Imagine for a moment that instead of engineering, you had chosen law. You're no ambulance chaser, but the latest journals are full of cases involving sick buildings and mold,1, 2, 3 and you dream of a facility that could generate the most opportunities for an honest litigant such as yourself.
What would you ask for? How about noxious odors, chemical irritants, and a constant source of moisture to feed that fungus? And because buildings don't file lawsuits, people do, you need folks to get sick. But not just anyone, the most vulnerable and sensitive bunch you can find: children, the elderly, and health conscious people who are the most aware of unhealthy conditions? Plus, just to make sure you get in the papers, wouldn't it be grand if the mayor stopped in once and awhile?
Last, what could you call this place so that people would be drawn in regardless of the potential hazards? Would you be going too far if you called it a health and recreation center? Obviously not, because this litigator's dream-come-true is an everyday nightmare for more and more designers as townships across the country add a recreation center to its list of civic amenities.
Sports ArchitectureAmerica's expanding waistlines not withstanding, the sports architecture industry has boomed over the last 25 years. Municipalities once satisfied with seasonally restrictive outdoor pools are now jockeying for themed aquatic centers with water slides, fountain features, and a lazy river for the, well, lazy. Instead of a couple hoops under a streetlight, now you see air conditioned gymnasiums with elevated running tracks and fitness areas with amenities that would challenge the likes of Jack La Lanne.
Quite often, these facilities become the icon for their respective communities. As the modern equivalent of the town square, they are a communal focal point with the Rotary Club meeting in the multipurpose room and mothers watching their toddlers in the splash pool while the mating ritual of the often maligned North American preteen is acted out around the snack bar.
Floor plans and budgets vary, but the typical center is generally comprised of three areas, each programmatically and environmentally unique: fitness, aquatics, and communal (e.g., multipurpose rooms, offices, meeting space, etc.).
These spaces radiate from the building's core, where the lockers, and in some cases food service, can be found. The architectural intention is a natural flow from one space to another with an openness that defies demarcation. In fact, it is quite common to have a running track winding through the complex, underscoring the oneness of the assembly.
These facilities present unique design challenges to the HVAC designer including:
- Moisture and vapor control to avoid mold and mildew;
- Drafts, chloramines, and excessive evaporation in the pool environment;
- Various and varying occupancies occurring within the same space; and
- Control and isolation of odor.
A Propensity for PropagationAt the ASHRAE Winter Meeting in January, there were no fewer than five programs directed at mold and mold mitigation4. Needless to say, none of the 20 some odd speakers recommended enclosing an open body of warm water within your building as a means to avoid spores. A typical condition within a natatorium is 85 degrees F and 60% rh. That equates to a dewpoint just under 70 degrees. If that warm moist air makes it into the conditioned adjacent space, you can bet that there will be some surface temperature at or below dewpoint and condensation will occur. It may be a structural member with no thermal break to the outdoors. It could be a domestic water line passing behind a wall. Regardless, once moisture exists, the propensity for mold propagation is established, and problems will ensue. Would you allow an aluminum garage door to be used as an exterior wall for a conditioned space in a humid climate like Houston or Orlando? Hopefully not, but there is a recreation center that actually won an award for architectural design that has garage doors separating the pool from the fitness area. As nifty as the idea may seem, you would never want to work out in a gym as warm and humid as the adjacent pool, so the doors will rarely be open, and the paths for vapor migration are too numerous to count even when the doors are closed. Vapor pressure is nature's water pump, and vapor is going to go where the laws of physics demand. So it is key to physically and environmentally separate swimming environments from other conditioned environments. That means vapor barriers for the adjacent spaces and keeping the natatorium under negative air pressure relative to the rest of the building5. One excellent example of meeting this environmental criterion of separation while still exercising a sense of openness is the Athletic and Recreation Center in Columbia, MO. Here, the lobby is visually open to, but physically separated from, the swimming pool by an expansive glass wall. Further, the running track actually passes through the natatorium at the second level, but the joggers are environmentally isolated within what the architect likes to refer to as a human-sized "habitrail."
Natatorium Nitty-GrittySo now that you have isolated the pool from the rest of the building, how do you treat this unique environment? It has been said that when you understand a problem, the solution becomes obvious. So it makes sense to list the primary problems associated with IAQ in natatoriums:
- Chloramine concentration at the water surface and deck;
- Excessive evaporation;
- Physiological discomfort due to drafts and temperature variance; and
- Condensation and the resultant rust and mildew.
What is interesting about the problems listed is that the application of a conventional solution to one may actually exacerbate another. For example, one might think that to eliminate chloramines at the water level, one must affect numerous air changes, and to do this, direct more air at the deck level to "sweep" the irritants away. However, air directed at the deck will inevitably sweep across the water level as well, in turn causing drafts and increasing the evaporation rate.
The key to controlling chloramines is knowing that they are heavier than air; in turn, they tend to collect at the surface of the water and on the swim deck. But returning all of the air at the deck may lead to excessive drafts, and since the irritants only need a "drain" to avoid pooling, returning only a portion of the air at the deck is all that is necessary. With approximately 30% of the air returned low, the remainder can be returned at ten feet or more, which avoids excessive air movement across the surface of the water and assists the break up of stagnation at the upper elevations.
Regarding stratification, while it is often encouraged in high-bay designs, it is counterproductive in a pool environment. It is true that stratification would help on the thermal load front, but stagnant moist air at the roof deck may lead to corrosion and mold. Therefore, on the supply side it is recommended to split the air supply high and low.
