In our previous outing, we focused on the process of engineering and the need for a plan to get from A to B most efficiently and effectively. We highlighted that reinventing the wheel or flying by the seat of your pants may not always be the most efficient ways to develop standard operating procedures. Tom Cruise stopped by (really, read the article) and helped drive home the point that the P100 offers valuable tools and insights that can make the design process more productive and less painful.
In this article, we pick up the thread and move over to the P100's approach to systems in an effort to hammer home last month's argument. Specifically, that regardless of whether you are working in the public or private sector, there is value in the P100.
Challenge is a Good ThingThe P100 can enlighten or confound, depending on your perspective and even what chapter you may be reading at the time. That is partly due to the fact that the P100 stakes a position on just about everything. And in so doing, there is probably something to test just about everyone.
But even if your opinions differ with the P100, you cannot deny that a well-stated position, pro or con, can either educate the technical tenderfoot or confront the nay-saying expert. Either way, the challenge leads to professional growth, and ultimately a collective progression in our field.
So what does the P100 say, anyway? Well, it provides specific direction on ventilation strategies, moisture control, central plant sizing, air distribution, piping systems, and building pressurization to name just a few topics. And the system selection process is built around an acceptable baseline design premise. Simply put, there is a preferred mechanical baseline system, and any deviations from that baseline must be vetted and approved before being allowed.
Now the GSA is bright enough to know that no single design is appropriate for every public building across the fruited plain. For example, moisture issues in Florida differ greatly from those in say, Arizona. Energy recovery may be more cost effective in a climate with extremes like Michigan than in Southern California's more moderate climes. So the P100 allows certain deviations within the baseline. But the basic building blocks for a baseline system remain consistent for all.
The Baseline SystemsThe baseline mechanical system in the P100 is important for a number of reasons, not the least of which is that it defines a system that, for the most part, is proven. Like the shoes you only wear on the weekend, parts of the P100 design are comfortable because they are broken in and have worn well. But comfort can breed complacency, so there are other parts of the P100 baseline that push the envelope just a little bit.
First, it should be noted that the P100 divides the building into perimeter and interior zones and mandates unique systems for each. While this makes sense in most buildings (a distinct core and border/perimeter) there are some footprints that don't lend themselves to such a simple separation. For example, long slender buildings tend to be too shallow to support two systems, and deviations from the two-system approach are appropriate. But in most cases, the core and border strategy is useful.
For the perimeter zones of new construction, a completely independent DOAS is required. In addition to this system (which we will discuss in greater detail later), the border zones also require a separate heating and cooling system. The P100 allows the designer to choose from three alternates:
- A traditional overhead VAV system with shut off boxes and finned tube radiation (FTR) heating;
- An overhead VAV system with fan-powered boxes and hot water heating coils; or
- In cases with low latent loads, an underfloor air distribution (UFAD) system, supplemented with two-pipe perimeter FTR (above the floor) for heating.
For the interior zones, a similar assortment and configuration is allowed with the one difference being that the DOAS can be ducted to the return airside of the VAV air-handling systems. In addition, enthalpy heat recovery is required for interior systems with an outdoor air requirement exceeding 30% of the total; most units over 3,000 cfm are required to have enthalpy economizers; and, where feasible, waterside economizers are encouraged.
Now it doesn't take a psychic to figure out which of these systems are "comfortable" and which are less so. Overhead VAV is the old-slipper equivalent of air distribution systems. DOAS and UFAD, on the other hand, are systems that are just now gaining acceptance in the North American market.
Doin' DOASOne area where the GSA has taken a progressive stand is in the area of ventilation. In practice such systems are usually difficult to sell to the owner or architect because of preconceived notions of higher first cost.
With the GSA, no sale is needed. Demand-based dedicated ventilation systems for the interior zones and the perimeter zones are required. These systems address ventilation in a simpler manner, facilitate latent cooling, and maintain proper building pressure.
Regarding the simplification of ventilation design, by providing dedicated ventilation units in lieu of bringing in outside air at every AHU, the ventilation strategy is simplified significantly. With one or two ventilation units, outdoor air modulation, filtration, and tempering are centralized instead of disbursed throughout the building. And with the heightened sensitivity to bioterrorism, the minimization of outdoor intakes and the monitoring required is not just easier to accommodate architecturally but is safer as well.
Because outdoor air represents the largest contributor to latent load in many climates, providing a centralized DOAS provides an opportunity to control humidity from one unit. In particular, the DOAS is controlled to deliver ventilation air at a significantly low dewpoint to facilitate latent cooling when supplied directly to a space (perimeter zones) or mixed with the return air of the VAV (interior zones).
In most cases, this saves energy because the entire airstream does not have to be cooled down and then reheated. But it also keeps the moisture associated with dehumidification concentrated primarily in one place (the DOAS), thus eliminating or reducing the propensity for mold growth in the rest of the systems. Simply put, dry drain pans don't slime up.
And last, the GSA takes a very proactive and unique approach to building pressurization and off-hour control of same.
Handling the PressureAs stated above, the P100 calls for some design elements that many consider less than ideal. One area where I have witnessed a "too much of a good thing" attitude from designers in particular is the proactive building pressurization strategy described in the P100.
