3 Ways to Confirm Airflow in a Smoke Control System

Our team of experts offers regular updates on FPE and life safety best practices, code modifications and more.

Pierce Lushinsky, P.E.


April 9, 2021

When your project requires a smoke control system in accordance with Section 909 of the International Building Code (IBC), there are several prescriptive requirements that form the fundamental basis of a reliable and code compliant system. As a life safety system, a smoke control system is held to a high level of reliability and one of the ways to ensure and demonstrate that reliability is through verification of components of this system. Section 909.12.1 of the IBC, Verification, states “Control systems for mechanical smoke control systems shall include provisions for verification. Verification shall include positive confirmation of actuation, testing, manual override and the presence of power downstream of all disconnects.” But how do we provide verification and how does the system tell us it is actually doing what we want it to do?

To keep this discussion brief, we will focus on verifying that the supply or exhaust fans of the smoke control system are in an operational-ready state both pre- and post- being called for action. One of the most important aspects of verification is monitoring for power downstream of the last disconnect. This is usually accomplished through the use of a voltage monitoring device on the “hot” lead of the fan prior to the motor controller or VFD, but after the service disconnect. The voltage monitoring device then controls a relay that sends a “yes” or “no” signal to the Firefighter Smoke Control Panel (FSCP) which displays a “Normal” or “Fault” status based on the condition that it reads. The voltage monitoring will tell us that the fans have power so they are able to operate when they are activated. But how do we know they are actually moving air after the system tells them to run?  A fan can have power and be “running”, but a thrown belt could result in loss of flow.

There are three typical methods of confirming airflow: a sail switch, a differential pressure switch, and a current transducer. These methods all fall under the umbrella of end-to-end verification. End-to-end verification is intended to confirm that the fan is flowing air, not just that it has power.

Method 1: Sail Switch

A sail switch is generally a paddle or vane that is located in the air stream of the fan that depresses a switch when air is flowing over it. With no airflow the vane relaxes and the switch is no longer depressed.

Method 2: Differential Pressure Switch

The second method is through a differential pressure switch; this switch compares the relative pressure difference between the intake and discharge side of the fan. If both sides are equal, there is no flow through the fan.

Method 3: Current Monitoring

The third method is a current monitoring device; this approach is a little trickier to ensure that it is going to provide the correct information back to the Firefighter Smoke Control Panel. A current monitoring device is only looking at the current draw of the motor driving the fan. The typical assumption is that the motor will draw a specified current when under load (the blades are spinning) and a lower current when the motor is spinning with no load (typically indicative of thrown belts or coupling shear). The critical assumption is that if the motor is under load, the blades are spinning, and therefore they are moving air. Depending on the specific details of the fan equipment being utilized, conditions that would reduce or restrict airflow could arise and not be detected by a current monitoring device. For example, a clogged air filter or intake grille could reduce the amount of airflow through the fan, but the blades are still spinning.  Validating the set point for this current switch can be difficult to perform in the field due to the issues described above.

Check out our past work with smoke control systems!

Read the full text of Section 909.12 of the International Building Code!

Reference code sections based on the 2018 International Building Code:

International Building Code Section 909 – Smoke Control Systems

International Building Code Section 909.3 – Special Inspections and Test Requirements

International Building Code Section 909.12 – Detection and Control Systems

International Building Code Section 909.16 – Fire Fighter’s Smoke Control Panel

NFPA 92 – Standard for Smoke Control Systems