RP-742 -- Direct Digital Control System Documentation Practices PDF

The purpose of this research was to identify, analyze and test methods of direct digital control system documentation available both within and outside the heating, ventilation, air-conditioning industry.

Several general issues were addressed during the identification phase of the research. First, literature and standards focusing on direct digital control systems, documentation recommendations, and documentation problems were reviewed. Second, example projects from design engineers, control vendors, and equipment manufacturers were reviewed for direct digital control system documentation completeness. Third, control vendors who produce and support direct digital control system software were asked to submit their application software for review. Finally, direct digital control system documentation surveys were conducted with design engineers, control vendors, equipment manufacturers, and end users to assess and evaluate current direct digital control system documentation methods.

In the next phase, analysis of the documentation methods, the focus shifted to issues related to specific direct digital control system documentation problems. First, the application software packages received from several vendors in the identification phase, once operational, were evaluated using several criterion. Also, direct digital control system control logic documentation for a variable air volume system was solicited and evaluated from control vendors who had submitted application software for review.

The beginning of the test phase was marked by gathering and summarizing information from the identification and analysis phases, and developing preliminary documentation recommendations for design engineers, control vendors, and equipment manufacturers. Testing of these preliminary recommendations revealed that engineers estimated an average increase in effort of 14 to 22%, depending on the complexity of the project, if the documentation recommendations were adopted into standard office practice. For control vendors, the average increase in effort was estimated to be 30 to 35% over current practice, again depending on the complexity of a project. Equipment manufacturers, because they need to follow the recommendations only when initially creating the documentation, estimated increases in effort which ranged from 13% too as much as 200% over current factory preprogrammed equipment documentation practice. For equipment manufacturers, this was dependant on equipment-type/equipment combination.

The results of this study, at first glance, seem to increase the total building cost and documentation work load quite significantly. However, that is not the case. In particular for a building which costs $150/square foot, where $25/square foot is devoted to control system cost, the estimated percent increase in overall project cost would be only 1.4 percent of the total cost. Equipment manufacturers, on the other hand, need only create the documentation once for factoryshipped equipment. Therefore, when amortized over the number of units sold, the increase in documentation work load is minimal. Once tested, the preliminary recommendations were then modified as suggested by individuals in each of the fields. This testing process was a "reality-check" as to the applicability of the direct digital control system documentation recommendations for each field. The final recommendations, therefore, reflect the opinions, suggestions and practical solutions by each field toward the industrywide improvement of direct digital control system documentation.

The work load will undoubtedly increase for all the disciplines. Possible benefits, however, must also be considered. They include increased documentation quality, flexibility, coordination, clarity, automation potential, standardization, and applicability in all phases of a building mechanical system construction project. All of these benefits are valuable because they avoid several of the problems that exist with current DDC system documentation. However, end users and owners of these systems must understand that the benefits justify the additional cost.