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Integrated Design Intent

The basis of a strong commissioning program has always been a strong Design Intent Document (DID). Commissioning has also been a process focused on building “systems” as opposed to individual pieces of equipment. As such, most of the DIDs I’ve seen over the past few years have been organized in terms of systems; i.e., one section for the HVAC system, one section for the fire alarm system, one section for the lighting system, etc.

This is a great improvement over the first generation of DIDs which were organized around pieces of equipment; i.e., fans, pumps, heat exchangers, generators, transfer switches, light fixtures, etc. The owners and designers who thought in those terms were really regurgitating equipment schedule information and thinking it was a DID. A true DID is something that defines the owner’s desired performance from groups of individual pieces of equipment working together as systems. Examples include temperature, humidity, indoor air quality, noise levels, light levels, smoke removal rates, etc.

As I have been working with systems-based DIDs, however, I am seeing a tendency for them to be compartmentalized and divvied up between the various disciplines, particularly during the design phase. For example, only the HVAC engineers are looking at the HVAC system DID criteria and, conversely, the HVAC engineers are only looking at the HVAC system DID criteria. Similarly, the electrical engineers are focusing only on the electrical DID “sections,” fire protection engineers are only looking at the fire protection system “sections,” and so forth. Perhaps most common is the misconception that if the “building envelope” is not being commissioned, the architectural designers need not be concerned with the DID performance criteria at all.

This can and has led to one of the very problems commissioning is trying to help prevent, i.e., poor coordination between the disciplines. Whenever we try to break down a building’s performance criteria into discipline-specific sections, we encourage non-integrated design. One of the typical challenges in this approach is determining where certain criteria belong in the DID. For example, if you need exhaust fans on backup electrical power, do you put that redundancy criteria in the HVAC or electrical section?

I propose that DIDs need to take a larger view and present the functional performance criteria in terms of an integrated building. What is it that the owner is really looking for? In some of the more complex facilities the critical criteria varies from space type to space type, and that’s how the owner will be judging the adequacy of the final product. The criteria may also vary from end-user to end-user, i.e., a certain piece of equipment needs specific pressure, amperage, ventilation, etc. Let’s consider organizing our DIDs in this format. The benefits should be a more integrated design approach and a DID that owners may be able to relate to better.

One example would be to define the performance criteria for an auditorium which is often one space within a larger building. Instead of defining the HVAC, electrical, lighting, life safety, and architectural features all in discipline-specific sections, try defining exactly how the auditorium needs to operate as an integrated space.

Noise: Noise is often one of the most critical criteria for an auditorium. Architectural finishes and furnishing (carpet, wall covering, ceilings, seats, curtains, etc.) have a huge impact on final space noise levels. Of course, the HVAC system is another system that has a major impact on noise levels. If you were to bury noise level criteria into a single discipline (HVAC is the typical section), are you putting the onus for noise levels in the auditorium onto the mechanical designer alone? The mechanical engineer won’t take on that responsibility and, in many cases, will simply use the DID noise criteria as the “selection point” for duct sizing and diffuser selection without coordinating with the architects at all.

Lighting: Lighting is another system that always needs to be integrated, regardless of what type of space is being served. Achieving light level criteria is heavily dependent on ceiling heights; wall, ceiling, and floor colors; and electrical power supply. The lighting designer can not be made solely responsible for achieving foot-candle goals without collaborating with the architects and electrical engineers.

Relative Humidity: Some auditoriums, particularly those housing costly musical instruments, may have different relative humidity requirements than the rest of the building. That seems like an obvious criteria to sit in an HVAC-only DID section. However, that mindset ignores the fact that a different relative humidity level will necessitate a vapor barrier around the auditorium, and that is an architectural responsibility.

Today’s facilities demand a Design Intent Document that reflects the integrated nature of all building “systems”.


Engineered Systems, August, 2004

Rebecca Ellis, PE, LEED AP, CCP, CxA
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318