Commissioning: Outline Objectives With DID (July 2000)
Testing CriteriaThe DID is the standard by which all systems are judged upon completion of the project. One of the highlights of the commissioning process is testing of the systems. The DID provides the criteria by which the systems are deemed to pass or fail the tests. As such, it is imperative that these criteria be quantitative and verifiable. It is not sufficient for the DID to require a “comfortable” building; it is necessary to define what “comfortable” is for each particular project. One example would be by defining indoor temperature and relative humidity requirements for each type of space. Indoor air quality is quantitatively defined; not just stated as “good indoor air quality.” This can be defined in terms of outside airflow per person, outside airflow per sq ft, percentage of carbon dioxide, etc.
Without such measurable goals, there will inevitably be disagreements at the end of the job about whether a space is comfortable and safe, since there are as many opinions about these topics as there are people in a building. Commissioning testing is designed to prove that the design and construction team has achieved each and every goal defined in the DID. If it can’t be tested and unequivocally verified, it has no place in the DID.
As an aside, there is also a document called the Basis of Design that is the place where the designers provide a narrative of how they plan on achieving the design intent goals. It provides information regarding which codes and standards are referenced, what design outdoor air conditions are used in load calculations, what type of systems (e.g., variable air volume, multizone, dual duct) will be designed and how they will be controlled, how the building will be zoned, etc.
Keeping on Track or Jumping the RailsThe DID is by necessity a dynamic document, as the owner’s expectations for a building may change throughout the course of the design and construction. It is ideal for the owner to have a DID prepared prior to contracting with the designers so that the designers understand exactly what the owner is requesting of them. As the design develops and cost estimates are prepared, it may become evident that the original DID cannot be achieved within the owner’s budget. That means that the owner either needs to find more money or the DID criteria needs to be loosened in order to allow for a more affordable system. In fast track projects, ideal performance goals may be unrealistic because of the long lead times for some equipment. In this case, the owner needs to decide between the performance requirements (DID) or the project schedule. These scenarios are all too familiar to the design engineers of the world. Their designs are torn apart by project managers who need to meet budgets and schedules and who start “value engineering” the professionals’ designs during a few meetings. Although the design engineers are a part of these meetings, there is typically such a panicked atmosphere that when they speak up and say, “Sure we can change that, but it will result in …” the owner hardly even hears what will be lost as a result of the design change. It isn’t until the end of the project when the systems don’t perform as originally anticipated that the consequences of the re-engineering effort become evident to the building owner. At that point, owners have a hard time remembering the engineers’ warnings during the design phase of the project, and the finger pointing begins.
The DID is a vehicle by which this miserable experience can be avoided. Whenever budget or schedule dictate a design change which sacrifices some aspect of the original design intent goals, this change is documented in a revised DID. The owner must then approve the revision as the “new” criteria for building system performance. This documentation helps to protect everyone on the commissioning team from a bad end to a good project.