Sprinkler-vent interaction

Sprinkler-vent interaction is the mutual influence of automatic sprinklers and smoke and heat vents that are installed in the same space. Vents can change the flow of hot gases that should trigger the sprinklers, while sprinkler sprays cool the smoke that the vents must remove and mix it with cleaner air. Simply combining both systems, each designed as if the other were absent, therefore does not guarantee that their benefits add up.

The subject has long been controversial. When the third edition of the SFPE Handbook of Fire Protection Engineering was prepared, no generally accepted technical guidelines for combined designs existed. A review of 34 position papers concluded that venting does not harm sprinkler performance and does not change the number of sprinklers that operate. The main design concerns are the cooling of smoke by sprinkler sprays, which reduces the flow through vents, and vigorous mixing of the smoke layer, which can fill the whole space with smoke. Cooper's example guidelines therefore include the early, coordinated opening of all vents in the compartment of fire origin.

Two systems with different aims

Smoke and heat exhaust ventilation, or smoke and heat venting, removes hot smoky gases from the upper part of a space and lets cooler, clean air into the lower part. The flow can be driven by buoyancy through vents, by fans, or by both, and roof vents may be opened by fusible links. Venting aims to help people escape, to let fire fighters enter and see the seat of the fire, and to limit damage from smoke and heat. A typical setting is a large, open warehouse-type space with roof vents, possibly divided into compartments by draft curtains that hang from the ceiling.

A sprinkler system has a different aim. Control-mode sprinkler systems are designed to control the growth and spread of a fire, and full extinguishment may need the fire service. In a warehouse fire, the wetting of stored goods by sprinkler water also resists the jump of fire across aisles. Cooper notes that venting becomes significantly more complex when sprinklers operate in the same space.

Both systems may be found in the same building. In the Netherlands, indoor car parks larger than 1,000 m² generally have facilities that limit fire spread, such as sprinklers, and/or facilities that help an interior attack, such as smoke and heat removal. The Dutch standard NEN 6093 contains calculation models and design and installation guidelines for smoke and heat exhaust systems. For a fire service attack in a space with such a system, it sets limits such as a visibility of at least 100 m to light-emitting objects up to a height of 1.8 m. The calculated smoke layer may then not exceed 200 °C where goods are stored in the layer, or 500 °C otherwise.

How the two systems influence each other

Sprinklers and fusible-link vents respond to the same flow. Both are triggered when the hot ceiling jet heats their fusible links or glass bulbs. The physical phenomena behind each system can be expected to interact, to a greater or lesser extent, when both are present. Vent design can therefore change sprinkler performance, and sprinkler design can change vent performance.

Sprinklers and roof vents in one curtained compartment. The ceiling jet triggers both; the sprays cool and mix the smoke layer that the vents remove. Based on Cooper (2002b), pp. 3-219, 3-236 to 3-237 and 3-239.

In one direction, sprinkler sprays cool the smoke that rises from the fire and fills the space. A vent design based only on the hot, uncooled smoke does not account for this cooling, so less smoke flows out of the vents than intended. If the reduction is significant, the vents must be larger.

In the other direction, ventilation can change where the hot gases go. The data sheets of the Australian Building Codes Board note that air flows can direct combustion products towards or away from detection devices, including sprinkler heads. They also warn that a vent or exhaust system may remove hot gases before the sprinklers activate. This could compromise the sprinkler system, especially one designed for early suppression. Fast make-up air inlets close to a fire may increase its heat release rate or deflect its plume.

Effect of sprinkler cooling on vent flow

Flow through a roof vent is driven by the buoyancy of the smoke layer. The hot layer creates a higher pressure at the ceiling than outside, and this pressure difference grows with the depth and temperature of the layer. A hotter layer pushes harder but is also less dense, so the mass of smoke removed does not keep rising with temperature. Cooper combines these effects into one expression for the mass flow per unit vent area. It assumes negligible pressure losses at the inlets, which typically holds if the low-level inlet openings are at least 1.5 to 2 times the vent area.

$$\frac{{\dot{m}}_{\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{t}}}{A_{\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{t}}}=0.61{\rho }_{\mathrm{a}\mathrm{m}\mathrm{b}}\sqrt{\frac{gh}{2}}{\left[4(1-\frac{T_{\mathrm{a}\mathrm{m}\mathrm{b}}}{T_{\mathrm{l}\mathrm{a}\mathrm{y}}})\frac{T_{\mathrm{a}\mathrm{m}\mathrm{b}}}{T_{\mathrm{l}\mathrm{a}\mathrm{y}}}\right]}^{1/2}$$

