Finding Smoke in HVAC Ducts

Are smoke detectors in HVAC ducts effective? Research has settled some questions over whether their installation is worthwhile.

Most building codes, including the National Building Code of Canada, require smoke detectors in HVAC ducts to shut down the HVAC system during a fire to minimize the circulation of smoke through a building.

However, there have been many questions about the effectiveness of duct smoke detectors and whether their expense is worth the degree of protection they might provide.

To address the doubts that have surfaced in past decades about the usefulness of HVAC smoke detectors, research was conducted at the National Research Council of Canada’s Institute for Research in Construction (NRC-IRC) in Ottawa, and at the University of Maryland. NRC-IRC conducted full-scale experiments using its 10-storey building test facility, and the university did small-scale experiments and modeling studies.

Most codes require duct smoke detectors in the supply air duct downstream of the fresh air inlets, filters and fans (see figure). Some codes also require detectors in return air ducts. Installation requirements for HVAC smoke detectors are provided in such standards as CAN/ULC-S524, Standard for the Installation of Fire Alarm Systems.

Confirmation: shut down HVAC fans in a fire

The research investigated whether the smoke movement created by HVAC fans is significant compared to that resulting from the fire itself and other effects. The tests confirmed 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. 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 the distribution of smoke to floors where there was no fire.

It was found that HVAC system shutdown is particularly important for small, compact buildings because smoke distribution through the ductwork is a larger proportion of smoke movement than in tall buildings, where stack effect and other mechanisms are large factors.

Smoke dilution and different detectors

There has long been concern that the concentration of smoke in HVAC ducts might be too low to activate duct smoke detectors in time to allow evacuation. To address this question, experiments were made using four different types of commercially available duct smoke detectors: ionization, photoelectric, sampling and multi-sensor.

It was found that all four types of detectors would respond to smoke in the HVAC system once it reached concentrations comparable to those used as criteria for safe building evacuation. In other words, HVAC smoke detectors are indeed capable of shutting down HVAC systems at smoke levels that still allow time for the evacuation of building occupants.

Smoke aging

There has been speculation that “smoke aging” might influence the effectiveness of HVAC smoke detectors. As smoke moves away from a fire source, it cools and changes in terms of smoke particle size, shape and colour, and it was suspected that the farther smoke is from the source, the less detectable it becomes. The full-scale experiments determined that this “smoke aging” did not adversely affect smoke detection.

Effect Of HVAC Filters

HVAC filters remove larger particles of smoke and there have been questions about the possibility of the filter reducing smoke density and making it undetectable. The filters are situated in the air supply downstream of the fresh air inlet, the filter, the conditioning area, and the fan.

Testing of Group 1 (Glass fibres in a cardboard frame) and Group 2 (Extended area, pleated wet-laid cellulose) filters showed that filters do indeed affect smoke detection, but the detectors will respond while evacuation is still possible. However, ionization detectors are recommended in systems with Group 2 filters. Multi-use detectors that combine photoelectric and ionization detection are better yet.

The NRC-IRC test facility was not configured to test high efficiency particulate air (HEPA) filters, but it is known that HEPA filters would further reduce smoke reaching detectors and therefore multi-use detectors should be used.

Detectors with sampling tubes

It was found that most buildings have air flow velocities that fall within the range over which smoke detectors are tested. Smoke detectors with sampling tubes were shown to be effective in this range.

Stratified flow

The research addressed how the location of a smoke detector in a duct affects detection. The research indicated that at low air velocities, buoyancy causes smoke to concentrate in the upper part of the duct. However, the results suggest there is no justification as required in some standards for requiring duct smoke detectors to be located at set distances (3 -10 duct diameters) from bends, inlets and outlets. Updates to standards that require this might consider this information.

It is recommended that HVAC smoke detectors be located at the top of the ductwork where the smoke concentration would be highest if stratification did occur. They should also be placed at the mid-point of straight runs, and sampling tubes should be installed vertically.

Dr. G. D. Lougheed is a principal research officer in the Fire Research Program at the National Research Council Canada Institute for Research in Construction in Ottawa.