Duct Smoke Detectors: The Impact of Various Factors on Their Effectiveness

This Update presents the results of research on smoke movement through HVAC systems and the effectiveness of duct smoke detectors. It provides guidance to practitioners and regulatory authorities in the context of North American code requirements for such detectors.

Requirements on the use of smoke detectors in HVAC ducts are included in most building codes, including the National Building Code of Canada (NBC).¹ The intent of the requirements, based on a recommendation made by the National Board of Fire Underwriters in 1939 ², is that the HVAC system be shut down during a fire to minimize the circulation of smoke through the building by the HVAC fan system. In most cases, the detectors must be located in the supply air duct downstream of the fresh air inlets, filters and fans (see Figure 1). In some jurisdictions, detectors must be located in the return air duct as well. Installation requirements are provided in standards, including CAN/ULC-S524.³

Figure 1. Typical constant-volume, single-zone HVAC system

Because there had been little or no research or data analysis to support this requirement, the Fire Detection Institute* undertook research on duct smoke detection to provide technical data to codes and standards committees and system designers. The research, conducted jointly by NRC-IRC and the University of Maryland (UMD), examined the use of duct smoke detectors both as a means of detecting fires or smoke within the HVAC system and as part of a building's smoke management system. UMD conducted small-scale experiments and modelling studies while NRC-IRC conducted full-scale experiments using its ten-storey test facility.
* The Fire Detection Institute has recently merged with the Fire Protection Research Foundation.

The research project addressed issues that had been raised regarding the need for duct smoke detectors and how they are installed and used. The key question was: Does duct smoke detection work and is it worth the added cost, considering the poten­tial for false and nuisance alarms? The specific issues investigated were the comparative driving forces of the HVAC fans relative to those produced by the fire; how detection is affected by smoke dilution, smoke aging, type of HVAC filter, and stratified flow in the HVAC ducts; and the efficacy of sampling tubes used for duct detection.

Comparative Driving Forces

The shutdown of the HVAC system is intended to minimize smoke transport through the building by the HVAC fans. It does not eliminate smoke movement through building shafts (elevators, stairs and service) and ductwork as a result of pressure differences produced by the fire and ambient conditions (stack and wind effects). The research examined the issue of whether or not smoke movement created by the HVAC fans is significant relative to that resulting from the fire itself and other effects.

It was found that the HVAC-related pressure differences were generally larger than those stemming from other factors, including the fire itself. These greater pressure differences also led to higher flows and the distribution of smoke to floors where there was no fire. The results confirm that unless an active smoke management strategy is in place, the HVAC fans should indeed be shut down upon detection of a fire, as required by codes and standards. The extent of the advantage of shutting down the system depends on the specific characteristics of the building in question. This can be quantified using computational methods.⁴

Dilution Effects

Engineers and code officials have long been concerned that the concentration of smoke in the HVAC ducts might be too low for detectors to respond reliably to the fires they are generally expected to detect. To address this concern, NRC-IRC researchers conducted experiments using four different types of commercially available duct smoke detectors — ionization, photoelectric, sampling and multi-sensor. They compared the analog output* (signal) from each detector with the optical density of the smoke measured at the same location in the return air duct in the HVAC system. The measurements related to dilution and aging effects were made in a return air duct. However, the results would be the same for detectors located in the supply duct.
* The analog signal from the detector is sent to the alarm control unit, where the levels at which the alarm signal would sound are selected. Each detector used in the research had a different range of outputs. For comparison purposes, the outputs were converted to a percentage scale based on the maximum output for each detector.

As shown in Table 1, the output of the detectors was proportional to the optical density of the smoke. The analog output at which the detectors would produce a trouble signal or an alarm is set at the control unit and is typically less than the maximum output from the detectors. As indicated in Table 1, all the detectors would respond to smoke in the HVAC system once it reached concentrations that are comparable to those used as criteria for safe building evacuation.

Source : http://irc.nrc-cnrc.gc.ca