Symbol	Description	Unit
${\dot{m}}_{\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{t}}$	mass flow of smoke through the vent	kg/s
$A_{\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{t}}$	area of the ceiling vent	m²
${\rho }_{\mathrm{a}\mathrm{m}\mathrm{b}}$	density of ambient air	kg/m³
$g$	acceleration of gravity	m/s²
$h$	depth of the smoke layer below the ceiling	m
$T_{\mathrm{a}\mathrm{m}\mathrm{b}}$	absolute ambient temperature	K
$T_{\mathrm{l}\mathrm{a}\mathrm{y}}$	absolute temperature of the smoke layer	K

The factor 0.61 is the flow coefficient of the vent. The term in square brackets reaches its maximum of one when the absolute layer temperature is twice the ambient temperature, which is 586 K for an ambient temperature of 293 K. If the layer is more than about 90 K hotter than ambient, the flow stays within 80 percent of this maximum. Much cooler layers lead to significant reductions in vent flow.

Relative mass flow through a roof vent as a function of the ratio of absolute layer and ambient temperature, with the layer at 100 K and, after spray cooling, 20 K above 293 K. Based on Cooper (2002b), pp. 3-223 to 3-224.

Worked example

Cooper considers a smoke layer 3 m deep and 100 K above an ambient temperature of 293 K. The pressure difference across a roof vent is then 9.0 Pa, the smoke leaves at 2.7 m/s, and each square metre of vent removes 2.4 kg of smoke per second. Suppose that sprinkler sprays cool the same layer to 20 K above ambient without changing its depth. The term in square brackets then falls from 0.76 to 0.24. Its square root, which is the fraction of the maximum possible flow, falls from 0.87 to 0.49, and the vent removes only about 1.4 kg/s per square metre, or 56 percent of the original flow. To remove the same mass of smoke, the vent area would have to grow by a factor of about 1.8.

Mass flow per square metre of roof vent for a 3 m deep smoke layer at different temperatures above an ambient 293 K, and the vent area factor needed to keep the flow of the 100 K case. Based on Cooper (2002b), pp. 3-223 to 3-224.

Spray-driven mixing and smoke logging

Sprinkler sprays act on the smoke layer in two ways: they cool it by evaporation, and they dilute it by mixing cooler air from below into the layer. As long as the loss of temperature and buoyancy is not too great, Cooper judges that additional vent capacity can offset moderate cooling and mixing. He adds that spray-driven mixing can apparently be so effective, even without much cooling, that the volume of smoke grows very rapidly while its temperature drops. If that happens, even impractically large increases in vent area would bring little benefit.

The extreme result is smoke logging, in which the sprays mix the fresh smoke from the fire plume continuously with all the gas in the space. In preliminary tests in a large shopping mall, a manually operated sprinkler spray could, under some conditions, bring a smoke layer down, after which smoke logging occurred rapidly. Under other conditions, however, the smoke layer was not disturbed by the spray. A vent design that meets its aims must therefore be based on an analysis that accounts for smoke logging and avoids it.

A model of a single spray in a two-layer fire environment identified six possible configurations of spray and smoke layer. Which one occurs depends mainly on the thickness and temperature of the layer and on the momentum, spread angle and droplet size of the spray. Four configurations favour a combined design, provided the vents keep the layer relatively thin and not too buoyant. The spray then carries the limited amount of smoke it entrains into the lower layer, where it becomes highly diluted. Two configurations could lead to vigorous mixing, rapid growth of the smoke layer and possibly smoke logging. Shortly after the first rapid-response sprinklers discharge, the layer is still thin and not too hot, and smoke logging is then never observed.

Effect of vents on sprinkler operation

Studies and opinions on combined systems have produced claims in both directions. In favour, it is claimed that vents limit the spread of combustion products, reduce the number of sprinklers that operate and help fire fighters locate the fire. Against, it is claimed that vents increase the burning rate, delay sprinkler activation, increase the number of operating sprinklers and remove too little smoke to matter.

After evaluating the evidence, Beyler and Cooper concluded that venting has no negative effect on sprinkler performance. Sprinklers do not depend on reduced oxygen concentrations, and venting changes neither the activation times of early sprinklers nor the total number activated. A fire directly beneath a vent may slightly delay the first sprinkler activations, but there is no evidence that this will have a significant impact on sprinkler performance. Venting does limit the spread of smoke and heat, which improves visibility for occupants and fire fighters and reduces damage. If the sprinklers fail, venting remains a valuable aid to manual fire fighting.

