Consider Alternative Control Methods for Operational Continuity in Critical Environments

By Jason Newhuis, PE, BSCx, CEM, LEED AP, Sr. Project Manager, and Jerry Bauers, P.E., Vice President, NV5

Data centers, commercial office buildings and, especially healthcare facilities, all have one thing in common—mission critical operational space that would suffer from a lengthy shut down or poor operation of HVAC or other mechanical/electrical system. Further, each of these critical environment areas has a specific set of environmental and code compliance requirements for mechanical and electrical systems that must be met by facilities teams to sustain the business. These systems impact production, research conditions, occupant comfort and safety.

As an example, an inpatient surgical environment must rely on a correctly designed, constructed, commissioned and operated space that functions correctly every time. In these spaces, the air is clean; pressurization is sustained; the temperature and humidity are stable and reliable; and, power and lighting are dependable. Perhaps the most critical criterion is that these facilities operate without substantial interruption in any operating condition. System reliability is put in place to support the number one driving factor: patient care. The facilities team supports this mission by delivering, operating and maintaining facilities so that healthcare professionals can do what they do best—practice medicine.

While many facility managers rely on hard safety control points to manage equipment operations, we would argue that there is a better way. The following outlines alternative control methods for responding to failures and deviations in operating conditions as they apply to HVAC systems in healthcare.

Alternative Activation

Hard safety control points are often used to interrupt equipment operations and protect the equipment from damage. Common hard safeties in ventilation systems include:

  • duct static pressure limit switches
  • low temperature limit switches (aka freeze-stats)
  • discharge air humidity limit switches
  • duct smoke detection

Standard hard safety limits act as an on-off device similar to a standard light switch. In effect, you either have power or you don’t. Standard limit switches are typically hard wired into a fan or other device via a relay and are either latching—meaning once the switch closes it requires manual intervention to reset—or non-latching where once the fault condition clears the safety limit, it resets itself. On-off devices serve as a great way to protect equipment from potential malfunction, but are a very poor way to sustain a critical operation. In fact, the activation of an on-off safety device is in essence a failure condition as opposed to a continuous operation solution where a single variable may be out of range.

With the exception of smoke detection, safety control points can all be approached in an alternative manner to promote continuity of operations while still providing equipment protection. An alternative approach to managing critical deviations entails analog alarm and alert conditions that allow the operator to recognize and respond to “failing” conditions before they become catastrophic.

Here’s how this alternative approach might work.

Reconfigured Loops

The NV5 team was brought in to help correct deficiencies with operation of the Department of Veteran Affairs medical and surgical intensive care center. These deficiencies had delayed occupancy for months. The spaces had consistently unreliable environmental control, specifically uncontrolled space humidification.

On our first visit, condensation was raining down windows and beading on the ceiling diffusers. It was clear the ventilation systems were out of control. The primary ventilation air handling unit was equipped with a steam humidifier that had a single control valve operating to achieve zone humidity with sensors located in the space. The system was controlled with a space sensor, a discharge air humidity sensor and a discharge air high humidity limit switch.

Through testing and observations, our team determined that, there were two primary issues. First, the control loop for the humidification sequence was very slow and would over humidify the space far beyond the setpoint causing the observed condensation on chilled surfaces. Secondly, the high humidity limit would activate unnecessarily due to poor steam atomization and small overshoots in discharge humidity.

We determined that as the demand for humidification in the space increased, the operation of the humidifier would approach the high humidity limit setpoint. This high limit safety would then activate the safety switch causing the humidifier modulating control valve to close. The humidification control loop would then “wind up,” so once the high limit humidity in the ductwork cleared, the steam valve output signal would open the steam valve quickly, raising the duct humidity again near the discharge humidity limit and trip the steam valve—creating poor valve control and humidification failure due to repeated activation of the humidity limit over time. The repetition caused radically fluctuating zone humidity condensation in the ductwork as the humidifier cycled from the off position to full steam valve stroke in repeated short bursts.

As a solution to the problem, we reevaluated the use of the high limit safety cutoff. The control loop was reconfigured to include both a space humidity proportional-integral-derivative (PID) control scheme in parallel with a duct humidity PID loop to limit high humidity in the ductwork replacing the high limit switch.

The single, oversized steam control valve for the humidifier was replaced with two valves in a 1/3 – 2/3 configuration. The two PID control loops, which controlled the valves from the lowest output signal, were configured to avoid initiation of the limit switch in the absence of a physical failure of the control valves. While the high limit safety switch was retained, the analog control of duct humidity eliminated steam valve cycling and excessive duct humidities. The high duct limit safety switch now acts to protect the system from equipment failure rather than normal control system oscillations. Environmental control was restored to the intensive care spaces, making them once again useable.

Modulating Limits

The alternative control methodology was used in a different scenario at the Richard M. Ross Heart Hospital, located on the campus of Ohio State University in Columbus, Ohio. The hospital specializes in cardiology and its heart program is ranked #20 in the US. On this project, continuity of operations was at the forefront during the design effort for our commissioning team. Mitigating potential infection risks during open heart surgeries required clean air changes and pressurization without failure. To avoid nuisance fan failures, the project team had to ensure that the supply fan static pressure limit was not allowed to trip in the absence of a real hardware failure.

In this case, the project team used an analog static pressure limit transducer to allow the control system to modulate fan speed and provide safety alarm functions. As the static pressure approached the limit setting, the fan speed signal would be trimmed to the variable frequency drive slowing the fan until it was under the limit pressure—as opposed to the more conventional approach of shutting the fan off with a hard limit. Through testing in multiple operations and failure conditions, we demonstrated that an analog static pressure limit transducer provided the protection needed for the equipment and continuity of airflow to meet the mission.

Whether looking to improve current operations or planning to meet future needs, the first step is to define the key environmental requirements that are necessary to meet the mission of your business. Small modifications during design or operation can have a huge impact on sustaining the function and performance of critical facilities. Changing your approach to implement safety controls can not only prevent nuisance faults, but provide a high level of protection for both equipment and operations, resulting in a lower risk of failure in critical conditions. ss.


About the authors

Jason Newhuis, PE started his career in 1998 as an HVAC technician and later, after graduation from Purdue University, transitioned to consulting engineering in 2005. His primary focus is commissioning, retro-commissioning and project management. Jason specializes in field services trouble-shooting and optimization, drawing on his early hands-on career. He currently maintains professional certifications through NCEES (Professional Engineer), NEBB (Building Systems Commissioning), AEE (Certified Energy Manager, Certified Demand Side Management Professional and Green Building Engineer) and USGBC (LEED Accredited Professional).

Jerry Bauers, PE, has delivered building design, commissioning, and retro-commissioning services for overt 40 years. He is a NEBB Certified Professional in building system commissioning, an NEBB past president and the former chairman of NEBB’s building system commissioning committee. Jerry specializes in commissioning and retro-commissioning services for pharmaceutical manufacturing facilities, research and teaching laboratories, and data and telecommunication facilities.


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