the Manual D design or at least the airflow and static pressure targets.

See Heat pump buying checklist and How to read a heat pump quote.