Douglas Krantz - Technical Writer - Describing How It Works

Can Just Any-Old Input on an AHU be Used for Fire Shutdown?

By Douglas Krantz | Life-Safety

Can Just Any-Old Input on an AHU be Used for Fire Shutdown?

Can Just Any-Old Input on an AHU be Used for Fire Shutdown?

Greetings Douglas,

Please let me know your opinion about "how" the fire alarm system must execute the shutdown of air handling systems during a fire alarm event.

The issue I am seeing more and more in the field is that the contractor, through its fire alarm subcontractor, is being told that the correct way to wire and execute the fire alarm shutdown sequence is to wire the fire alarm signal, from the fire alarm relay to an "input" to the HVAC system AHU controller. This engineer believes this is NOT correct on many levels, and wants this bad practice to stop.

I am seeing more and more fire alarm shutdowns of air handling units and other fans being performed by fire alarm subcontractors wiring into an "input" of some AHU manufacturers' controllers.

I also see them wiring in series sometimes with safety devices such as pressure switches, drain pan water switches, freeze stats, etc. In most cases, I "hear" they are directed by some manufacturer representatives who are "saying" it is ok. I am saying it is "not ok".

We must always remind ourselves that smoke usually kills before the fire ever gets there. Rated firewalls help to delay/stop the fire. F/S dampers help to stop the smoke through the ducts. Proper UL-rated control systems stop the fans, the spread of smoke, and save lives.

The fire alarm relay and associated wiring must cause the shutdown of the AHU/fan directly and not through an HVAC controller software.

The AHU/fan must shut down no matter what mode the HVAC controls are in, automatic or manual. There can be no override to the fire alarm shutdown; it is the highest priority. Historically this has been accomplished by breaking a power source to the motor either directly or by breaking a relay/contactor that supplies the power, through a hand-off-auto switch, or control power.

Similarly; when I look at VFD submittals, I see only "multi-purpose" wiring connections for "generic shutdown", but nothing specifically called out for fire alarm or life safety rated. But, what I do see is that some VFD [Variable Frequency Drives], "start/stop/interlock" inputs actually cut a power source on the VFD board which is more appropriate than depending on a HVAC controller software algorithm to react. It would be important to have a fire alarm shutdown signal for the VFD and bypass unit if a bypass is installed.

I like to say building codes are minimum requirements, but they must be met 100% to meet "code".

Thank You, RM

I'm speaking as a technician. I have had to deal with certificates of occupancy, and final testing by fire marshals. What I have always done is the "how-to" part, and the fire marshal and other AHJs deal with the "so-prove-it" part.

Air Handler and Smoke Damper Sequence

When looking at the meaning of the codes regarding air handlers and smoke dampers, the purpose of the rules and codes is to show what is required to prevent the spread of smoke.

In essence, the fan motors in air handling units have to stop whenever smoke is detected at or in the air handling unit. That "Stop" has to occur no matter what. Also, if smoke is detected inside a plenum passing through a smoke wall or fire wall, the smoke damper has to close. That closing has to occur no matter what.

The rules and codes talk about the detection of smoke, and the stopping of the fan blades or closing of the damper; the rules and codes don't talk about the alarm relay contacts or the wires.

Many times, though, the fire marshal or some other AHJ requires more than the bare minimum shown in the rules and codes. Sometimes, the AHJ also requires that all fans stop and all dampers close with any fire alarm.

Many of AHJs also know about the plenums ballooning out or being crushed when the dampers close at the same time as the fans shut off (without throwing a crowbar into the fan assembly to instantly stop the fans, the fans do take time to actually stop), so they will require a :10 second delay between stopping the fans and closing the dampers.

Authority Having Jurisdiction

See the article: Just Who Is the Authority Having Jurisdiction (AHJ)? at for more clarification.

As a representative or designated agent of the owner, you can be considered by the National Fire Alarm Association (NFPA) to be an AHJ, with authority over the design of the fire alarm system.

On the other hand, the contractor, being paid to install the fire alarm system, does what they're told to do. If the AHJ says to do more, the contractor does more. They do get their money, don't they?

Circuit Classifications

See the article: NFPA's 7 Classes of Fire Alarm Paths for a summary of the major fire alarm paths.

The issue with the air handlers is that the total pathway is really from the alarm circuitry within the fire alarm control panel, all the way to the fan blades in the air handling unit. For life safety purposes, the total pathway is end-to-end; it's from the circuit in the control, to the fan blades. The pathway cannot really be considered to be only the wires in the pathway.

