“This is usually the least expensive,” said the University of Nebraska-Lincoln professor.
“This involves having a heart-to-heart talk with the owner and making sure that when a building is operating or being built, it involves contractors using good IAQ practices. It means storing material in their proper place, avoiding mildew problems, making sure all return air entrances are monitored.…”
In other words, if you don’t bring in contaminants, it’s easier to provide healthy IAQ. That’s about as green as you can get.
Of course, Wentz, who serves on the faculty of the Mechanical Contractors Association of America’s (MCAA’s) National Education Initiative (NEI) and Institute for Project Management (IPM), knows just using common sense and solid IAQ practices doesn’t always cut the mustard. One can be careful during the building process, but that will never guarantee 100 percent perfection, that is for sure.
To gain Leadership in Energy and Environmental Design (LEED) classification in new construction, one has to conform to ASHRAE Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality.
To get a point in the classification process, one has to exceed the standard by 33 percent. Since it is a challenge to produce this level in an energy-efficient manner, many HVACR contractors and engineers are turning to using dedicated outdoor air systems (DOAS) in conjunction with sensible cooling at terminal units.
“It is more costly,” said Wentz, “but you get better results.”
REASONS FOR CHOOSING DOAS
Stanley Mumma, Ph.D. and professor of architectural engineer, College of Engineering, Penn State University, is a strong advocate of DOAS. In his estimation, four major problem areas, common to most air systems, are overcome with properly designed DOAS:• Ventilation air distribution in all air variable-air-volume (VAV) systems.
“Engineers cannot be sure where the ventilation air, in all-air systems, is distributed once brought into the building and mixed with return air at the air-handling unit,” noted Mumma.
“Proper distribution is a function of VAV box minimum settings, space sensible loads, local exhaust and exfiltration, short circuiting paths, and interzonal air transfers. Complete knowledge of all of these factors at design is not possible, much less in real time. Therefore, engineers cannot really defend compliance with ASHRAE Standard 62.1-2004 when using all-air VAV systems.
"On the contrary, dedicated, 100 percent outside air systems place the proper ventilation air quantities into every space.”
• Excess outside airflow and conditioning required for all-air VAV systems.
According to Mumma, when the multiple spaces equation of Standard 62 is used, generally 20-70 percent more outside air is required in an effort to ensure proper ventilation air distribution in all air systems than is required with a DOAS.
“Cooling and dehumidifying the high outside air quantities in the summer and humidifying and heating the air in the winter is an energy-intensive proposition,” he said.
• VAV box minimum settings must be surprisingly high.
“Perhaps contrary to current practice, VAV box minimums must reflect both the ventilation requirements of the space and the fraction of ventilation air in the supply air,” said Mumma.
“For example, a space requiring 200 standard cubic feet per minute (SCFM) of ventilation air and served with supply air that is 40 percent ventilation air, will require a box minimum setting of 500 SCFM.”
Mumma added that when the box minimums are properly set to satisfy the ventilation requirements, the potential for considerable terminal reheat becomes an issue. “Therefore, properly operating all-air VAV systems will always use more terminal reheat than dedicated outdoor air systems supplying air at the same temperature.”
• Inability of most all-air VAV systems as currently designed to decouple the space sensible and latent loads.
In the eyes of Mumma, the inability to decouple the space’s sensible and latent loads leads to high space relative humidity at low sensible loads in the occupied spaces. “Properly designed dedicated outside air systems can accommodate 100 percent of the space latent loads and a portion of the space sensible loads, thus decoupling the space sensible and latent loads,” he said.
According to the professor, a parallel sensible-only cooling system is then used to accommodate the sensible loads and not met by the dedicated outdoor air systems.
“When all of the outside air load, all of the space latent load, and up to 30 percent of the space sensible load is accommodated by the dedicated outside air system, only about 40 percent of the design chiller load must be borne by the parallel sensible-only cooling system,” explained Mumma. “There is, therefore, a strong incentive to control the space latent loads independently of the space sensible loads to avoid moisture-related IAQ problems.”
DECOUPLING LATENT, SENSIBLE LOADS USING DOAS
In order to decouple the space latent and sensible loads using the small volumetric flow rates of outside air, the supply air dew point temperature must be suppressed more than is typical with an all-air VAV system, said Mumma. If the bulk of the latent load is from the occupants, and each occupant receives 20 SCFM of outside air, “then the change in humidity ratio is approximately 15 grains,” he explained.“Clearly, the supply air dew point must be at least 15 grains less than the space target humidity ratio,” said Mumma.
“This approach to meeting all of the space latent loads requires that the building envelope be tight, and in conformity to ASHRAE 90.1-2004. Large and unpredictable variations in the envelope transmission of moisture, either through infiltration or movable sash, will lead to a loss of space design conditions. Such a loss of control could result in unwanted condensation at the distributed parallel sensible cooling equipment and an associated distributed microbial problem.”
The DOAS must be cooled and dehumidified in the summer, and humidified and heated or cooled at other times of the year. In the simplest form, Mumma said the preconditioning could be achieved with a preheating coil, a cooling coil, a reheating coil, and a humidifier.
However, Standard 90-2004 requires that “individual fan systems that have both a design supply air capacity of 5,000 cfm or greater and have a minimum outside air supply of 70 percent or greater of the design supply air quantity shall have an energy recovery system with at least 50 percent effectiveness. Fifty percent energy recovery effectiveness shall mean a change in the enthalpy of the outdoor air supply equal to 50 percent of the difference between the outdoor air and return air at design conditions.”
Consequently, Mumma added, since DOAS are by definition 100 percent outside air systems, energy recovery is required to supplement the simplest form of preconditioning equipment mentioned. ASHRAE Standard 90.1-2004 also requires that a total energy recovery device be used to reduce the load on the cooling coil. Mumma also noted that if the supply air is to be delivered at a temperature above the required supply air dew point temperature, a sensible energy recovery device is required.
SUMMARY
Both Wentz and Mumma admit that the paradigm shift in the design of building comfort control systems is in its early stages. In the eyes of Mumma, the requirements of the new paradigm include:• Separating the outdoor air system from the space-conditioning systems through the use of a DOAS “to ensure proper ventilation air delivery into all occupied spaces.”
• Use of DOAS to handle all of the outdoor air load, all of the space latent load and as much of the space sensible load as possible.
• Maximizing the cost-effective use of energy recovery equipment, therefore complying with ASHRAE Standard 90.1-2004.
Mumma is also an advocate of radiant ceiling sensible cooling as the parallel system of choice for occupant sensible thermal control.
“As more and more dedicated outside air design systems are placed in service, the dedicated outdoor air paradigm will expand rapidly,” stated Mumma.
For more information, Mumma has put together a Website dedicated to the science of DOAS: http://doas.psu.edu/.
Publication Date:04/21/2008
