Exhaust air systems remove air from within a building and discharge it to the outdoors. For commercial buildings, exhaust air systems are generally used to:
• Provide general exhaust for odor-producing areas to prevent odors from migrating to other areas of the building.
• Provide ventilation for spaces that do not require cooling but for which it is desired to keep the space temperatures below a maximum limit and also to keep the spaces under a negative air pressurization to prevent odor migration. Because this type of exhaust air system is normally utilized for equipment rooms, we will refer to this as negative-pressure equipment room ventilation. • Provide the exhaust airflow required for certain types of equipment.
General Exhaust Applicable codes and the authority having jurisdiction may require exhaust from certain areas within commercial buildings in order to prevent odor migra- tion. For example, Table 403.3 in the 2009 International Mechanical Code requires public toilet rooms to be exhausted at a rate of 50 cubic feet per minute (cfm) per water closet15
or urinal for normal use and 70 cfm per water closet or urinal for heavy use (theaters, schools, sports facilities, etc.). The code also requires sports locker rooms to be exhausted at a rate of 0.5 cfm/ft2 of net occupiable floor area. In addition to exhausting these
spaces with the code-required airflow, sufficient negative air pressurization must be established within these spaces with respect to adjacent spaces to ensure proper con- tainment of odors, which is achieved by maintaining the space under a 0.02-in. w.c. negative air pressurization with respect to adjacent spaces. In situations where the space under concern is separated from adjacent spaces by one or more (closed) doors, the required 0.02-in. w.c. negative air pressurization can be achieved by exhausting
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Figure 5-10 Schematic diagram of a combustion air makeup system.
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approximately 100 cfm per 3-ft-wide (closed) door16 more from the space than what is
supplied to the space.
There may also be other spaces within the building for which exhaust is desired for odor containment but for which it is not required by the applicable codes or the author- ity having jurisdiction. For example, the 2009 International Mechanical Code does not require exhaust from janitor’s closets. However, it is desirable to keep janitor’s closets under a negative air pressurization to contain the odors produced by the cleaning agents that are often stored in these rooms. Therefore, each janitor’s closet should be designed with an exhaust airflow of 100 cfm in order to properly contain these odors (supply airflow is not required for janitor’s closets).
Often, the air systems providing general exhaust for commercial buildings serve multiple spaces and multiple occupancy classifications. For example, a four-story office building that has toilet rooms and janitor’s closets in the same location on each floor would normally have one exhaust system to serve all of these toilet rooms and janitor’s closets. However, physical separation of the spaces requiring general exhaust, differing hours of operation, and multiple tenants are all reasons why separate exhaust systems may be required to meet a building’s general exhaust requirements.
Makeup air for public toilet rooms and janitor’s closets is normally transferred from adjacent spaces that are served by the building HVAC systems. Typically, the outdoor air supplied by these HVAC systems to meet the occupant ventilation requirements is suffi- cient to provide makeup air for the toilet room and janitor’s closet exhaust air systems.
There may be some spaces within a building for which the general exhaust airflow is a significant percentage of the supply airflow that is required to maintain the space temperature. One example is a large sports locker room. The 0.5-cfm/ft2 exhaust air-
flow required for this occupancy may be as high as 50% of the supply airflow required for cooling. For this reason, sports locker rooms are normally heated and ventilated only. However, if cooling is required in addition to heating, the HVAC unit must have dehumidification capabilities in order to properly dehumidify the high percentage of outdoor airflow that will be required. Another alternative would be to design the HVAC system serving the locker rooms to utilize a maximum of 15% outdoor airflow and transfer the remaining airflow required for exhaust to the locker rooms from adjacent conditioned spaces.
General exhaust air systems normally operate when the building is occupied and are shut off when the building is unoccupied. It is common for exhaust fans serving general exhaust to be electrically interlocked with the operation of the HVAC systems serving the adjacent areas so that the exhaust fans can only operate when the HVAC systems are operating. If the building is designed with a computerized building auto- mation system (BAS), the exhaust fans would be controlled by the BAS and can be assigned a different operating schedule than the associated HVAC systems, if desired.
Negative-Pressure Equipment Room Ventilation Negative-pressure equipment room ventila- tion is provided for spaces that do not require cooling but for which it is desired to keep the space temperature at a maximum of about 100°F, which is 5 to 10°F higher than the summer design outdoor temperature (Figs. 5-11 and 5-12). Equipment room ventilation systems can positively pressurize the space they serve, which is the requirement for equip- ment rooms containing gas-fired or fuel-burning appliances that use the room air for com- bustion (refer to the Positive-Pressure Equipment Room Ventilation section earlier). However, it is more common for equipment room ventilation systems to place the spaces they serve under a negative air pressurization to prevent odors from migrating to adjacent
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spaces. In these situations, an exhaust air system that draws makeup air from an opening in the building envelope, such as a wall louver or roof-mounted intake air hood, would be designed. The exhaust air system would be controlled by a space thermostat and be ener- gized whenever the space temperature rises above the setpoint of the space thermostat, which is normally about 85°F. It is necessary for equipment room ventilation systems to incorporate a normally closed motor-operated damper on the outdoor air opening so that the damper closes whenever the ventilation system is off. This prevents outdoor air infil- tration to the space when ventilation is not required.
