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Fire Science Question


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Anybody ever wonder about the logistics of attached garages that contain natural gas (open flame) burning devices, such as furnaces, clothes dryers, and water heaters in the presence of our bikes.


When I lived out east the common mode of construction placed these appliances inside the primary dwelling. Now I am on the West Coast (Oregon) and the above appliances are often placed inside an attached garage and elevated 16” above the floor of the attached dwelling. I have never been completely comfortable with this set up, but I have accepted it, as the probability of a gasoline leak in one of my bikes is minimal.


However, I question the logistics of appliances that burn natural gas in close proximity to vehicles containing combustibles.


My question(s): Is this safe, from a "fire science" point of view? Does the 16” elevation matter. How does the 16” elevation work at providing safety? Is battery charging advisable under such conditions?


Thanks in advance for your response(s).



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Yes, it is safe.


The elevation or raised platform of a water heater places the open flame above where the majority of flammable vapors accumulate. This is to say that most garage present flammable vapors are heavier than air and therefore sink to the floor. Gas and propane would be two to consider.

Those that are lighter than air will conversely rise and would typically be found up high. Natural gas is one example here.

The other aspect to consider is upper and lower explosive limits. Just because the gas is present does not mean it is capable of igniting. Just as in an engine, there must be enough fuel for combustion to occur. Too lean and it doesn't ignite. Too rich and it doesn't ignite.

Where the issues come in to play in a garage is primarily from owner modifications to designed in safety features. Proper venting that is not obstructed or improperly routed. Space ventilation by blocking vents or adding forced ventilation that could cause a draft through the appliance vent sufficient enough to blow out the pilot or burner.

As long as there is sufficient ventilation and the in place appliances are properly engineered and are in good working order there is no real risk.

Vehicles and batteries are not an issue under these circumstances.

It is always good practice to have a functioning fire extinguisher suitable for the type of potential ignition sources that are present. "A" for normal combustibles such as paper and wood. "B" for flammable liquids. "C" for energized electrical circuits. These are readily available and are most often rated for multiple fire types such as a 10A-2BC extinguisher for example. I'll try and dig up a link that shows what the various extinguisher ratings mean.

More important to having an extinguisher of the correct type and size is the training to use it and the knowledge to know when to use it and when to call the fire department and for you to get out and not get yourself hurt. That and placing it in an appropriate location that is accessible and not blocked by stuff.


Yes, it's very safe to have these appliances in the garage. In my professional experience it is almost exclusive to homeowner or poor installation screw ups/modifications that cause issues.


My best example of this would be to find a garage that has had the sheet rock removed or "access" punched in that compromises the wall design to hold a fire in the garage and not let it in to the attic or living quarters. Same holds true for removing the self closing hardware on the door that leads from the house to the garage. Now the fire has a straight shot to the house and more fuel and air.


Hope this helps in your question.

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Slainte - Phil is most probably right in his summation BUT if I had a choice I'd err on the safe® side.


I appreciate Phil's recommendations on fire extinguishers but that is a defence after or during the event of a fire as are details dealing with drywall fire blocking pursuant to building code. While it's good strategy to have plans to deal with the outset and spread of a fire I believe stringent prevention of having a fire start are as or are more imperative.


As we move between municipalities working at our trade (drywall) we find that building departments interpret the building code differently.


For peace of mind I would consider appliances with sealed burner units that draw their fresh / combustion air from outside. That's what we use in our shop where combustible chemicals ie. cleaners, solvents, paints etc. are used.


I realize they are rare but fires due to volatile vapours in the presence of open ignition sources do occur. One house we worked on burnt to the ground just after the tenant moved in. They weren't happy with how easily the slate tile floor marked under traffic so they had the flooring contractor return to apply a sealant. The floor was sealed, the contractor left (the owner was away) and the vapours from the drying sealer eventually ignited. The fire marshal suspected that the vapours were ignited by the fan in the furnace that was being used to circulate the air to promote drying. A second house we worked on caught fire after the owner had been working on his riding lawnmower in the attached garage. Gasoline was spilled in this activity and (supposedly) cleaned up. He closed the garage and shortly after a fire erupted resulting in the gutting of not only the garage but also the house due to smoke and water damage.


While these events are rare I think it's best to plan for the worst possible scenario when it comes to the potential of fire. Part of the plan is the consideration that not only do we have combustibles present ie. laden gas tanks both in vehicles and stored portable cans, but also that we tend to use our garages as work shops thereby introducing various ratios of combustible vapours sometimes in confined, unvented spaces.


I share your "never been completely comfortable" with open flame appliances in the same space with volatile liquids. I would plan beyond the building code to minimize the potential of fire in the first place.

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I do home inspections as part time job, and one of the standards we are trained to inspect to for in garage appliances (hot water tank, furnace not common in Canada, garage heaters more so) is the 40cm (16”) raised rule. Both, as Phil mentioned; for most normal space use ignitable vapours (including those from a leaking appliance) will pool below that, but also to reduce the potential for impact damage to the appliance.


Are there certain circumstances where the amount of accumulated ignitable vapours can overcome this design precaution? Sure. But under normal use of the space; issues are exceedingly rare.


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skinny_tom (aka boney)

Here on the left coast our garages are not heated. On the exterior walls, vents (as Phil mentioned) must be installed when it is constructed and must not be blocked by the occupants. These vents are lower than the 16" platform upon which the house mechanicals are placed allowing for the vapors to escape prior to reaching the level of the burners.


If I'm not mistaken, many houses back east have heated garages. The danger of having the mechanicals in the garage is obvious because of the sealed nature of a heated room and the recirculation of air. However, from a medical standpoint, there is a far greater danger of poisoning yourself with carbon monoxide gas with all that stuff in the house instead.

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Boney makes a good point about carbon monoxide.


Our building code identifies the issue of carbon monoxide in detached homes and doesn't really deal with fire control issues until either there is a) defined habitation space above a garage or b) for attached dwellings. CO abatement measures get a gimme under fire control codes although more in sealing definitions than venting as Boney outlined. Apparently, we don't vent 'cause we have winter issues.


Again, I would go (well) beyond the building code to prevent fire in the first place and then address means to mitigate the spread of a fire that might happen. Ergo, I continue to share your concern about the open flame appliances in your garage cum workshop.

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