Firefighting 101                                                         by:  Dan Howell

Forced Ventilation

Forced ventilation can be used to supplement both horizontal and vertical ventilation. Forced ventilation is the process of mechanically moving air inside a structure. The three primary methods of forced ventilation are hydraulic, negative, and positive pressure. It is the goal of this article to begin the discussion of forced ventilation by examining the different tools, techniques, advantages, and disadvantages of hydraulic and negative pressure ventilation.

Hydraulic ventilation is a simple procedure that is normally performed by firefighters operating from the interior of a structure. The only tools required are a hose line and a fog nozzle. The process begins after firefighters have successfully knocked down the fire in a room. At this time there will be poor visibility and a large amount of smoke, steam, and heat. Firefighters advance to a window or door that was hopefully already vented to assist fire attack. If not, firefighters will have to clear the vent opening completely. Then, the hose line is positioned a few feet from the vent opening and the nozzle is set to a wide fog pattern. The nozzle is opened and the fog stream positioned so that it covers most of the opening. At this time, heat and smoke are drawn into the stream and forced out of the structure. The process should only continue for a few seconds until visibility in the room has been improved. Firefighters may then resume other operations and repeat the procedure if necessary.

The effectiveness of hydraulic ventilation is based on the size of the ventilation opening and the amount of water flowing from the nozzle. When ventilation begins, the nozzle should be fully opened so that maximum ventilation can be achieved. Finally, firefighters should also ensure that there will be no one struck by the stream or any debris that are forced from the structure.

The primary advantage to hydraulic ventilation is that it is quick and simple to perform. Firefighters do not have to set-up fans or make time-consuming vent openings. Hydraulic ventilation is also very effective at clearing a single room after knockdown has occurred. This technique allows firefighters to improve conditions in the immediate area. Hydraulic ventilation has the potential to increase water damage to the structure and, if over used, may strain water supplies. Hydraulic ventilation is a good option for firefighters but should only be used as necessary to improve visibility for other continuing operations.

Negative ventilation is a method of forced ventilation that is employed by pulling air out of a structure. The tools necessary to use negative ventilation are an electric vent fan, a power source, a fan jack, and possibly salvage covers. Negative pressure ventilation is normally used during overhaul once the fire has been knocked down. This is necessary since negative pressure ventilation involves pulling smoke and combustion gases through the vent fan. Negative pressure ventilation begins by selecting the vent site and hanging the fan in the vent opening. Normally, the vent opening will be a window or door. It is also possible to place vent fans over vertical openings to support vertical ventilation as well. Vent fans are typically supported by hooks on the fan or hung from a ladder or fan jack.  Once the fan is properly positioned, it must be connected to a power source. After this basic set-up is complete, the fan is started and ventilation begins.

The efficiency of negative ventilation is primarily determined by the amount of “churning” that occurs at the vent fan. Churning occurs when air pulled into the intake of the vent fan is discharged and then pulled back through the intake again. This circulation of air reduces the amount of smoke that is cleared from the structure. There are number of options that can be implemented to reduce the effects of churning. First, the area around the fan can be sealed with salvage covers. This prevents air that is discharged from the fan being pulled back into the intake. Second, natural air currents in the structure can be used to assist the fan. This is possible since negative pressure ventilation is performed during overhaul and firefighters have more time and options to determine where they wish to ventilate. Fresh air openings should be made in areas where smoke needs to be removed so that a draft will exist that will help the fan to pull smoke from those areas. The easier firefighters make air movement inside the structure, the less churning will negatively effect the operation.

In addition to churning, there are several other disadvantages.  Negative pressure ventilation requires more resources and time than other methods of forced ventilation. A power source is necessary and if churning becomes a problem the fan has to be properly sealed. Negative pressure fans are also smaller in size, which physically restricts the volume of air they can move. Finally, the fan will frequently be hanging in a doorway, which becomes a hindrance to firefighters.

Although these disadvantages exist, the use of negative pressure ventilation should not be forgotten. There are still several advantages to its use. Negative pressure vent fans can be used in smoke filled environments. This allows them to be used where other vent fans cannot. This is very helpful because negative pressure ventilation can be used to assist other methods of forced ventilation when ventilating areas with limited vent openings. Furthermore, negative pressure ventilation will not feed large volumes of air to any remaining hot spots and hidden fires. A final advantage to the electric vent fans used for negative pressure ventilation is that they do not add carbon monoxide to the environment. This advantage is very important during overhaul and when the fan must be operated on the interior of the structure.