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Design Intent Document

The design intent document (DID), as noted in previous columns, is the backbone of the commissioning process. The DID is the definition of how the owner expects the building systems (HVAC, for example) to perform upon completion of the project. It represents the objectives of the project from a systems performance standpoint and is intended to improve communications and keep everyone on the team, i.e., designers, owner, contractors, commissioning consultant, operations and maintenance staff, etc., working towards a common goal.

The 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 the DID 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. For example, define indoor temperature and relative humidity requirements for each type of space. Quantitatively define indoor air quality; don’t just say “good indoor air quality.” This can be defined in terms of outside air flow per person, outside air flow per square foot, percentage 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. The 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 their load calculations, what type of systems (e.g., VAV, multizone, dual duct, etc.) will be designed and how they will be controlled, how the building will be zoned, etc.

The DID is necessarily 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 can not 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 revision must then be approved by the owner 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.

Finally, a good DID can become an invaluable operations and maintenance (O&M) reference throughout the life of the systems. If the O&M staff understand what the systems where intended (and not intended) to do, then they can more intelligently address occupant complaints. For example, if an occupant complains because the HVAC system can not provide him with a 68oF space, the O&M people can look up the DID and find out that the space temperature design criteria was 72oF. They need to tell the occupant that the system is incapable of achieving his desired condition, and that, if he has a problem with that, he should bring it up with the building owner who set the design criteria.

It is NOT the O&M personnel’s responsibility to apologize for design decisions nor to “fix” systems that are not broken. Too many building systems, especially HVAC, become all messed up after years of the O&M staff trying to meet various occupant requirements by “tweaking” the system out of balance and out of control. The DID can be a tool that helps them maintain the building systems as intended and not try to “squeeze” more out of the systems than can be expected.

 

COMMISSIONING: GETTING IT RIGHT

Engineered Systems, July, 2000

Rebecca Ellis, PE, LEED AP, CCP, CxA
President
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318
rteesmag@QSEng.com