Synergistic Systems
The Complexities of Modern Laboratory Ventilation
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In many laboratories, a building automation system (BAS) performs functions that are critical to safety for workers inside the lab and out. When it works as intended and is used well, a BAS helps execute a laboratory ventilation management program. Too often, though, the BAS becomes an obstacle instead, typically due to its complexity and the differing perspectives of the professionals who use and depend on it.

A BAS is conceived, constructed, and operated by many people, and it serves many more. It is a major component of building infrastructure, the primary tool for operating a building, a special-purpose computer network, and a part of the Internet of Things (IoT). Its components include sensors for physical quantities; computers that report and regulate those values; physical actuators that move dampers, spin fans, and fire boilers; and various display devices.
The BAS covers many different spaces in the building—offices, classrooms, the cafeteria, collaboration spaces, laboratories, and lab support spaces—as well as the exhaust fans on the roof and the cooling plant in the basement. In many cases, the BAS reaches far beyond one building, tying together an entire campus.
These many functions are important context for the role of a BAS in lab ventilation. Typically, the BAS is the place where we tell the mechanical system what we need: the air change rate for a room, the face velocity for a fume hood, the alarm limits for a user’s display, the schedule for the workers coming and going, and so on. The BAS dynamically adjusts every movable part of the ventilation system to fulfill those requirements and tells us whether the system is working or which parts are not. Finally, we need to use the BAS interactively to troubleshoot and resolve problems.
Lab room controls and fume hood controls may be “part of” the BAS, meaning they are part of the same product family, installed and maintained by the same contractors; or they may be “integrated to” the BAS, the same way the fire alarm system is integrated. Expectations for data access and control performance should be the same either way.
THE MECHANICAL SIDE OF LAB VENTILATION A lab ventilation system usually has central air handling units, each supplying conditioned air to multiple rooms, through a branching duct network called the air distribution system. Complementing the supply is a central exhaust system with large fans that draw air from multiple rooms and exhaust devices such as fume hoods and storage cabinets through a separate, converging network of ducts. Both the supply and exhaust sides of the system have control terminals, which are mechanical devices that physically restrict airflow. An airflow control system governs each terminal and fan as though it is independent from the others. Usually, it works to think about it that way. Sometimes we need to think about the interactions.
Each terminal regulates flow to the amount needed for the function it serves: fume hood exhaust, room air supply, storage cabinet ventilation, and so on. Most air terminals have an actuator, a damper, and a sensor. The actuator receives an electrical position command and moves the damper, which partially blocks the air path to regulate airflow. The sensor measures the airflow. Many varieties of sensor exist; the most common measure “velocity pressure” and differential pressure.
Every air terminal responds to a controller programmed to set the flow required to satisfy the purpose of the devices. The controller repeatedly determines the flow setpoint, measures the airflow, compares those two values, and calculates and commands an adjusted position for the damper. Some controllers handle more than one air terminal.
When the fan or any terminal in the system changes its operating state, all the others are affected by changing duct pressure. The airflow sensor indicates this change, and the controller adjusts the damper, restoring airflow to the setpoint. Because of this behavior, the air terminal is said to be “pressure independent.”
Terminals that use mechanical feedback instead of sensor feedback are called “mechanically pressure independent.” Usually, these terminals are installed without an airflow sensor. The control system infers that if the actuator is in the right place, the airflow has approximately the right value, due to the action of the mechanical regulator. This affects the information available for monitoring and troubleshooting. If, for some reason, the terminal does not deliver the desired airflow, the control system doesn’t receive that information and can’t record it or pass it to users.
THE FUNCTIONAL (SAFETY) SIDE OF LAB VENTILATION Lab exhaust equipment includes fume hoods, other exposure control devices, and some lab apparatus. For each exhaust device, it’s important to establish what airflow rate is required, how it is selected, and how it is controlled. First, ask if the best way to operate the device is constant flow, switched on and off, or variable flow. Make sure lab HVAC designers and facility operators all use the same answer. If variable operation is selected, get a clear, complete list of the dynamic factors that set the flow rate. We need this information to design, test, or use the lab.
To combine effective containment and energy efficiency, we want the exhaust flow for a fume hood to respond to the sash. A fume hood whose exhaust flow increases as the sash is opened and decreases as it’s closed is known as a variable air volume (VAV) fume hood. Usually, this behavior is accomplished by controlling face velocity—that is, the velocity of air at the hood opening. Facility engineers, HVAC designers, and safety officers collaborate to select the intended face velocity according to the hazards involved, the containment characteristics of the hood, and regulatory requirements. The same stakeholders need to set the minimum airflow drawn when the sash is closed. The implications of these quantities are explained in the ANSI/ASSP Z9.5 standard, Laboratory Ventilation. For VAV fume hoods, the fume hood controller communicates with the room ventilation controller, and together, they dynamically coordinate airflow rates in and out of the room.
