Would you use Positive Pressure Ventilation

Would you/could you use Positive Pressure Ventilation in a backdraught situation?

Published:  01 September, 2008

Two experts, one from the US and one from the UK, answer this tricky question. Chief Jim Powell is based at The University of Nevada, Reno Fire Science Academy (USA), Gary West (WMA BAHons) is with Lancashire Fire and Rescue Service (UK).

My view: Chief Jim Powell , The University of Nevada, Reno Fire Science Academy (FSA). The accepted remedy for known backdraught conditions would be top side ventilation first, prior to opening of the compartment/structure. Ventilation timing and gaining entry become paramount. The solution, a well-coordinated attack with top side ventilation completed first, coupled with the opening of the compartment and nozzles flowing in a power cone configuration prior to entry.

Theoretically speaking (in my mind) PPV at this point could be used to force the products of combustion towards the vent opening and thus defusing the combustible mixture (by allowing the heated gases out of the compartment) once top side ventilation is completed. Location, size of the compartment and fan capability are factors, however this is not without risks.

My experience with PPV during backdraught conditions at this point is zero as I relied on the hydraulic force of the power cones (roughly 60 degree fog patterns/automatic gpm nozzles at 100 PSI) to push the combustible mixture towards the vent opening. As a first in company officer I responded to a reported mobile home fire around 09:30 AM on a cool spring morning and upon arrival the structure presented with the following: no wind, temperature 42 degrees F, frost on the windshield of the car in the drive way and several small tricycles in the yard. This double wide mobile home had blackish brown smoke lightly pushing and sucking around the windows with the classic black greasy drip on the inside of the windows. Black smoke was “drifting” upwards around seams in construction at the back half of the structure with the major concentration around what appeared to be a bedroom window.

I established command and gave a radio report of a backdraught and ordered my firefighters to pull two attack lines (1 and 3/4”) and place one near the front door the other to take aloft. I ordered the second firefighter to ladder the structure and get the saw and an axe.

I radioed the next due company to come to the scene and take a position on the hose line at the front door as well as purge the air out of the line. My crew and I gained roof access, found the hot spot on the roof and opened it up (4”x 8”) with the saw and axes. Once the hole was opened I advised the second crew to make entry for search and rescue as well as suppression.

The interior crew swung right upon entry and away from a dim glow, searched the front half and then to the rear where they extinguished a smouldering fire in a rear bedroom. The mother who resided there with her two children had had vehicle problems that morning and had gotten a ride with a neighbour to day care and then to work. Bottom line here was the top side ventilation worked very well and this was basically a text book operation. In this case the use of PPV with the unknown rescue would have been prohibitive in my mind.

My actual combat experience with backdraughts is limited to around six, however I remain convinced that topside ventilation is the preferred priority. The use of horizontal ventilation is an option depending on what the compartment is constructed of. If that tactical course of action is selected then maximum firefighter safety must be considered. I have seen only one backdraught in a high rise situation and it found air prior to us preventing it. Once again the tightness of the compartment relative to the construction type and materials versus the ability of crews to open it up dictate winners and losers in my opinion.

As to the direct question of using PPV while in a known backdraught condition/situation: in my opinion the benefits would have to outweight the risks substantially. In addition to the risk to human life (civilian and firefighters) should that operation fail, firefighters need to be prepared to transition into defensive mode.


Gary West, Lancashire Fire and Rescue Service

The risks that a modern day firefighter faces on a daily basis are still, even in the days of safer working practices, more substantial personal protective equipment, and better standards of firefighting equipment, extremely hazardous.

Of the hazardous phenonena that the firefighter may have to face the most common is the flashover and unfortunately, the  situations where a firefighter may have to deal with a backdraught are becoming more regular.

As is widely known, a flashover would be dealt with using conventional firefighting techniques. Linked with effective use of natural wind conditions, the risk of firefighters suffering injury is reduced. For the operational crews who work within a fire service who are actively applying Phase 3 PPV at an incident, the risk to the firefighter is negligible, providing that all of the appropriate procedures are being carried out correctly. Should this not be the case, then the use of PPV is potentially life threatening. These procedures are numerous, such as ensuring a good seal around the inlet, ensuring that there is a route through to the exhaust, and ensuring that other windows and doors that may affect the ventilation process are closed. Debatably, perhaps the biggest risk of all however, is using PPV in a fuel rich compartment, otherwise referred to as a backdraught.

As we know, a fire (or flammable) gas explosion (FGE) is missing only one thing, an ignition source. A flashover is a vented fire, having all three elements of the “triangle of combustion” (heat, oxygen and fuel), plus of course, the chemical reaction necessary for combustion to occur. A compartment with a backdraught situation present is lacking one part of the triangle, that being oxygen. With the introduction of oxygen, a deflagration will occur, with little chance of survival for casualties and firefighters alike.

