Air handling unit safety devices, or “limit” devices as they are alternately referred to, are devices that serve to protect a built-up air handling unit from unsafe operation, by putting a limit on the variable that they are sensing, and “tripping” when that limit has been exceeded, thereby halting the normal operation of the air handling unit. A typical VAV air handling unit may be equipped with some or all of the following safety devices (and perhaps others as well): 

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Low limit temperature controller
High limit discharge static pressure controller
Low limit suction static pressure controller
Duct smoke detector(s)
Fan vibration switch(es)

Low limit temperature controllers protect hot and/or chilled water coils from excessively low temperatures. High and low limit static pressure controllers prevent fan housings and ductwork from pressure extremes. Smoke detectors monitor for smoke in the airstream. And fan vibration switches monitor for excessive fan vibration. All of these controllers are used to protect the equipment in the event of abnormal system operation. 

Safety devices such as these are typically electromechanical or electronic in nature, and the setpoint or “trip” point adjustment is integral to the device. Also typical to such devices is the requirement for manually resetting them once they trip, generally accomplished via a manual reset pushbutton on the device itself.

Most often a device serving as a safety or limit will have a single set of contacts to wire to, whether it be a single-pole-double-throw contact, or simply a normally closed contact that opens upon a trip of the device. Generally speaking, in a Building Automation System (BAS), the devices are specified to be “hardwired” to shut down the supply fan, the return fan (if applicable), and interrupt power to the outside/return air damper actuators and hot/chilled water valve actuators. Interrupting power to the outside and return air damper actuators would cause the outside air damper to spring closed and the return air damper to spring open. Interrupting power to the hot and chilled water valve actuators would (typically) cause the hot water valve to spring open and the chilled water valve to spring closed. This of course is providing that the dampers and control valves are equipped with spring-return actuation.

These devices could be wired in series, such that a trip of any device in the “limit circuit” would interrupt 24-volt power to all of the above components (fan starter relays, damper and valve actuators). However defeating this concept is the general requirement that a trip of any safety device needs to register through the BAS, in order to flag the condition and generate an alarm. Common practice dictates instead that each safety device powers a low voltage double-pole-double-throw (DPDT) relay. Power for the relays is commonly sourced from a single control transformer, perhaps located inside the control panel that houses the AHU control module. Each relay is powered through the normally closed contact of its respective safety device, such that when power is applied and all is well (no safety device trips), all relays are energized. Now the hardwire interlock “limit circuit” can be accomplished via series connection of one pole of each of the double-pole relays, using the normally open contacts (which are closed when the relays are energized). As long as all safety devices are untripped, and hence all relays are energized, the “limit” circuit is intact, and the AHU is allowed to operate as intended. A trip of any device de-energizes its respective relay, and the limit circuit is broken.

The other pole of each relay is used to report safety device status to the BAS. Though preference may vary, the normally closed contact of this pole can feed a binary input on the AHU control module. This is done for each device. Since normal operation of any safety device results in its relay being energized, each binary input normally reads an open value. When a safety trips, its relay de-energizes, and its respective binary input reads a closed value. The BAS is programmed to recognize this and register a safety device trip alarm.

Tip of the Month: Design and fabricate a control panel specifically for the safety device interlocking. House the common control transformer and all of the relays within this panel, and label each relay as to its function. Add a terminal strip for all incoming and outgoing wiring. Wire all safety devices back to this panel. Wire the limit circuit within the panel, from relay to relay, and out to the devices that are to be affected (fan starter relays, damper and valve actuators). Locate the panel adjacent to the control panel that houses the AHU control module (next to it, underneath it, etc.), to facilitate wiring between the control module binary inputs and the relays. Or simply upsize the AHU control panel to accommodate these relays and terminal strips. Label the terminals and all incoming/outgoing wiring appropriately.

While this may seem more labor-intensive than field-mounting the relays, it pays dividends when it comes to system commissioning, troubleshooting, and general maintenance.

[an error occurred while processing this directive] About the Author

Steve Calabrese attended the University of Illinois at Chicago (UIC) and graduated in 1990 with a B.S. degree in electrical engineering. He began his HVAC career as a design engineer at Air Comfort Corporation, a family-owned mechanical services company. In 1993, Steve went to work for Midwest Mechanical Construction Company, a large, full service mechanical contractor serving the greater Chicagoland area. During his eleven years there, Steve migrated through various roles as mechanical systems designer, control systems engineer, controls group manager, and engineering manager.

In 2003, Steve’s book, Practical Controls: A Guide To Mechanical Systems, was published. Geared toward the HVAC professional, the book details practical methods of controls and defines the role of HVAC controls in an easy-to-understand format. Steve brings his mechanical contracting experience to this writing, and offers practical approaches to control systems issues.

In 2004 Steve left the mechanical contracting end of the industry to go to work for Chicagoland’s most tenured Automated Logic dealer. As a project manager-engineer, Steve’s day-to-day operations include control systems design and consultation, parts selection/procurement, installation management/support, startup and commissioning support, and project fiscal and schedule management.

As a member of several professional associations, Steve actively participates in local seminars and training endeavors. He attributes his mechanical HVAC knowledge and expertise, which includes pipe/duct sizing, load calculations, equipment selection, and shop drawings generation, to working many years for a mechanical contractor. Steve couples his broad mechanical knowledge and experience with a strong background in the area of electricity and electronics. His control systems expertise runs the range from estimating and engineering to programming and commissioning, and he is well-versed in electrical and electronic stand-alone controls, as well as microprocessor-based direct digital controls (DDC) and networked Building Automation Systems (BAS).

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