Selected Highlights of the Labs21 2008 Annual Conference

Constant Flow, Variable Flow, and All the Space Between

Jim Coogan, P.E., Siemens Building Technologies, Inc.


Truly constant volume laboratory ventilation systems are almost obsolete, but there is a wide range of “nearly constant” systems that respond in steps to users' activities and needs. These systems, sometimes called two-state or two-position constant volume systems, offer many options to ventilation designers.

The ventilation strategy should fit and support the safety programs for the laboratory. Ideally, the ventilation designer and safety officer work together to produce a coherent program of exposure control.

Four Steps to Designing Two-State Ventilation

The following four design steps will lead to a system that ventilates a lab efficiently and works with the institution's safety programs.

      1. First, identify the conditions in the room that call for differing flow rates. Often, the presence or absence of people in the room call for differing flow rates. The need to use a particular exhaust device, or some other aspect of the lab users' work, can also necessitate differing rates.
      2. Second, determine the flow rates for the device and other flow devices that coordinate with it. The ventilation engineer should share this task with the health and safety professional. Typically, each one has information and expertise to contribute. To set flow rates that effectively protect workers, designers should consider the principles of industrial hygiene, along with issues of room air distribution and ventilation efficiency. These concepts, however, do not lead directly to simply calculated answers. Designers should resist the temptation to fall back on rules of thumb. Instead, they should consider the intended laboratory tasks and document any assumptions about lab use. The process is actually simplified by the fact that they are selecting two different flow rates, rather than trying to cover all circumstances with one.
      3. Third, select the control system inputs to trigger flow rate changes. Although this step is closely related to the first step, there is an important distinction. The question in step one was, “What conditions change the requirements for airflow?” Now the question is, “How does the control system detect the changes?” The answer might be manual inputs from the user or something more passive. The building automation system is likely to execute some control logic to select the high or low flow setting. For example, there are usually timers involved to limit unnecessary changes of state. If there is more than one input involved, then a process that combines their effects is necessary.
      4. Fourth, design an indicator that informs lab users of the ventilation system's state. Users should understand that the system has multiple flow states and recognize how that supports their work. They can verify, at any time, that the system is operating in the appropriate mode. If it is in the wrong state, they can take steps to restore safety.

Figure 1. Typical Flow Indicator on a Two-State Fume Hood

Figure 1. Typical Flow Indicator on a Two-State Fume Hood


For an exhaust device, the indicator is typically on or at the device. For a two-state room, indicators might be within the room or in the corridor outside.

It is important that the information sent to the user is based on measured airflow, not just the state of the control device. Since mechanical systems can be complex, the airflow is not always what the controller commands. Consider an actual case of a two-state fume hood system using a two-speed fan motor to set the flow. The exhaust was directed through a barometric damper. When the fan ran at the low speed, the damper closed, causing the hood to leak. The system indicated that the hood was working at the low flow rate since the motor was running, when in fact there was effectively no flow and no containment.

Two-State Rooms

Some systems reset flow rates room by room. Safety professionals reduce the ventilation rate for some rooms if they are confident that workers are not inside executing procedures that create hazards.

If the flow rate is based on occupancy, a familiar set of methods for the building automation system can determine the rate, such as schedules, manual switches, people detectors, and combinations of these. It is important to select methods that make sense for the particular room and the way people use it. For example, in a teaching lab, a schedule might be appropriate. On the other hand, the instructor might need to take responsibility for manually selecting high or low ventilation.

Designers should carefully examine their assumptions about what users will do. For example, sometimes a light switch is used to indicate occupancy. In a facility with good daylight, that might not work. People detectors are not always effective either; it depends on the kind of sensor and the activity of the people.

Some lab rooms change state for reasons other than occupancy. Emergency ventilation is one example. If the lab workers have an accident, they can exit the room and initiate the emergency mode. They may press a switch to signal the emergency, or make a phone call. The ventilation system then switches over to a set of flow rates selected for the emergency condition. In emergency operation, it is important to avoid the pitfall of pressurizing the room to the extent that it becomes difficult to open the doors.

Two-State Fume Hoods

When airflow reduction applies to a fume hood, a health and safety official and a ventilation engineer identify two operating states:

  • Sash open with a worker at the hood
  • Sash closed

Determining the flow rates required to contain the hazard in each state requires consideration of the hood's containment characteristics as conditions change in the actual working environment. This means paying attention to the “As Installed” and “As Used” performance.