Specifically, the P100 calls for a perimeter DOAS that incorporates the following design features:
- 100% OA with filtration;
- VAV modulation;
- Leaving wetbulb of 50°F;
- Leaving drybulb between 70° and 78°; and
- A flow station indicating supply quantity at all times.
During occupied hours, the unit operates to simultaneously provide the code dictated ventilation quantity and maintain a positive building pressure (perimeter zone relative to outdoors). During unoccupied hours, the system operates at 40% capacity, still providing sufficiently dry air to pressurize and dehumidify the building and envelope. If the outdoor dewpoint drops below 37°, when moisture infiltration is no longer a concern, the unit has the capacity to maintain a neutral building pressure.
The interior DOAS has similar design constraints, except that it is used purely for ventilation, not pressurization, and during unoccupied hours it is shut down.
We all know the value of proper building pressurization, especially in humid climates where mold and moisture have created a whole new area of expertise for America's trial lawyers. The catch is that not every locale requires the aggressive system posture mandated by the P100.
But again, in this author's opinion, there is value to all designers in understanding the baseline ventilation strategy regardless of the approach you ultimately implement. The P100 design is a worst-case scenario design that, when properly implemented, will combat the problems which often lead to sick buildings and sick occupants.
But on the design continuum, there are problems that exist in one climate that don't exist in others. In those cases, the P100 DOAS strategy can be modified and possibly simplified to address only those challenges that are relevant.
UFAD a Fad?No article about the GSA and HVAC systems could be complete without a discussion of underfloor air distribution systems. The story is a circuitous one, but nonetheless telling, because it speaks to the strength (in my opinion) of the P100 as a road map for safe and sound design.
In 2003, the P100 was revised to include UFAD systems as a baseline approach for "office type loads and other spaces with low latent loads." Progressive to be sure, but not groundbreaking stuff. Remember that just two years ago, most if not all articles and discussions regarding UFAD were glowing. A new paradigm, it was called. I seem to even recall an article claiming UFAD cured arthritis.
Now I'm fibbing about the arthritis claim, but combine that positive press with a plethora of feel good testimonials of UFAD's success in Europe, and you could safely surmise that not allowing UFAD would have been deemed a bigger risk than allowing it.
But a funny thing happened on the way to the A/C forum. As North American designers began to implement UFAD in various configurations, some problems arose. The most obvious culprit for most failures could be laid at the feet of inexperienced engineers and contractors, and the lack of a national consensus on load calculations and things as seemingly benign as construction details.
But the GSA was also experiencing its first Federal Courthouse with UFAD, and the initial returns weren't very promising. Reports varied, but the powers that be at the top of the GSA were reaching the conclusion that UFAD, at least UFAD in the untested North American market, was at best a shaky substitute for overhead VAV, and at worst, a security, control, and energy-burning nightmare.
In turn, the GSA issued interim UFAD design guidance, elements of which are included in the sidebar. While this retrenchment does not ban UFAD out right, it does raise design issues that render UFAD a tough proposition at this time for GSA projects.
So you might ask, "Why is this a good thing?" First, the GSA had the courage to put UFAD in the design guide in the first place. And second, even though the pendulum has swung too far against UFAD at this time, it will eventually swing back. And when it does, UFAD design in North America will have progressed from the end-all-be-all yet unproven panacea it was, to a slightly more "comfortable" design option for all projects, public or otherwise.
ConclusionThe P100 is imperfect. Heck, we all are. But it presents a basic design concept that can be used to develop or hone your own personal standards. Personally, I consider the P100 base design, warts and all, as the starting point for my own designs. In the end, most of what I do only bears a slight resemblance to the P100 base, but I know I have started from solid ground.
We have talked about just a few of the tools and ideas resident in this standard, but there is significantly more to be discovered. Analyze your designs and procedures through the prism of the P100. And with your mind wide open, endeavor to improve what you do and how you do it.
And who knows, maybe when you are through you will recognize that good enough for government work is, in reality, pretty darn good. ES
To download the P100, access the GSA website at www.gsa.gov/Portal/gsa/ep/channelView.do?pageTypeId=8195&channelPage=%2Fep%2Fchannel%2FgsaOverview.jsp&channelId=-15012.
Thinking Over UnderfloorUntil the P100 is revised in 2005, the GSA has provided some interim design guidelines for UFAD systems. Only after the concerns listed below (there are more, this is a partial accounting) have been addressed in exhaustive detail, is a design allowed to proceed. Unfortunately, at this time, the cumulative effect of the conservative measures dictated often renders UFAD untenable from risk, cost, and operational standpoints for many GSA projects.
- Architectural and structural issues such as deck-to-deck heights; locations and relationships of functional spaces; and minimum floor cavity heights for interface clearances, etc.
- Seismic, vibration, and acoustic control including additional seismic restraints and controls required for the height of the floor cavities/plenums.
- Water, mold, and contaminant (IAQ) control including minimizing sources and pathways for liquid water, water vapor, and contaminant incursion into raised floor plenums.
- Life-safety, fire, and smoke management in raised floor plenums that contain water pipes and/or electrical, and IT wiring/cabling.
- HVAC control including methods of calculating and ensuring sensible and latent loads are properly addressed at all operating points.
- Accessibility and maintainability including adequate sizes and locations of mechanical equipment room and accessibility of equipment and components.