One example of this type of exhaust air system is one that serves equipment rooms without gas-fired or fuel-burning appliances (or the appliances have direct connections to the outdoors for combustion air). Another example is the exhaust air system serving an electrical equipment room, particularly one that contains heat-generating equipment, such as transformers or uninterruptible power supplies (UPSs).
Equipment rooms can also be ventilated by transferring makeup air for the exhaust air systems from adjacent spaces that are conditioned by the building HVAC systems. In this situation, the maximum allowable space temperature within the rooms would still be 100°F,17 but the makeup air would be the temperature of the air within the
adjacent conditioned spaces, which is typically 75°F during occupied operation of the Figure 5-11 Schematic diagram of a negative-pressure equipment room ventilation system.
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building HVAC systems. Because the temperature rise of the transfer air is 25°F, the exhaust airflow required to maintain the equipment rooms at 100°F is 2.5 to 5 times less than the exhaust airflow required to maintain the equipment rooms at 100°F when outdoor air at the design summer outdoor temperature is used to make up for the exhaust airflow.
Refer to Fig. 7-5 in Chap. 7 for a graphical representation of the exhaust fan, exhaust air louver, outdoor air louver, and motor-operated dampers for a negative-pressure equipment room ventilation system as they would be shown on a floor plan drawing.
Equipment Exhaust Finally, exhaust systems are used to remove undesirable or potentially harmful contaminants from the building at the point where these contaminants are gener- ated. An example of this type of exhaust air system is a kitchen hood exhaust system. The size, quantity, and location(s) of kitchen exhaust hood(s) are usually determined by the kitchen equipment consultant. The exhaust airflow through each exhaust hood depends upon the type and input rating of the appliances located under the hood. The kitchen hood exhaust system must be designed in accordance with NFPA Standard 96—Standard for
Ventilation Control and Fire Protection of Commercial Cooking Operations.
Air Pressurization Calculations
Once all of the air systems have been defined for a building, it is necessary to perform air pressurization calculations for the overall building as well as for individual spaces within the building in order to:
• Ensure that sufficient outdoor makeup air is available to meet all exhaust airflow requirements within the building without placing the building under an overall negative air pressurization.
• Ensure that appropriate air pressure relationships exist between adjacent spaces within the building.
Generally, commercial buildings should have about 5% positive air pressuriza- tion; that is, the sum of the outdoor airflows supplied by the various HVAC and ventilation air systems to the building should be 5% greater than the sum of the exhaust airflows removed from the building. However, some judgment needs to be applied in the building air pressurization calculation, such as in the case where an equipment room is exhausted by an air system and makeup air is supplied to this room through an opening in the building envelope. In this case, the equipment room exhaust airflow would not be included in the air pressurization calculation for the overall building since the airflow through this room is actually separate from the rest of the building.
Also, appropriate air pressure relationships need to exist between adjacent spaces within the building. The air pressurization of a particular space within the building is equal to the sum of the airflows supplied to that space by the HVAC and ventilation air systems minus the sum of the airflows removed from that space. The air supplied to each space in the building includes both the supply air from the HVAC system(s) serving the space plus outdoor air delivered directly to the space by the outdoor air ventilation system(s). The air removed from the space includes both the return air to the HVAC system(s) serving the space plus exhaust air removed from the space by the exhaust air system(s).
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Because some spaces of the building will have a more positive (relative) air pres- surization than other spaces, there will be a transfer of air from spaces of greater air pressurization to spaces of lesser air pressurization. The transfer of air from one space to another needs to be evaluated to ensure that air is flowing in the proper direction. Typically, it is desirable for air to flow from spaces that are clean to spaces that are less clean or from spaces that are odor-free to spaces that are odor-producing. Section 403 of the 2009 International Mechanical Code defines the requirements for pressure relation- ships between various occupancy classifications within buildings.
It is very helpful in this regard to prepare an air pressurization schematic diagram of each floor of a building showing (with airflow arrows) the directions that air flows between the spaces of the building. The diagram should also show the resultant air pressurization of each space and the overall air pressurization of the floor, both in terms of cubic feet per minute.