Coastal Florida solves this problem with the FEMA flood plain codes
Water heaters, AC units and such are required to be above flood plain which usually gets them up a couple feet off the garage floor that is usually that far below the FF of the house. (typically a 4 course stem wall)
The same applies to coastal NJ & flood plain areas for 'new' construction.
I guess it is a federal rule ... everywhere but New Orleans ... but I digress.
That would also apply to building a new garage here and there has been a discussion about replacements, particularly HVAC units. They are starting to make all of the HVAC condensers go up but I am not sure about water heaters. I doubt most electric units get permitted anyway.
Joe's question is interesting in the uplands tho. It appears the IRC would prohibit a gas water heater, dryer or whatever if the burner is below 18". I imagine a lot of people get surprised when they get the report from the home inspector. (I imagine where Joe got this)
I would say no. Not that it would hurt anything to raise them up off the floor, but I think that the situation your descibing is the reason that all of the new gas water heaters have the FVIR "Flammable Vapor Ignition Resistant" design. I may be wrong, but I think itís been required since around 2003.
On a similar note, I have heard of some inspectors requiring the receptacle outlets in Resi garages to be located above 18-inches from the finished floor for that same reason though.
Caper: I did as well, and I am sure that there are many jurisdictions around the country who have a rule in place that covers this. Article 511 covers commercial garages and is not to be considered here, I am talking about a residential garage. Let's spill gasoline on the garage floor in that, or any other garage, and see what could happen when a spark ignites the puddle!
The NEC is without a specific rule on this, but common sense and field experience by qualified persons, and those who understand electrical systems and inspections (not political hacks) must be considered.
You say..."The NEC is without a specific rule on this, but common sense and field experience by qualified persons, and those who understand electrical systems and inspections (not political hacks) must be considered."
I agree that 'common sense' should be considered...but it is not within Article 100!
I must also add that spilling gasoline anywhere and having an ignition source is a real danger, but the direct answer to your OP is as far as I can go within the limits of the NEC which is what I have to enforce, as adopted within the NJ Uniform Construction Code.
BTW, my garage has not had any vehicle inside in 10+ years; no lawnmowers, snow blowers, etc.
Joe, do you really think you're the first to have concerns about water heaters in garages? I doubt it- since you hail from California, and they've been putting water heaters on pedestals for at least a quarter-century now.
The plumbing code addressed this issue long ago. Water heaters in carports and garages, or that draw their ventilation from them, need to either be on a pedestal, or be of a 'flammable vapor ignition resistant' design. FVIR. Easily found everywhere water heaters are sold - and at a nominal price difference. UL listed and all that stuff.
Oddly enough, I am dealing with this exact situation in my house. Since the place was built (1957) there have been many changes to the requirements for water heaters. Venting, drainage, mounting ... lots of changes. And, if the water heater is in a closet, no storage is allowed there.
No need to re-invent the wheel, and get all worked up about 'classified locations.' The plumbers got there first.