Back to Basics: November 2012
Ventilation saws are great tools to use when cutting open a roof or the side of a gable end for vertical ventilation.
November 1, 2012 By Mark van der Feyst
Ventilation saws are great tools to use when cutting open a roof or the side of a gable end for vertical ventilation. These saws speed up the ventilation process, allowing for quicker evacuation of hot gases, smoke and unburned particles of combustion. Vent saws are also useful for special operations such as RIT, in order to create an opening for rapid extrication of a mayday firefighter. Using a ventilation saw requires training to understand how to optimize the saw’s performance.
|Photo 1: A close-up of a bullet chain, manufactured specifically for the fire service. Photos by Mark van der Feyst
|Photo 2: The intake port of the ventilation saw is very distinctive, as it has a black foam cover around the filter.|
|Photo 3: Once the thickness of the roof is known, the gauge on the ventilation saw can be set so the roof joists aren’t cut all the way through.|
|Photo 4: Holding the saw vertically will cause the chain to stutter when it comes into contact with strong material, such as a roof joist. This is particularly useful if the thickness of the roof is unknown, or if the saw gauge is broken.|
|Photo 5: These black marks and small notches are from the bullet chain coming into contact with the roof joist.
Ventilation saws manufactured specifically for the fire service differ greatly from regular chainsaws used for household or commercial functions. The biggest difference is the type of chain: on a fire-service ventilation saw, a specially designed chain, called a bullet chain, is used in place of a regular chain. The bullet chain has a beefier look to it and has different cutting properties built into it. In photo 1, you can see a close-up of a bullet chain. The bullet chain needs to be pushed into the material, rather than being pulled into it, as you would with a regular chain. The bullet chain produces a fine sawdust, instead of wood chips that are produced by a regular chain.
A bullet chain will cut through all types of materials when operated at a consistent speed. The speed needed to cut through three inches of material – be it wood, shingles, insulation, plastic pipe, metal pipe, nails or wire – is 74 feet per second (ft/s). This speed allows the chain to drive through the material. To achieve 74 ft/s, the ventilation saw needs to consistently operate at 10,200 rotations per minute (rpm). When the speed drops to 73 ft/s, the chain bounces off the material because it does not have the speed or power to cut into and through it. This effect, called chain chatter, can lead to kickback. The bullet chain is designed so that it will not produce kickback.
Two other distinctive features on a fire-service ventilation saw are the exhaust and intake ports. In photo 2, you can see the intake port; it is distinctive as it has a black foam cover around the filter. The air intake has been moved to the top of the saw because placing it there helps to increase performance speed and allows the saw to work in environments that are smoke-filled. The air is drawn in from behind the firefighter where it is cooler and cleaner than the air in front of the firefighter. The exhaust port is also on top, which blows smoke and heat away from the saw. The exhaust port works in conjunction with the black plate on the front of the saw body to deflect heat and debris up and away.
The ventilation saw also has a depth gauge attached around the bar of the saw that allows the firefighter to adjust the depth of the cut. This gauge works well when the roof’s thickness is known. Adjusting the depth can be accomplished by cutting an inspection hole with the gauge set back all the way. An inspection hole can be created by cutting a small triangle in the area where the vertical vent opening will be made. The triangle cut can be the width of the chain bar and can be made by plunging the blade of the saw straight into the roof in a three-sided fashion. Remember to operate the saw at full throttle to ensure its optimal performance. When the saw is operating at half throttle or thereabouts, the chain will not work as it is intended to and the saw engine will work a lot harder.
Once the thickness can be observed, the gauge can be set to cut the roof’s material without cutting the roof joists all the way through, as in photo 3. But what if the depth gauge malfunctions? What happens if the silver knob that tightens the gauge becomes loose, falls off or is missing altogether? This can be a reality as equipment/tools sometimes lose vital parts during operations. The saw can still be used to cut the roof without cutting the roof joists all the way through. This is where the positioning of the saw while cutting comes into play, along with the use of the bullet chain.
As you can see in photo 4, the firefighter holds the saw vertically rather than on an angle, as in photo 3. This vertical position allows the chain on the saw to come into complete contact with the roof joist. The depth of the joist comes into complete, full-surface contact with the chain. For example, if the roof joist is a 2×8, then the full seven-and-a-half inches of the wood joist will come into contact with the chain. Remember that the bullet chain must be pushed into the material and will want to repel from the material naturally. When the saw is being moved across the roof line during the cut, as it comes into contact with the roof joist, the operator will feel the saw resist the joist; this allows the operator to bring the saw up to roll over the joist and then plunge it back down. When the saw is on an angle without the depth gauge, the operator will not feel the resistance of the joist and will be more likely to cut all the way through it.
In photo 5, you can see black vertical marks on the roof joist. These black marks are from the bullet chain coming into contact with the joist. Notice the top of the joist where there is just a small notch cut out from the saw rolling over it. This small notch will not compromise the joist as much as it would had it had been cut almost all the way through.
A ventilation saw looks like a regular chainsaw but the differences require know-how. Training will accomplish this.
Mark van der Feyst is a 13-year veteran of the fire service who works for the City of Woodstock Fire Department in Ontario. Mark instructs in Canada, the United States and India. He is a local-level suppressino instructor for the Pennsylvania State Fire Academy and an instructor for the Justice Institute of B.C. Contact him at Mark@FireStarTraining.com
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