The air supplied low should be directed from the floor and up the exterior walls, especially where windows are present6. This bathing of the envelope with dry air will eliminate the possibility of condensation forming on the fenestration. Remember the openness that the architect is trying to achieve in a recreation center? If there is condensate on the windows, there may as well be wall there. The air supplied high should be directed horizontally across the pool and never down onto the surface of the water. In most cases, the recommended ratio of high supply to low supply is approximately 80/20.
Another key to controlling the odor and problems is to provide sufficient outdoor ventilation. ASHRAE recommends 4 to 6 ach, which has proven adequate over the years. Using runaround heat pump technology, the designer can bring in sufficient outdoor air while reclaiming both the energy available in the exhaust air stream as well as the heat of compression in the compressor. This heat is used as reheat, or if needed, it can be redirected to the pool water.
Varying VentilationOnce you step out of the natatorium, the second challenge can be found in the gymnasium and other fitness areas. Here the problem is occupancies that can vary not only in number but also in comfort requirements. A gym, for example, is almost empty most of the day but filled with 50 or so sweaty youths vying for pick-up time on the courts after 3 o'clock. Then, once a month or so the place may be packed with hundreds of people watching a playoff game or a community theatre production of "Shakespeare on the Courts." The fact is you have to provide a system that can address these various loads efficiently. Simply picking a unit based on that peak occupancy and its associated ventilation requirement is lazy design at best and negligence at worst. If the system is grossly oversized, its latent capacity will be severely compromised at off-peak conditions due to short cycling with DX and limited flow with chilled water coils7. The problem of insufficient latent capacity is exacerbated when you start throwing in copious amounts of outdoor air. More and more moist air gets pumped in, and even though the drybulb may be within an acceptable range, the humidity rise will make the space unbearable. So what's a designer to do? For starters, consider VAV even if the space is served from a dedicated unit. Instead of cycling or excessive throttling, the amount of air being delivered to the space is scaled back as the load allows. Not only does this save fan energy, it also lessens the latent capacity problems associated with constant volume systems. Second, only provide the amount of ventilation air you need when you need it. While ASHRAE 62 may allow the designer to take advantage of variable occupancy and cut the ventilation rate in half, a drawback to this exception is that you are still providing too much ventilation during those times the space is only partially occupied, and not enough when the space is maxed out. Instead, consider providing equipment of sufficient capacity to supply the full ASHRAE-dictated amounts, and use a CO2 control strategy to reset the outdoor air rate on an as needed basis. Using CO2 allows the system to adjust to varying levels of activity and occupancy, and a sensor in the space or return set at 700 to 1,000 ppm is a proven strategy to meet indoor air requirements, avoid over ventilation, and save energy8.
Pathways for Pong and the Core StrategyHave you ever stepped into a building and immediately established that it housed a chlorinated pool, a locker room, or that stuffed peppers were served within the last 24 hours? Of course you have. Do you have the telepathic powers of the Amazing Kreskin? No, you simply smelled your way to the unknown. Hark once more to the need for openness in a recreation center. Unfortunately for the HVAC professional, this openness introduces opportunities for odor migration, or as they might say in the United Kingdom, pathways for pong. In general the following pressure relationships should be established and maintained (Figure 1):
- The communal portions of the building should be net positive to the outdoors and all other spaces;
- The natatorium should be negative in relationship to the rest of the building and slightly negative to the outdoors; and
- The locker and service core should be negative in relationship to every other space.
It should come as no surprise that the bulk of the building is under positive pressure, but quite often the need for the natatorium to be negative is overlooked or misunderstood.
Knowing that moisture in the pool area will, in most cases, be traveling out of the building due to vapor pressure, in addition to combating odor migration, a slight negative pressure will facilitate drying within the exterior envelope at those locations where the barrier is compromised (and it will be somewhere).
Last, to ensure that the locker core is always negative to the rest of the building, fan terminal units can be implemented to "pump" makeup air from the common corridors into the lockers, where it is then exhausted. ASHRAE 62 allows the transfer of ventilation air brought in for other occupancy requirements to be used as makeup air in this way9.
Experience has shown that exhausting most of the occupancy driven outdoor air through the core, works out to 1.5-to 2-cfm/sq ft in locker spaces and has two benefits. The first is the negative pressure relationship already discussed. The second is that you now have a single exhaust source, which makes it easier to apply energy recovery such as an enthalpy wheel.
The design for a recent facility demonstrates this final point. The ASHRAE dictated ventilation quantity for the communal spaces was approximately 6,000 cfm. In lieu of exhausting the core directly at 4,000 cfm (1-cfm/sq ft) and providing conventional relief of the remaining 2,000 cfm, the 6,000 cfm was transferred and exhausted from the core (Figure 2).
In addition to increasing the stale air turnover rate to 1.5-cfm/sq ft, this fixed quantity was ducted back to the AHU where an energy wheel was incorporated to reclaim the energy. Another AHU provided active pressurization of the building with a relief fan modulated to maintain a fixed differential pressure in relationship to the outdoors. This decoupled the ventilation control from the building pressure control, as it should be.
The Finish LineIn summation, we can see that sports architecture has some unique requirements, including:
If the ventilation systems are not designed or operated properly, a recreation center's dreadful air quality will reach out and touch someone before the perky attendant behind the control desk can offer a friendly how-do-you-do. And at once, a negative impression is made, negating everything the design team has done to communicate an open and engaging gathering space. Thankfully, a designer can avoid this pitfall without having to go overboard on controls or complicated strategies. By applying and adapting the tips described herein, one can provide appropriate systems that allow these facilities to be the healthy and inviting places they are intended to be. ES