ACCA swiftly followed with its own standard, "Manual N (Revised) - Load Calculation for Non-Residential Buildings, Three Stories and Less."

The proposed ASHRAE document would cover all sizes of commercial buildings, and this is really the heart of the issue. ACCA wants a document that is specific to its core membership, the light commercial contractors, and felt that it already had a potential standard in Manual N.

As both associations argue the merits of these standards, some folks on both sides acknowledge that the commercial market and system designers may be better served by not having a standard. They point out that once a standard mandates what must be done, there is the possibility of increased litigation.

A meeting scheduled to be held June 8 is designed to clarify the issues for both organizations.

Can Bridges Be Built?

"I want to do everything in my capacity to build the bridges rather than the walls," said Greg Leisgang, 2005-2006 chairman of ACCA. "When ASHRAE released their standard, it kind of set a clock ticking, and we had to respond."

Leisgang said he is hopeful that in the June 8 meeting or shortly thereafter, these two organizations can reach "an understanding of what's in the best interest of the industry."

"We've got to work doggone hard to do what's right for this industry," he said. "We have got to re-establish a foundation of trust."

Ron Vallort, P.E., president of ASHRAE, said, "ASHRAE cannot speak to ACCA concerns about the standard. I can say that we have been and will continue working with ACCA in developing commercial load calculations. One of my goals this year is that ASHRAE must look outside our foundation to strengthen ourselves. We continue to work with ACCA to develop this and other standards."

The ASHRAE point of view is rooted in many years of experiences with its membership.

"ASHRAE published its first manual on cooling and heating load calculation nearly 30 years ago. Since that time, ASHRAE has funded research on this topic, which has been incorporated into its handbooks.

"So, ASHRAE has a long history of providing guidance in this area. The next obvious step was to write a standard to establish a minimum level of care that would apply to any method. That standard is now out for public review, and ASHRAE will respond to the needs of its members on whether they prefer the requirements in the standard or would prefer to receive guidance in the form of information in the handbook, a guideline, or special publications.

"We also must be responsive to comments from members of other organizations such as ACCA, to make sure the needs of the industry are met as well as the needs of the public."

Vallort responded to those who might question the need for any standard at all.

"ASHRAE's goal in providing this standard is to establish the criteria of an appropriate calculation process to ensure an accurate estimate of the air conditioning and heating load in a building.

"The load serves as a basis of the selection and sizing of the systems and equipment that are required to accomplish inherent psychrometric processes, such as conditioning for outside air, reheat, dehumidification, and humidification. ASHRAE thinks ensuring a more accurate estimate would benefit not only contractors but building occupants as well," said Vallort.

Load Calculations

In a press release on its proposed standard issued by ASHRAE on May 17, Chris Wilkins, chair of the ASHRAE committee writing the standard, said, "The biggest challenge is allowing for enough flexibility that an engineer can choose the appropriate tool (method) for calculating the load while at the same time being strict enough that an appropriate level of accuracy is attained for all methods."

He further explained, "Standard 183P does not dictate a particular method or program but does establish a minimum level of care that would apply to any method. The standard draws information from the society's handbook and publications and assigned criteria is based on accepted practices for determining peak loads."

Computer vs. Hand Calculations

The society's document contends that representative building loads cannot realistically be calculated without a computer. According to 183P's foreword, "The requirements in this standard that relate to the load calculation method are highly technical as they relate to these simplifications of the fundamental heat transfer interactions. If a method oversimplifies the problem, then an inaccurate load estimate can result.

"Complying with this standard will require knowledge of the underlying principles of the methods used and the techniques that these methods use to address the fundamental heat transfer interactions."

Can "credible" load calculations for light commercial buildings be done by hand? Manual N says that yes, hand calculations are acceptable in many instances, although there is certainly nothing wrong with using computer-generated procedures if you prefer.

Manual N's stated purpose and intent is "to establish minimum requirements for credible load-estimating procedures for light commercial nonresidential buildings three stories in height and less."

"The intent of this standard is to provide pragmatic guidance that is transparent to HVAC contractors and code officials. ... to validate procedures that are typically presented in an accredited classroom or a professional development seminar. ... [and] to validate procedures that size HVAC equipment for occupant comfort and system efficiency."

ACCA's document also points out the distinctive nature of light commercial buildings. According to the Manual N preface, "Light commercial buildings are a subset of commercial buildings, but they have a number of distinctions that effect procedural requirements. Some of the primary differences relate to practitioner expertise, mechanical equipment, customer expectations, and construction turnaround times.

"Additionally, for the bulk of small buildings - churches, convenience stores, small office suites, retail, garages, etc. - project economics do not justify the services of professional engineers for new construction or system upgrade. Therefore, mechanical contractors and other design professionals must produce solutions (equipment sizing, equipment selection, equipment location, etc.) that balance occupant comfort, efficiency, and IAQ with customer expectations and budgets."

The document admits that for large buildings such as high-rise offices and other such structures, "There are thousands of variables, considerations, and nuances that affect load calculations. ... For these buildings, the approach needs to be rigorous and is the purview of professional engineers, architects, and other design professionals.

"For light commercial buildings, it is necessary and appropriate that the ‘large building' procedures be distilled to useful, simplified, time-saving formats that effectively produce reasonable and defensible estimates of equipment sizing loads."

The ACCA standard is also said to "determine if a load calculation manual, software product, and such tools meet a minimum level of performance and comply with industry standards." The standard allows both the use of "sophisticated variable-intensive, computer-only approaches" and "more formulistic approaches," some of which include "credible procedures that can be performed by hand."

ACCA's specific commentary on the standards, as well as the proposed Manual N Revision, is posted online at www.acca.org/tech/ansi/. Proposed ASHRAE Standard 183P may be downloaded from www.ashrae.org/template/PDFDetail/assetid/43750.

Publication date: 06/06/2005