The overriding objective of all forms of tactical ventilation is to improve interior tenability for occupants and firefighters through purposeful flow path management.

Flow path management for control and extinguishment is best achieved by creating ventilation openings as close to the source fire as is possible (e.g., not in a remote location) and in coordination with the fire attack team (e.g., vent ahead of rather than behind the attack team).

Openings are made either horizontally or vertically and utilize the differences in pressure and density of heated gases or mechanical or hydraulic assistance to generate the flow path, as shown in Figure 23.

When utilising ventilation tactics and interior attack, firefighters must continually control their environment with hose stream techniques to cool and dilute the highly combustible smoke.

Effects of Ventilation

Underwriters Laboratories (UL) and the National Institute of Standards and Technology (NIST) have scientifically demonstrated that ventilation no longer equals cooling. The mass of hot gases leaving a compartment needs to be replaced with an equal mass of (relatively) fresh replacement air. This replacement air leads to an increase in HRR and may trigger RFD. Occupants and firefighters within the fire compartment are at increased threat to life safety if fire control is not in place. The effects of ventilation on both fuel- and ventilation-controlled fires are listed in Table 3.

Table 3: Effects of Ventilation

Fuel-Controlled	Ventilation-Controlled
Smoke and heat are drawn towards the exhaust opening	Smoke and heat are drawn towards the exhaust opening
Buildup of hot smoke may be reduced	Heat release is increased (potential for rapid fire development)
The fire may grow larger before becoming ventilation-controlled (potential for flashover)	Addition of fresh oxygen (potential for rapid fire development)

Natural Ventilation

Natural ventilation refers to the movement of the products of combustion based on differences in pressure and density, as well as ambient wind conditions, to ventilate a structure. The advantages and disadvantages of natural ventilation are listed in Table 4.

Table 4: Natural Ventilation

Advantages	Disadvantages
May use existing openings	Depends on natural air currents
Uses natural air currents	Flow path may expose other parts of the building
Uses the physical properties of smoke (buoyancy and expansion create pressure differences)	Flow paths used for ventilation may be escape routes
Removal of smoke and heat	Will increase fire development
Works best on 'hot' smoke	May be negatively influenced by wind conditions
	Has limited effectiveness on 'cold' smoke

Forced Ventilation

Mechanical ventilation refers to the movement of the products of combustion based on a pressure differential generated by the use of blowers and smoke ejectors to ventilate a structure. The advantages and disadvantages of mechanical ventilation are listed in Table 5.

Table 5: Mechanical Ventilation

Advantages	Disadvantages
Speeds removal of smoke and heat	Can significantly increase rapid fire development
Can overcome natural wind effects (to some extent)	May rapidly spread the fire into voids
Can be used to move 'hot' or 'cold' smoke	Mechanical mixing and injection of air into smoke can result in an ignition that generates a violently destructive overpressure

Figure 25: Forced ventilation - mechanical

Hydraulic Ventilation

Hydraulic ventilation refers to the movement of the products of combustion based on a pressure differential generated by the use of a fog pattern or a straight stream with an 'O' pattern that is directed out a nearby opening to ventilate a structure. The advantages and disadvantages of hydraulic ventilation are listed in Table 6.

Table 6: Hydraulic Ventilation

Advantages	Disadvantages
Speeds removal of smoke and heat	Uses water
Can overcome natural wind effects (to some extent)	Not usually used in initial fire attack because hose is in use
Can be used to move 'hot' or 'cold' smoke	Can cause water damage if used without care
Only needs a hoseline	Nozzle operator must remain in the heated/contaminated area
Most useful during overhaul

Figure 26: Forced ventilation - hydraulic

Impact of Differences in Elevation of Openings

As demonstrated in both the horizontal and vertical ventilation tests conducted by UL,, the greater the height difference between the inlet and outlet, the greater the flow of both air and smoke when utilizing these ventilation methods. Given the buoyancy of hot smoke, making an exhaust opening above the inlet increases the effectiveness of both horizontal and vertical ventilation.

It is important to remember that the increased inward air flow can result in an increase in HRR. As a result, the rate of fire growth can quickly outpace the capability of the desired exhaust opening, returning the intended inlet to a bi-directional flow path. This can increase the risk to firefighters entering for fire attack or search, largely due to rapid fire spread towards their entry point. In the specific case of horizontal natural ventilation, it has been previously noted that the smoke layer failed to lift following ventilation and smoke tunnelling and rapid air movement in through the front door was observed.

Tactical recommendations of this UL research report include:

• Ensuring that the attack team is in place with a charged line, and ready to attack the fire before initiating horizontal ventilation if water has not already been applied; and
• Cooling the upper layer any time that it is above 100oC (212oF) in order to reduce radiant and convective heat flux, and to limit potential for ignition and flaming combustion in the upper layer.

Note that this research project did not examine the impact of smoke cooling, but examination of the temperatures at the upper levels in this experiment (and others in this series) point to the need to cool hot smoke overhead.

