Sealing the Cracks in IAQ

Editor’s Note: The following is excerpted and adapted from “Commissioning Buildings in Hot, Humid Climates: Design and Construction Guidelines,” by J. David Odom, vice president of IAQ Services, and George Dubose, mechanical engineer, with CH2M Hill, Orlando, Fla. In addition to passages from the manual’s introduction and a section on mechanical considerations at the schematic design stage, comments from an interview with Odom are also included.  
 
Just months after occupying their new, multimillion-dollar municipal building, employees of a Florida county began complaining of chronic sinus problems, allergy attacks, headaches and asthma—classic signs of sick-building syndrome and building-related illness. The architects, engineers and microbiologists tasked with finding the cause of these symptoms identified a problem that is becoming widespread nationwide: severe microbial contamination.

Mold and mildew were growing unchecked throughout the building’s heating, ventilation and air-conditioning (HVAC) system and in many spaces within the building. The excess moisture was the direct result of a combination of rainwater leaks and an HVAC system that pulled moist outside air into the building during the hours when the cooling system had cycled off. Once the HVAC system became infected, it dispersed spores throughout the building.

So, only a few years after opening, the building underwent a major overhaul. Ultimately, repairs and other associated costs would exceed $20 million.

Unfortunately, this problem is not an isolated one. Rainwater leaks occur in every climate, and in this case, the leaks alone would probably have led to significant microbial contamination and building evacuation. The real devastation arose from the less obvious cause: improper interaction between the building envelope and the HVAC system.

In essence, IAQ problems are symptoms of a broken process. The complexity of the building-construction process and the failure to recognize interrelationships between different disciplines often worsens existing problems.

Key factors in the schematic design of the mechanical side of building systems in humid climates are as follows:

• Maintaining building pressurization through proper control of exhaust, makeup air and ventilation.

• Properly selecting HVAC and control systems for adequate dehumidification and filtration.

During hot, humid months, outside air with a large moisture load can be drawn into a building through the wall system and into the interior space. Because airflow always follows the path of least resistance, air can be carried down interior walls of a space if they are connected to the exterior envelope. As air flows through, moisture condenses and is deposited in wallboard and other building materials. Moisture accumulation increases with decreasing interior temperatures and with increased negative pressures. Figure 1 shows that pressure differentials in hot, humid areas—even one negative Pascal, which many consider insignificant—can create a problem in a building with an average envelope and an interior temperature of 74°F.

• Control of mechanically induced depressurization.

• Proper distribution of makeup air within the building spaces.

• Building designs that overcome any depressurization from stack, wind and fan effect.

Figure 2 - Effects of Positive Pressurization


An important consideration in achieving positive pressurization is that interior building partitions not adversely affect the distribution of air.

The way buildings operate today is as a multitude of pressure vessels: interstitial spaces in an exterior wall; plenums; dropped ceilings; rooms that typically have doors closed between them. These different pressure relationships—some of which are very well connected to the supply air, some that are not—include pressure-starved areas and some pressure-excessive areas.

Most building codes establish minimum ventilation requirements in relation to occupancy or space function. These requirements are usually based on ASHRAE Standard 62-1989, “Ventilation for Acceptable Indoor Air Quality.” This standard specifies the minimum acceptable outdoor air requirements for occupied spaces.

Providing conditioned outside air not only helps pressurize a building but also dilutes chemicals or particulate pollutants generated in the space. Outside air can also be induced in the space by the HVAC system as ventilation air. If the HVAC system introduces air into the space, the system must continuously dehumidify the air. Of course, adequate dehumidification should not be sacrificed for adequate ventilation. If the air is not continuously and adequately dehumidified, the moisture added to the space might be greater than the HVAC system’s ability to remove it. This moisture source normally results in moisture-related mildew problems on the interior surfaces of the building (that is, interior finishes and the surface of furnishings).

To provide proper dehumidification, an HVAC system must:

• Fully dehumidify the air that flows across the cooling coil.

• Provide sufficient run time to remove moisture from the interior air despite the satisfaction of interior temperatures.

To fully dehumidify the airflow across the coil, cooling coils must be sized properly to meet the sensible load (load associated with dry-bulb temperature) and latent load (moisture in air associated with wet-bulb temperature). This includes the combination of both outside air and return air. This air must be brought to a temperature that causes the moisture in the air to condense for latent heat (or latent energy) removal. Simultaneously, the cooling coil is reducing the sensible temperature of the air to offset the sensible energy generated in the space (lights, solar, people, equipment, etc.). A common range of temperature for the cooling of this air is between 50°F and 55°F. At this temperature, most HVAC system airflows will be at 100% relative humidity (RH) and will effectively condense moisture from the air. Air provided to a space under these conditions has the best chance of maintaining interior conditions of 75°F dry-bulb (°Fdb) and 60% RH.

If the system cannot provide sufficient dehumidification while it reacts to temperature control alone, it must continue moisture removal without affecting interior temperatures and occupant comfort. This can be accomplished by reheating—a form of simultaneously cooling and heating to continue dehumidification while not overcooling the occupants.

Methods of reheating include direct or indirect gas-fired heating; hot-water heating; hot-gas reheating for refrigeration-based units; and for parts of the country that allow it—electric.

Devices added to the equipment, such as wraparound coils, can also provide a means of reheating. Wraparound coils simply transfer energy from the incoming cooling coil air stream to the exiting cooling coil air stream. These coils are available in a passive refrigeration-based unit or as a water-based system that uses pumps to move the water through the system.

In conventional HVAC systems, two dehumidification methods are used. The first is a cooling-based system cooling air below its dewpoint. Moisture condenses on the cooling surface and is removed from the air. For example, a cooling-based system can cool an outside air stream from 95°Fdb (55% RH) to 77°Fdb. At 77°Fdb the air is at 100 percent RH. If it is cooled below 77°Fdb to 55°Fdb, 68 grains of moisture per pound of dry air are condensed out of the air and onto the cooling coil.

The second method involves the use of a desiccant that attracts moisture to its surface by introducing a low vapor pressure at the desiccant surface. The vapor pressure of the moisture in the air is higher, so moisture travels from the air to the desiccant. The desiccant then must be recharged through a heating process, allowing the moisture to be driven from the desiccant and discharged to another location besides the cooling air stream.

One of the best strategies is a combination of desiccant and cooling systems, particularly for 100% outside air streams such as makeup air systems. Since air exits a cooling-based system at saturation, it only moves to a lower RH once it mixes with the room air and heat is added to it. The desiccant, on the other hand, enters the space with very low RH, and its RH increases to the room’s RH level once the two air streams reach equilibrium.

Filtration should be selected according to the amount of outdoor air supplied by the air handler, the ability of the filter to capture airborne contaminants and the relative pressure drop (as it relates to equipment static pressure capabilities and energy use). Chapter 25 “HVAC Systems and Equipment” of the ASHRAE Handbook provides guidance and recommends filtration capacity. No matter how well designed a filtration system is, most moisture-related IAQ problems cannot be avoided by filtration alone.

The greater the amount of outdoor air, the more efficient the filtration system must be, and the greater the required dust-holding capacity (see Figure 3). Makeup air units will require filters with at least 60% efficiency (per ASHRAE Standard 52) and with a high dust-holding capacity; these are referred to as bag-type filters. Recirculated systems with prefiltered fresh air may be selected for filters with 25% to 30% efficiency.

Figure 3 - Filter Factors

Fiber-glass and other synthetic nonbiological-promoting filter media should be considered, together with the proper support. Use of paper products is not recommended because these products, when wet, become an excellent food source and habitat for biological growth.