The same review notes that in many fires, current design practice such as the guide NFPA 204 of the National Fire Protection Association is likely to limit the vents that open to one. In a successful sprinkler operation, the vents may not open at all. Vents with fusible links also suffer from vent skipping, the counterpart of sprinkler skipping: a random delay caused when water droplets in the ceiling jet cool their links. Because far fewer vents than sprinklers are installed per unit floor area, Cooper suggests a safety factor on the total vent area. The Australian data sheets are more cautious: they warn that venting before sprinklers activate could compromise early suppression systems. They ask for special attention wherever a design departs from the standards for suppression and detection systems.

Sprinkler activation in performance-based design

Some performance-based methods tie the design fire to the moment of sprinkler activation. In New Zealand, the Verification Method C/VM2 (second edition, 2025) lets the design fire for life safety (ASET) analysis in sprinklered buildings grow only until the sprinklers activate. After activation the fire is assumed to be controlled, with a constant heat release rate. Activation is calculated with values prescribed in a table of C/VM2. For a standard response sprinkler these are an RTI of 135 (m·s)^1/2, a C-factor of 0.85 (m/s)^1/2 and an activation temperature of 68 °C. If the chosen sprinkler standard requires a higher activation temperature, that temperature must be used.

In Australia, the data sheets of the Australian Building Codes Board take the maximum heat release rate of a controlled fire as the rate when the last sprinkler within the assumed area of operation activates. In both methods the moment of sprinkler activation fixes the size of the design fire. A consequence not stated in either document is that any delay in activation would enlarge the design fire, and with it the smoke that a vent design must handle.

Design approaches

Beyler and Cooper recommend design methods that make vents open early, with all vents operating together as a group. Sprinkler designs must treat draft curtains as obstructions, and the curtains should hang over aisles rather than over storage. Cooper suggests two general criteria for a successful vent design. The vents must remove the hottest and most hazardous smoke and slow or stop the descent of the smoke layer, for example above eye level. Where draft curtains are used, a significant additional benefit is possible to the extent that the venting keeps the smoke within the curtained compartment of fire origin.

Cooper also gives example guidelines for a design on which all parties could agree. The sprinkler design is first established in the traditional way, from full-scale tests without vents. Where the sprinklers are expected to control the fire, the vent objective is to keep the smoke within the curtained compartment indefinitely. If that is too ambitious, or the sprinklers only slow the fire, it is to do so for a set time, for example until the fire service attacks. All vents of the compartment then open early, for example together after the first sprinkler activates. The vent area is designed with a compartment fire model of proven capability for sprinklers, vents and draft curtains together.

Example guidelines for a combined sprinkler and vent design: sprinklers first, then the vent objective, ganged early opening and a model-based vent design. Based on Cooper (2002b), p. 3-240.

Both zone models and field models can support such designs. Zone models suit parametric studies and practical design, while field models suit the detailed study of specific scenarios. Examples include a draft curtain almost directly above the fire, or the sequence in which sprinklers discharge.

Limits and practical points

The evidence on combined systems is incomplete. Cooper wrote that no generally accepted guidelines existed at the time, and that past experiments had not tried to isolate the effects of sprinkler cooling. Knowledge of spray-driven mixing and smoke logging rests partly on visual observations that were reported only anecdotally. The vent flow expression above also assumes generous inlet openings and a well-defined smoke layer.

Three practical points follow from the sources. Vents and sprinklers designed each by their own rules do not necessarily combine their benefits. Extra vent area can offset moderate cooling of the smoke, but according to Cooper it would bring little benefit once vigorous mixing occurs. At the same time, there is experimental evidence that venting can control smoke logging. Finally, the review of Beyler and Cooper found that venting affects neither the activation times of early sprinklers nor the total number activated. The only exception noted was a possible slight delay of the first activations when the fire is directly beneath a vent.

See also

• ESFR sprinkler
• Response time index
• Sprinkler hazard classification
• Hydraulic calculation

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• Cooper, L. Y. (2002b). Smoke and heat venting. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-219–3-242). National Fire Protection Association; Society of Fire Protection Engineers.
• Hopkins, M., Schoenrock, J., & Budnick, E. (2005). Fire suppression systems (NIST NCSTAR 1-4B). National Institute of Standards and Technology.
• Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
• Tewarson, A. (2002). Generation of heat and chemical compounds in fires. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-82–3-161). National Fire Protection Association; Society of Fire Protection Engineers.
• Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). Fire safety of indoor car parks accommodating electrically powered vehicles. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.