Flow rate, or face velocity, is just one factor affecting the performance of a fume hood as a containment device. Before setting the airflow, designers should consider other factors, such as room air currents and equipment within the hood. For support, the design team can turn to the large body of published industrial hygiene research that emphasizes the other influences on hood performance (Ahn, Woskie, DiBerardinis & Ellenbecker, 2008). To automatically apply a rule of thumb would neglect worker safety and energy efficiency.

Detecting the opening of a sash is usually easy, with one or more switches that respond to sash movement. When a sash is opened, the system selects the higher flow rate. When the sash is closed, the system selects the lower flow rate.

Some designs switch the flow rate for all the ventilation equipment in the room (supply, general exhaust, and fume hoods) as a unit. In such a case, the controller checks each fume hood sash and the occupancy state of the room to select the flow setting.

Reduced Flow With an Open Hood (Not Recommended)

In some cases, designers propose to lower the hood flow rate while the sash is still open, but the worker is apparently not present. This approach has serious disadvantages. Whether the criterion is energy conservation or safety for workers, closing the sash is a better idea.

Consider the energy use first. (Energy use is not the most important issue, but typically it is the reason to reduce airflow.) Advocates of reduced flow for an unattended hood sometimes go as far as to achieve a 40 percent reduction (e.g., 100 fpm attended and 60 fpm unattended). Closing the sash, however, frequently enables an 80 percent flow reduction. That's twice the savings. Figure 2 illustrates a numerical example.


Figure 2. Sash Position and Airflow Rate

Figure 2. Sash Position and Airflow Rate


Clearly, closing the sash saves more energy than leaving it open and reducing the flow, but it is apparently easier said than done. Anecdotes abound of sophisticated ventilation systems that fail to save energy because the sashes are left open. There appear to be difficulties getting lab workers to use equipment correctly. That does not mean designers should give up. It sends exactly the wrong message when we improve the efficiency of unsafe practices.

Nearly any Chemical Hygiene Plan requires hood users to close the sashes whenever possible. This basic principle appears throughout lab safety literature, including standards from NFPA, ACGIH, SEFA, and AIHA. We need to take this idea seriously and address the real problem. There is a wide variety of approaches to sash management from which to choose. Solutions include the technical fix (automatic sash closers), human behavior changes, and a combination of the two (Coogan, 2008). There are many options, but giving up is not one of them. A sash management plan is a prerequisite in the Energy Performance Criteria from Labs21 (Labs21, 2008). Lab managers need to address it; lab designers ought to help them.


With attention to the design process, two-state ventilation control systems can be very efficient and fit well into an organization's safety program. This paper presents a four-step design process that encourages the ventilation designer to work with the institution's safety programs rather than against them.


ACGIH. (2004). Industrial Ventilation: A Manual of Recommended Practice (25th ed.). Cincinnati, OH: American Conference of Governmental Industrial Hygienists.

Ahn, K., Woskie, S., DiBerardinis, L., & Ellenbecker, M. (2008). A Review of Published Quantitative Experimental Studies Factors Affecting Laboratory Fume Hood Performance. J Occup Environ Hyg, 5, 735–753.

AIHA. (2003). ANSI/AIHA Z9.5-2003 Standard for Laboratory Ventilation. Fairfax, VA: American Industrial Hygiene Association.

Coogan, J. (2008, April). Green Lab Facilities: Steps Toward Sustainability. Technology report, Siemens Building Technologies, Inc.

Labs21. (2008). Labs21 Environmental Performance Criteria (Version 2.2). Retrieved October 10, 2008. Latest version is available at

NFPA. (2004). Fire Protection for Laboratories Using Chemicals, NFPA 45. Quincy, MA: National Fire Protection Association.

SEFA. (2005). SEFA 1-2005 Recommended Practices for Laboratory Fume Hoods. MacLean, VA: Scientific Equipment & Furniture Association.


Jim Coogan, P.E., is a principal in product development and applications for Siemens Building Technologies. He has over 25 years experience designing microprocessor-based controls for mechanical systems, with 19 of those spent in the HVAC industry. Jim has served chairman of ASHRAE Technical Committee 1.4, Controls and is an active member of TC 9.8 Laboratory Systems. Publications include technical papers on room pressurization and laboratory commissioning.