Discussion

The tactical implications identified in the UL Horizontal Ventilation Study and NIST Wind Driven Fire Study are interrelated and can be expanded to include the following key points:

• Heat transfer (convective and radiative) is greatest along the flow path between the fire and exhaust opening.
• Exhaust openings located higher than the fire will increase the velocity of gases along the flow path (further increasing convective heat transfer).
• Flow of hot gases from the fire to an exhaust opening is significantly influenced by air flow from inlet openings to the fire (the greater the inflow of air, the higher the HRR and flow of hot gases to the exhaust opening).
• A flow path can be created by a single opening that serves as both inlet and exhaust (such as an open door or window).
• Thermal conditions in the flow path can quickly become untenable for both civilian occupants and firefighters. As noted in an earlier NIST study examining wind-driven fires, thermal conditions can change extremely quickly under wind-driven conditions.
• Closing an inlet, exhaust opening or introducing a barrier (such as a closed door or smoke curtain) in the flow path slows gas flow and reduces the hazard downstream from the barrier.
• When the fire is ventilation-controlled, limiting inflow of air (e.g., through door control) can slow the increase in HRR and limit fire development to the growth stage.
• Multiple openings result in multiple flow paths and increased air flow to the fire, resulting in more RFDs and increased HRR.

Coordinated Tactical Operations

Understanding how fire behaviour can be influenced by changes in ventilation is essential. But how can firefighters put this knowledge to use on the fire ground, and what exactly does coordination of tactical ventilation and fire attack really mean?

As stated earlier, tactical ventilation can be defined as the planned, systematic and coordinated removal of smoke and heat from a structure, and their replacement with fresh air, due to the actions of on-scene firefighters through purposeful flow path management. Each of the elements of this definition is important for safe and effective tactical operations.

Ventilation (both tactical and unplanned) not only removes hot smoke, but also introduces fresh air, which can have a significant effect on fire behaviour.

Tactical ventilation must be planned. It should be undertaken to intentionally influence fire development and must take into consideration the flow path. Tactics to change the ventilation profile must be intended to influence the fire environment or fire behaviour in some way, such as raising the level of the interface layer to increase tenability. The ventilation plan must also consider the flow path and vent ahead of, rather than behind, the attack team. Ventilation should be conducted in the immediate area of the fire, not at a remote location.

Tactical ventilation must be systematic. Exhaust openings should generally be made before inlet openings, particularly when working with positive pressure ventilation or when taking advantage of wind effects.

As pointed out in the 2010 UL study on the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction, tactical ventilation must also be coordinated. Coordination of ventilation and other tactical operations requires consideration of sequence and timing.

Ventilation may be completed immediately prior to, during, or after fire attack has been initiated. The sequence will likely depend on the stage of fire development, burning regime, and the time required to reach the fire.

If the fire is small and staffing is limited, it may be appropriate to control the fire and then initiate ventilation, including hydraulic ventilation performed by the attack team. This approach minimizes potential fire growth.

In general, when a fire is ventilation-controlled-as those beyond the incipient stage are likely to be-ventilation should not be completed unless the attack line(s) can quickly apply water to the seat of the fire. In a small, single-family dwelling, this may mean that the attack team is on-air, the line is charged and the entry door is unlocked or has been forced and is being controlled (held closed). In a larger building, this may mean that the attack line has entered the structure and is in position to move onto the fire floor or into the fire area.

The key questions that must be answered prior to implementing tactical ventilation are:

• What influence will the ventilation tactics have on fire development?
• Are charged and staffed attack lines in place?
• Will the attack team(s) be able to quickly reach the fire?
• How will ventilation impact crews operating on the interior of the building?

Coordination requires clear, direct communication between companies or crews assigned to ventilation, fire attack and other tactical functions that are or will be taking place inside the building.

While not a tactical implication directly raised by the UL study, another important consideration is the hazard of working without or ahead of the hoseline. While a controversial topic in the North American fire service- where truck company personnel generally work on the interior without a hoseline-searching with a hoseline provides a means of protection and a defined exit path. Staffing is another key element of the operational context. If you do not have enough personnel to simultaneously control the fire and conduct search operations, in most cases it is likely the best course of action to control the fire to ensure a safer operating environment for search operations to ensue.

Air movement

The movement of air into and out of the structure has a vital role to play in structure fire development. To assess air movement, examine any inward air movement feeding the fire and the outward movement of the exiting smoke.

Consider:

• Air movement direction: identify where it is entering and where it is exiting;
• Velocity of the air flow, both inward and outward; and
• Wind direction and strength.

Remember: doors opened to gain entry provide air to the fire. Employ correct door entry procedures and do not open doors until crews are ready to enter the structure for fire attack. Consider the implications of window failure.

Ventilation Paradox

A paradox is a seemingly true statement, or group of statements, that lead to a contradiction or a situation that seems to defy logic or intuition. The 'ventilation paradox' can exist when we read out-dated training material relating to a fuel-controlled fire that is typical of the predominant burning regime at the time. In the past, the following statement was accurate:

Tactical openings made to release combustion products may serve to reduce smoke logging, lower compartment temperatures, prevent flashovers and backdraughts and generally ease the firefighting operation. - Grimwood, 2005

Conversely, modern building construction and furnishings have now upended this logic, and the predominant burning regime has changed to ventilation control. Thus, the following statement is now accurate:

However, it is also possible that such openings may achieve undesirable and opposing effects, causing temperatures to rise with resulting escalations in fire spread, leading to flashovers, backdraughts and smoke explosions. - Grimwood, 2005