The term “face velocity” gets confusing when the sash is closed or nearly closed. As shown in Figure 1, the shape of the openings in the hood and the air pathways from the room to the hood are irregular. The openings include the sill at the work surface of the hood, gaps in the superstructure, the bypass above the sash, and the grill that covers it. Hood manufacturers design these openings to direct air streams for the purpose of containing and removing contaminants. The irregularity of the openings and air pathways complicates any attempt to characterize the hood’s “effective open area” and to relate exhaust airflow to “average face velocity.” If the open area is underestimated and the exhaust flow is set accordingly, a hood can leak when the sash is closed.
Figure 1. Air pathways (blue arrows) into a fume hood with sash closed (left) and open (right).
When the sashes are closed, is the remaining open area large enough that maintaining the face velocity exceeds the minimum flow required to clear the hood? Or is the minimum flow large enough that the “average face velocity” through the openings exceeds the required value? It would be good to know which condition you have, but it might be hard to tell. In practice, it’s difficult to measure the exhaust flow or face velocity when the sashes are closed. Still, we need to take both into account when setting up a VAV fume hood controller.

Figure 2 illustrates the operation of two hoods with the same face velocity (80 feet per minute, or fpm) and minimum flow needed for clearance (300 cubic feet per minute, or cfm) but different amounts of open area when the sashes are closed. Hood A, which has a larger opening, works on velocity control even with sashes closed, because the corresponding airflow is greater than the value needed for clearance. Hood B, which has a smaller opening, operates at the clearance flow, which results in a greater velocity through the opening.
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Figure 2. The charts illustrate operation of two variable air volume fume hoods with the same face velocity (80 feet per minute), flow needed for clearance (purple line, 300 feet per minute), and bypass opening (red line, 4 inches) but different values for the minimum opening (green line, 3 square feet vs. 5 square feet). The charts show the way open area, exhaust flow, and inward velocity vary with sash opening height.
For both hoods, when the sash is raised to the full height (30 inches), the open area is at the maximum value, face velocity is on setpoint, and the exhaust flow is high. As the sash is lowered, flow and open area decrease, and the face velocity stays constant. When the closing sash reaches a height of 4 inches (red line), the bypass starts to open, and the area stops changing as the sash continues to close.
At this point, the flow for Hood A is still above the value (300 cfm) needed for clearance. The flow for Hood A doesn’t go any lower, so the clearance flow does not come into play, and face velocity is maintained through the entire range of sash movement.
For Hood B, when the closing sash reaches the height of 6 inches, the flow reaches the clearance value. This value is applied by the hood controller so further reduction in area as the sash closes does not reduce the exhaust flow, and the velocity starts to rise. Velocity continues to increase until the bypass starts to open and the area stops changing. Compared to Hood A, which has larger openings, Hood B runs at a lower flow and higher velocity when the sashes are closed.
Most face velocity control systems use one of two approaches: sash position sensing or sidewall sensing. When lab safety officers are evaluating or improving the performance of a hood, they probably need to know which type of control system a facility is using. Some hood control systems combine the two approaches, but that’s uncommon. For more information, refer to the ASHRAE Handbook.
Fume Hood Control by Sash Sensing The sash position sensing system uses physical sensors to determine the location of each movable sash. The controller is configured with measurements of the width of the main opening and other relevant dimensions. It combines these figures to calculate the effective open area from the current sash position readings and multiplies the calculated opening by the desired face velocity to set the currently required exhaust airflow.
If the airflow setpoint to maintain face velocity is less than the minimum flow needed to clear contaminants, the controller uses that minimum value instead. Then the closed-loop control process (measure, compare, adjust, repeat) drives the actual flow to the selected setpoint.
When it works right, the mechanical flow control device and the flow control electronics coordinate to regulate the airflow, and the fume hood operates as intended. Engineers describe performance of the flow control system in terms of speed, stability, and accuracy, while safety officers see the effects in terms of containment (for more information, refer to a report from Siemens). Flow control performance depends on the characteristics of the electronic controls as matched to the characteristics of the mechanical system. Achieving performance is a job for a control technician. The safety officer needs to remain aware that changes in the mechanical system can create a need to adjust control parameters. Different operating points of the complex manifold exhaust system can affect flow control and containment at individual hoods.
When problems are suspected in the fume hood control system, checking the exhaust flow setpoint is a good way to isolate them. If the exhaust flow setpoint is wrong, the problem is in the controller’s information about the fume hood. If the exhaust flow setpoint is right, the problem is between the controller and the mechanical system. The technicians working with you should follow that line of thought.