There is a difference between a non-supervised pathway and a supervised pathway.

Non-Supervised Class E Pathways: Class E pathways cannot be considered life safety pathways because the devices attached to the Class E pathways can fail to be controlled by the fire alarm system. Also, if there's trouble with the device, a trouble signal does not get back to the control panel.

A printer, for instance, that may not be even turned on most of the time, is not supervised: it's connected on a Class E pathway. In this case, even though the wires are connected, if the printer is turned off, the printer fails to print.

With a Class E pathway, a failure anywhere on the pathway doesn't even show a trouble on the fire alarm panel.

Supervised Class D Pathways: Air handlers and air duct smoke dampers, just like door holders, are annoyance supervised; they're Class D pathways. To be Class D pathways, if the circuit isn't working, like a wire comes loose or breaks, the fan will stop, or the damper will close, or the door will close.

People have a problem with the stopping of the fan or the closing of the damper. In other words, they get annoyed, and then demand that the problem will be corrected.

Commissioning Class D Pathways

These life safety tests should be performed at submittal time and at the time of commissioning.

With air handlers, Class D pathways are easy to test.

Step One is at Submittal Time: The first part of the test is checking out the bypass ability of the air handler. If the air handling unit doesn't have a listed "Fire Shutdown" input, then the air handler can't really be considered to be supervised in any way; when there's smoke, it can still spread the smoke around the building. In other words, It then fails to be a Class D supervised pathway and is actually a Class E unsupervised pathway. It is not a life safety pathway.

It's often possible to bypass just an "input". Remember that the air handling unit is not maintained by fire alarm contractors. If the air handling unit isn't working, the building's maintenance people or an HVAC contractor will sometimes bypass an input that is causing the air handler to fail to work, and then leave it bypassed because they can't see a problem with bypassing the input.

Step Two is at Commissioning Time: The second part of the test is performed at the panel. Disconnect one of the wires that should shut down the air handling unit, and if the fan keeps going, the circuit is a Class E pathway. It is not supervised.

When the circuit is a Class E pathway, if sometime the circuit fails, the failure won't be found until the shutdown is tested, or when there's a fire, whichever comes first.

After installation, the pathway from the fire alarm contacts on the fire alarm panel to the blades of the fan should be tested. The testing should be with all bypasses active for the air handling unit.


"Listing" is not performed by the manufacturer, it is not performed by the owner, it is performed by a company that is motivated to make sure that the equipment is safe.

UL, or Underwriters Laboratories, is one of many such listing companies.

An insurance company is an underwriter. They have to pay for damages when something is unsafe, so to reduce the payout money, they are safety motivated. Insurance companies have joined together to offer a testing laboratory, UL.

See: History of UL

If a testing laboratory, through their testing, finds that a particular model of equipment is safe, and that it can be used for specific purposes, then the testing laboratory puts that model of equipment on their "List" of equipment that can be used. The manufacturer can then label the model as "Listed".

The manufacture wants this listing on their equipment. To keep the listing, the manufacturer cannot say that the equipment can be used for, for instance, "Fire Shutdown" if the testing laboratory has not tested it for fire shutdown.

On the manufacturer's silkscreen for the screw terminals, or in the equipment's official Installation Manual, if it actually says that the input to an air handler is for "Fire Shutdown", then that has been tested and listed by the testing laboratory for proper fire shutdown of the air handling unit.

The alternative to using a listed fire shutdown for the air handler is to use contactors to disconnect the utility power for the fan.

Fire Shutdown

I have seen VFDs (Variable Frequency Drives), air handlers, and RTUs (Roof Top Units) that have the "Fire Shutdown" screw terminals. With those silkscreened on labeling of the screw terminals, in order for the fans to operate, there either has to be a wire jumper placed between the screws, or the screws have to be connected to the fire alarm system using Class D wiring techniques.

I've also seen the contactors being used to stop the utility power to the fan motors.

Fire shutdown is very technical. Be careful of using highly technical information from manufacturer representatives; manufacturer representatives are salespeople. The don't really have the full electrical and electronic training to know exactly how a system works, so their answers about fire shutdown might not be totally accurate.

On the other hand, the manufacturer's technical support team has to be accurate. Their answers are required to be aligned to the "listing". When you need to know details of how something works, like fire shutdown, call the manufacturer's technical support team. They have to be correct.

But I agree with you, just using any old input for fire shutdown isn't good enough; it has to say "Fire Shutdown".

Douglas Krantz
Life Safety
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