Limitations of fan filter units

Fan filter units are used in a T-grid ceiling system or Sheetrock ceiling. What is often overlooked is that fan filter units with certain constant-volume ECM fan motors are not recommended for use with all systems.


Fan filter units are used in a T-grid ceiling system or Sheetrock ceiling. Courtesy: AABCTesting in hospitals, pharmacies, and clean rooms often requires testing and adjusting fan filter units. A fan filter unit consists of a fan, a motor, and a filtration system in a single unit that is used to provide particle-free, laminar airflow to the room or working space. It may have an optional electronically commutated motors (ECM) fan motor available, which is designed to adjust the fan torque and speed to maintain a constant airflow and compensate for filter loading. Fan filter units are designed to be installed in a T-grid ceiling system or Sheetrock ceiling; 2x4 or 2x2 sizes are available.

Fan filter units are a popular choice of designers when room air-changes-per-hour (ACH) requirements are high, when the primary air handler is not HEPA-filtered but a space it serves requires high-efficiency particulate air (HEPA) filtration, or when the primary air-handler performance is not capable of overcoming the HEPA-filtration resistance. What is often overlooked is that fan filter units with certain constant-volume ECM fan motors are not recommended for use with all systems.

ECM fan motors and system compatibility

In their operation and maintenance manuals, manufacturers caution about directly connecting the supply ducts of terminal units, constant- or variable-volume, to their fan filter units. The airflow serving the fan filter units needs to be balanced, and a start-up sequence needs to be considered. The problem with the constant-volume ECM motor is that the motor is easily stalled if under- or overpressurized. Once an ECM motor stalls, it will shut itself off and require a voltage cycle for restart. One manufacturer states that a neutral static state at the fan filter inlet is the best condition.

Stalled ECM motors were encountered on a recent hospital pharmacy project. The fan filter units were served by venturi constant-volume terminal units. Contrary to typical balancing where the design airflow is the target, when balancing to pressure-sensitive and specific ACH requirements, balancing to the design airflow is only the start. Once at design airflow, the room pressures and airflow directions dictate if further adjustments are required. The room ACH requirements are just as important, but should be calculated prior to balancing to determine that they are sufficient.

The fan filter manufacturer was contacted for a possible strategy to eliminate the fan filter motor shutdown during generator testing. Courtesy: AABCThe pharmacy was balanced and all room requirements were met. The following Monday, it was reported that the pharmacy was experiencing room-pressure alarms. A quick check of the airflows indicated that the supply airflow was low and the supply duct static pressure serving the fan filters was high, an indication that the fan filters were not operating. The area surrounding the pharmacy was occupied, so the option of shutting down the air-handling unit (AHU), cycling the fan filters breakers, and performing an AHU restart was not possible. After several tries, the AHU was able to be slowed down to a speed that allowed the fan filters to reset and regain operation.

Why did the fan filter motors stop operating?

During a discussion with the maintenance staff, it was found that there was a generator test over the weekend. A true power outage would have a better outcome than the generator test procedure. Shutting down one panel at a time instead of all panels may have an adverse effect on mechanical system operations. If the fan filters are shut down after the AHU and smoke dampers are shut, the fan filters have nowhere from which to pull air; and if there are multiple fan filters on a system, the weakest fan filter will stall and shut down. If the fan filters are started up after the AHU, there is the likelihood that all the fan filter motors will stall, which was the case with this project.

The supply terminal serves the fan filter at a reduced airflow, and the remaining fan filter airflow is made up from a return air grille. Courtesy: AABCThe fan filter manufacturer was contacted for a possible strategy to eliminate the fan filter motor shutdown during generator testing. The manufacturer had not experienced this problem previously. However, the manufacturer's representative noted that problems with fan filter hunting was an issue and that is the primary reason that they do not recommend having terminal units supply fan filter units. If there is any fluctuation of airflow in either the fan filter or the terminal unit, hunting results due to two devices trying to control the same airflow. The manufacturer suggested that on systems served with terminal units, constant-torque motors should be used instead of variable-torque motors. Although it is not published, it was suggested that operation outside of ±0.20-in. wc static pressure at the fan filter inlet may stall a variable-torque motor.

The ECM motors on the project were reprogrammed for constant-torque operation. The automatic fan filter speed adjustment for filter loading was lost, but the terminal device should open to compensate. This has reduced the fan filter stalls significantly, but not in every case. During generator testing, the sequence of the electrical panel's shutdown is changed every month, which results in a different condition every month.

Variable-torque-motor fan filter units work very well in applications where high air changes are required but the room temperature loads are minimal. The supply terminal serves the fan filter at a reduced airflow, and the remaining fan filter airflow is made up from a return air grille. If the fan filter is under-pressurized by the supply terminal, additional airflow from the return source will make up the difference. Overpressurization may continue to be an issue if backdraft dampers are installed in the return duct to prevent unfiltered air from entering the space.

Future fan filter projects

For future fan filter projects, the following items are suggested to be reviewed:

  • Determine if the submitted fan filter is acceptable for operation with the AHU supply system. If terminal units are being used and the fan filter submittal states that it is designed to compensate for filter loading (constant-volume, variable-torque motor), it is not the correct application. A constant-torque fan filter motor is suggested.

  • If the fan filter is direct-connected to the duct system, what is the start-up sequence for the supply system and fan filter? A constant-torque fan filter will overcome additional pressurization/under-pressurization, but not in all cases. An interlock to start and stop the fan filters dependent upon the AHU status is suggested.

  • In most fan filter installations, the ceiling is hard (Sheetrock) and access to the fan filter controls are often not considered until required for testing. If the accessory fan filter remote control is not installed, access will be required for the fan filter controls as well as to the duct for static pressure measurements. Duct static pressure values are required to determine if the supply terminal/system is set up properly.

  • Building automation system fan filter status monitoring, and having all test data available when troubleshooting pressure or airflow problems, is very helpful. If the space is experiencing pressure issues with the same system static pressure being maintained, but the terminal unit control damper is 100% open instead of 75% noted in the test and balance report, there is a good chance the fan filter is not operating.

  • In the case of a hospital, inquire as to the procedure for routine generator and fire alarm testing. The method by which this testing is performed can have undesirable effects on the mechanical systems besides just fan filter motor stalls, such as duct failures, unexpected AHU shutdown, and smoke fire damper closure during AHU operation, which is another subject in itself.

Terry Wright is a senior project manager at Engineered Air Balance Co. Inc. This article originally appeared in Associated Air Balance Council's (AABC) TAB Journal. AABC is a CFE Media content partner. Edited by Joy Chang, digital project manager, Consulting-Specifying

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