WHY ACCA Manual S Means Superior Equipment Sizing

There are three good reasons why you should use Manual S from the Air Conditioning Contractors of America (ACCA): (1) It's the nationally recognized standard; (2) it prevents problems associated with equipment oversizing and undersizing; and (3) as of 2009, it's a requirement in the International Residential Code (IRC).

Most contractors know that ACCA's Manual J is used to calculate heating and cooling loads. However, not everyone is aware that Manual J8 (Section 10-4) guides HVAC system designers to use ACCA Manual S, Residential Equipment Selection, to choose equipment that's the right size once the loads have been calculated.

Manual S is the ANSI-recognized, national standard providing clear instruction for interpreting and applying original equipment manufacturers' (OEM) expanded performance data. It instructs designers on how to select equipment that meets the application requirements (heating, sensible cooling, and latent cooling) at the design conditions that were used for calculating loads.

Manual S also prescribes equipment sizing limits. These limits ensure capacities will maintain customer comfort while preventing equipment oversizing and undersizing. Oversizing can result in excessive humidity, callbacks from comfort complaints, higher building costs (larger equipment is more expensive, more materials are needed, and more labor is used to install it), and higher-than-necessary energy consumption. Oversizing may also cause increased wear and tear on the equipment. Oversized equipment satisfies the control's set point faster, which leads to more equipment starts and stops. The sudden jolt of electricity to start motors and compressors causes component wear. Excessive starting creates extreme stress on mechanical components.

Finally, Manual S is a code requirement. A code modification adopted in the 2009 IRC now reads, “Heating and cooling equipment shall be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies.”

Different Climates Need Different Equipment

“We're very proud of the IRC adoption of ACCA Manual S,” says Ray Isaac, president, Isaac Heating and Air Conditioning Inc., Rochester, NY, and 2008-2009 chairman of ACCA. He adds, however, that the code adoption should have little or no effect on professional HVAC businesses. “Those [businesses] that have been following the nationally recognized Quality Installation Specification (ANSI/ACCA 5 QI - 2007) have been doing this for years.”

Still, some HVAC designers incorrectly select cooling equipment based solely on the equipment performance data published in the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) directory. “This practice isn't valid for most areas of America, and is avoided by professional HVAC businesses,” Isaac says. “The current Manual J8 completely defers to Manual S and the original equipment manufacturers' (OEM) expanded performance data. Simply stated, Manual S directs HVAC system designers to use the OEM expanded performance data and the heating and cooling loads from Manual J8, to select equipment that best meets the home's comfort needs,” he adds.

At the heart of the matter: a 36,000 BTU/hour cooling load doesn't necessarily equal a 3.0 ton unit. In the past, many contractors would default to whatever capacity was listed in the old ARI directory. That meant contractors in Miami, Phoenix, and Seattle — all extremely different climates — would be using the same reference, and expecting the same performance. However, a unit doesn't work the same in each one of those climates. Manual S not only provides equipment selection guidance, it also explains the science behind the guidance.

Manual S has four steps for selecting cooling equipment:

• Establish the design parameters

• Simple and Helpful

  Estimate the target airflow

• Search for candidate equipment combinations

• Evaluate and select the acceptable candidates (more than one candidate may be offered to the customer).

Diligent HVAC system designers should consider different product lines and equipment options when searching for equipment candidates. In addition to package and split systems, there are other cooling systems that will serve small cooling and heating loads (e.g., ductless mini-split systems, packaged terminal air conditioners, etc.).

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