(NB The triangle of combustion is generally now referred to as a tetrahedron, in order that the “chemical reaction” is understood by all as to be included in the combustion process.)

There is no doubt that by using PPV in a backdraught situation, the backdraught will occur more quickly, and significantly with greater ferocity, as the ventilation unit is taking the mixture closer to the ideal mixture (Stoichometric mixture) where every gas will burn at it’s most vigorous. Even something as simple as directing a jet of water into a backdraught compartment will induce the backdraught, as air is being forcibly entrained. It is true to also highlight that natural wind speeds of any magnitude may also speed up the induction of oxygen into a fuel rich compartment.

If the above is the case then, why would it possibly be a consideration for an incident commander to use PPV in order to encourage a backdraught to occur? At any incident involving fire, it is essential that a fire team are able to recognise the signs and indicators of a backdraught or fuel rich compartment and therefore the following are issues that must be identified by the Incident Commander and his/her fire team.

• Ascertain the history of the fire (what fuel has been burning and for how long). • Are the windows smoke-blackened (carbon) and greasy (water vapour)? • Is smoke pulsing from gaps in doors and windows (mini backdraughts)? • Can whistling be heard around the gaps in doors (pressure)? • Does the smoke look fuel rich when the compartment door is opened? • Is there a rapid inrush of air at the bottom of the door when it is opened? • Can a dull, unusual silence be perceived (neutralising of pressure)? • A lack of visible flame would indicate the possibility of a backdraught.   Should a fire team or incident commander identify one or all of the above, then it is likely that the compartment is fuel rich, and as such a backdraught may occur, not always immediately.

Once it has been ascertained that there is a danger of a backdraught occurring, it would be suicidal to commit a breathing apparatus team(s) (BA) to that building or compartment without first dealing with that backdraught. The most effective way of doing so is by venting that compartment. This is generally done by smashing the window, externally to that compartment. Clearly the person or persons undertaking this task must do so from a safe position, and a safe cordon and covering jet must be in place prior to implementation of this task. It must be borne in mind that the blast wave may travel a significant distance, and therefore without a safe cordon and covering jet, there would be a risk to passersby and adjacent buildings/vehicles.

Upon completion of this task, and when the backdraught has occurred, the room that has been vented will now probably be back to the fully developed or flashover stage of combustion, and as such can be dealt with extremely effectively by using PPV in Phase 3 mode (offensive phase).   So why not speed up that process with the assistance of a ventilation unit? There is absolutely no reason why not. Clearly a covering jet and safe cordon would not only be needed at the exhaust, but now also at the inlet, and with a wide enough boundary to eliminate the risk of injury to both firefighters and other members of the public. It would also be preferable for the ventilation unit to be running on full throttle prior to the inlet being opened, which may be opened using a line, in order to further reduce the risk.

It must be highlighted however, that the use of PPV to speed up the ventilation process is a tactic used as a last resort when all other avenues have been exhausted, and when there is no saveable life, and also no firefighters are committed to the building.

What if it is not possible to ventilate a fuel rich compartment in order to initiate a backdraught?

This is uncommon, but not unlikely. For example, imagine the 16th floor of a high-rise building, where a fuel rich compartment has been identified, yet an external exhaust cannot be reached in order to initiate the backdraught. In this scenario, PPV is pretty much a non-starter. The force of the potential blast – should the windows in that apartment remain intact – would be extremely volatile and forceful, to the point where it would be apparent that not only  the apartment itself would be compromised, but also it is likely a that similar situation would occur in the lobby, corridor and staircase, probably on the fire floor only, but possibly above and below that floor also, depending upon the ferocity the potential blast wave, and the fire separation between these floors.

Should the opposite occur, and the windows deteriorate, then there would not only be a risk internally, but the debris that would project externally would create problems at ground level. But equally as problematic would be the fact that flames coming out of those windows would have the effect of a blowtorch, and this may then result in fire spread above the fire floor, in the absence of the availability of a protective water supply.

The favourable means of then dealing with a backdraught at high levels or in unvented compartments would be mass flooding of that compartment, and this may take a considerable period of time, and large amounts of water used in order to suppress the hot fire gases.

To conclude, and in answer to the question as to whether PPV can be used in a backdraught scenario, the answer is yes, provided that the necessary control measures are in place, that an exhaust can be created externally, and most importantly, that there is no saveable life or firefighters located within the risk area. As such, the tactical mode declared at the incident would be “defensive”, should sectorisation have been implemented at that incident.