7 reasons why you shouldn't use HVAC control systems for smoke ventilation

Smoke ventilation systems today require sophisticated control systems with highly complex sequences of operation. HVAC control systems could be seen as a viable solution, as they can be extremely flexible and offer a high degree of programmability. However, while they may be adequate for simple applications, there are a number of issues that mean they do not deliver on all the functionalities needed for more complex schemes.

Here are 7 reasons why:

1. Unreliability in occasional use

Smoke ventilation systems are only expected to operate if smoke is detected and will be unused for long periods of time. Most importantly, they must work first time, every time. HVAC systems are not designed for this and can be temperamental when not continuously in use. This means that they may not be sufficiently reliable for smoke ventilation control.

2. Inability to operate at high temperatures

The interface devices out on the network (I/O devices) are rarely designed to operate at high temperatures, which would present obvious problems in a fire.

3. Lack of error diagnostics reporting

HVAC systems rarely perform basic error checking routines to enable fault or communication error diagnostics to be reported across the network.

4. Vulnerability to interference

BMS or PLC based control systems can be more vulnerable to errors caused by background interference from other electrical devices in the building. They can also be too sensitive for smoke control purposes.

5. Insufficient failsafe capability

The I/O devices are rarely programmable, so if the HVAC system loses contact with the main panel, there is no designated failsafe state. This means that if the device loses communication with the main panel and is controlling the extract fans on a life safety system, the fans may not continue to run. This is a crucial issue: smoke control systems are reliant on a degree of failsafe capability from the system that controls them. Similarly, the smoke control system needs to operate on the first activation and continue to operate in that mode until overridden by the fire officer. The control system should be able to lock out subsequent signals that might distract the control system from its primary function. Also, the failure modes of HVAC control systems vary from system to system and without rigorous testing, may not be fully understood when being put into an application.

6. Security

The quality of maintenance of smoke control systems is not always as it should be. HVAC control systems are mostly open protocol, meaning that anyone with a little knowledge could adjust the programming without fully understanding the implications. So a system that is designed to operate in accordance with a predetermined cause and effect could be completely compromised by some inadvertent adjustment. Likewise, general operational commands should be overridden by the fire commands and all override facilities should be locked so that only fire service personnel or staff familiar with the operation of the system should be able to manage the override facility, and not leave it in its overridden state.

7. Continuity of Operation

A smoke control system is designed to operate, and continue operating in the most difficult of conditions. This means that the control system and its communications network must be able to do the same. If it is programmable, it needs to be able to communicate that programming continuously without interruption, so needs a backup power supply that maintains the operation and a failsafe mode if the communications are lost. Similarly, if system failure occurs, the system should identify this and attempt to restore operation quickly and most importantly, resume operating in the same mode as prior to the failure.

Conclusion

Typically, HVAC systems are not as reliable and robust as they need to be for smoke control applications; they require rigorous testing and a full understanding of their capabilities and limitations to ensure that their inability to acquire a degree of failsafe operation is overcome before being put to use as part of a life safety smoke control system. We would recommend using a system that is designed and developed for the application, rather than trying to pick something off the shelf that may or may not be fully fit for the purpose.