In Episode 5, check out UL FSRI Director, Steve Kerber, as he talks about how fire flows from high pressure to low pressure and how ventilation can impact the flow path.
When you’re on the fireground, it is important to understand the basic fire dynamic principle that fire flows from high pressure to low pressure. In a compartment, the fire acts like a pump. As the fire burns, hot fire gases (high pressure) collect at the ceiling and move toward low pressure, creating a flow path of heated fire gases to the outside. Outside air (carrying oxygen) is drawn into the base of the fire through openings in the building (doors, windows, roof). As long as the fire has air entering from the outside, the fire will continue to grow. A fire in a room with modern furnishings will grow in direct proportion to the available oxygen. The more oxygen that is available, the faster the fire will release heat energy and the greater the risk of rapid-fire development.
The flow path is the volume between an inlet and an outlet that allows the movement of heat and smoke from the higher pressure within the fire area toward the lower pressure areas accessible via doors and window openings. When you make ventilation changes to a structure, the pressure changes the flow path of the fire and can either be used to for your tactical advantage or your detriment by making the fire environment worse.