In firefighting, ventilation is an important part of structural firefighting tactics, and involves the expulsion of heat and smoke from fire building, permitting the firefighters more easily and safely find trapped individuals and attack the fire. If a large fire is not properly ventilated, not only will it be much harder to fight, but it could also build up enough poorly burned smoke to create a smoke explosion, or enough heat to create a flashover

Flashover

A flashover is the near simultaneous ignition of most of the directly exposed combustible material in an enclosed area. When certain organic materials are heated they undergo thermal decomposition and release flammable gases...

. Contrarily, poorly placed or timed ventilation may increase the fire's air supply, causing it to grow and spread rapidly.

Types of ventilation

In general, there are two types of ventilation; vertical and horizontal. Their names refer to the general locations of the intended exit points of the heat and smoke to be ventilated. Vertical ventilation takes place through holes cut in the roof, typically by truck companies during the early stages of a fire in a process known collectively as roof operations, while horizontal ventilation usually takes place through doors and windows. The goal of each is to clear heat and smoke to increase chances of survival for trapped occupants, and/or so that water lines can be advanced into the structure, to more effectively battle the flames. While their goals are similar, their applications are different, but still both require good timing and coordination so that increased air flow through a structure doesn't contribute to fire spread.

Methods

Mechanical fans can be used to provide positive pressure ventilation when used in tandem with either existing openings such as window

Window

A window is a transparent or translucent opening in a wall or door that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material like float glass. Windows are held in place by frames, which...

s, skylights or heat/smoke vents on the roof; or by cutting new exhaust vents in the building. If there is no suitable existing hole, firefighters may use their equipment to make one, such as specialised saw

Saw

A saw is a tool that uses a hard blade or wire with an abrasive edge to cut through softer materials. The cutting edge of a saw is either a serrated blade or an abrasive...

s for cutting a large hole in the roof. A conical hose-stream aimed around an opening -of a window or door, etc.- entrains smoke and thus increases the exhaust rate of smoke from the space. This is a process called "hydraulic ventilation". This strategy might be used when the fire is small and protecting property from smoke damage can be achieved safely. It can also be used more aggressively when a structure is "fully involved" and the smoke is obstructing the nozzlemans view of the hotspots.

High-rise buildings sometimes also incorporate fans to produce a positive pressure

Positive pressure

Positive pressure is a pressure within a system that is greater than the environment that surrounds that system. Consequently if there is any leak from the positively pressured system it will egress into the surrounding environment....

in stairwells and elevator shafts to reduce smoke infiltration into those spaces.

When glass windows in a burning structure burst from internal pressure and heat, or the fire burns through the roof, it may be said to have "auto-ventilated" or "self-ventilated."

External Links

• "Ventilation Basics" — http://www.firerescue1.com/fire-products/ventilation/articles/18506-Ventilation-Basics/
• "Positive Pressure Ventilation" — http://www.firetactics.com/PPV.htm