Fume Hood Control by Sidewall Sensing As with sash sensing, sidewall sensing uses a movable damper to restrict the flow. While the interface with the mechanical system is the same, the representation of the fume hood is completely different. A sensor installed in a special opening through the structure of the hood indicates the air velocity through that passage. This velocity is physically related to the velocity through the other openings, including the open or partially open sash. It’s not the same thing as the face velocity; it’s a different opening with vastly different characteristics. That sensor provides an indication of average face velocity, but we don’t want to mistake it for the actual average face velocity. If the sash opening increases and the flow does not change, the velocity through every opening goes down. The sensor indicates a reduced velocity, causing the control system to increase the flow.
Compared to a sash-sensing fume hood control system, a sidewall-sensing system offers the following advantages and disadvantages: • There is no need to identify and account for the fixed openings in the hood. The sensor “feels” their effect. • The working range of the sidewall sensor is small because the intended velocity is constant; the desired flow rate goes through a wide range, making sensor selection more challenging. • In a sidewall sensing system, we still need a way to maintain the minimum exhaust flow to remove contaminants. This could be accomplished by adding a flow sensor or making sure the bypass is large enough to draw the required flow under face velocity control. • Coordinating the hood with supply and exhaust flows in the room is harder because the sidewall sensing system has no airflow data to share.
AIRFLOW INDICATOR FOR FUME HOOD USER Various authorities require that fume hoods include a device to tell a lab worker whether the fume hood is working. These devices vary in several ways: • the indicator could be numbers on a screen, red-yellow-green lights, a needle on a gauge, or audible alarms • the physical variable indicated could be face velocity, exhaust airflow, hood suction pressure, or duct suction pressure • the source of the information could be the hood control system or a separate sensor
Regardless of the type of device, lab workers need to know what the indicator means and how to respond to it. If the device displays a number, workers need to know what values to expect and what to do when the number is out of the expected range. At a minimum, they should know who to contact and whether they should continue working. If the display is just a set of lights, lab workers need to know if work can continue and when to call for help.
The maintenance workers need a deeper understanding of the display depending on their role. The Z9.5 standard elaborates on this topic.
The safety officer needs to understand the source of the information because that affects how to use it. Is the control system reporting the same data it uses to operate the hood, or an independent look at what the control system is doing? Both are useful, but in different ways.
When certifying a hood or investigating problems, it’s useful to compare manual measurements with the information delivered by the monitor. For more information, see the article beginning on the next page.
FIXING FLOW CONTROL PROBLEMS If the hood controller can’t maintain the desired airflow, the problem can be in the local airflow control loop or in the central exhaust system. Either way, a technician works through the BAS, manually commanding dampers and fan motors to find and correct the problem.
If there is not enough suction in the duct, the flow is too low even when the hood controller opens the damper all the way. If the suction is too high, the local flow control might be too sensitive and fluctuate. If the suction pressure changes too much, then the individual flow values don’t settle and the controller is always looking for a new setting.
Managing the exhaust system is getting more complicated. The VAV exhaust system described earlier includes multiple exhaust terminals, each with its own flow controls, and one or more fans with a fan control system. Previously, the usual practice was to run the fan controls and each flow controller as though they were separate, independent systems; they were connected mechanically, but control algorithms were not connected to each other. Today those systems are increasingly connected. “Duct pressure reset” strategies save energy and can improve the action of the flow controllers. As usual, this engineering optimization adds complexity. The fan control algorithm is now connected to the flow controls, and they affect each other directly, as well as through the mechanical system. Values that would have been selected and held constant are now dynamically adjusted.
Due to this complexity, the HVAC staff, control technicians, and commissioning agents have more to learn, more to set up, and more places to look when problems arise. Commissioning agents should explicitly test the effects of the fan control system on flow controls for the fume hoods and rooms.
LONG-TERM MONITORING While the complexities of these systems are challenging, today’s computer network technology and capacity for remote data analysis opens startling possibilities for managing laboratory ventilation. The IoT presents new data sources; air contaminant sensing becomes more common and more capable; remote servers log HVAC operating variables. Periodic performance reports confirm normal operation and highlight anomalies. Newer analytic techniques recognize patterns and detect faults as they develop. The Z9.5 standard directs building owners to explore the possibilities and see what’s practical for them. Rather than living with the reality that buildings degrade as they age, we can enable them to get better.
JIM COOGAN, PE, is an engineer at Siemens in Buffalo Grove, Illinois, and a member of AIHA’s Laboratory Health and Safety Committee.
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American National Standards Institute: ANSI/ASSP Z9.5, Laboratory Ventilation (2022).
ASHRAE: “Laboratories,” chapter 17 in the ASHRAE Handbook.
International Institute for Sustainable Laboratories: “Best Practices for Building Automation Systems in Laboratory Environments” (webinar, September 2020).
Siemens: “Performance Report: Safe Airborne Hazard Control for Critical Environments.”