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Space at the bottom of a Fire Door

What do you mean by "space"? Are you referring to the gap between the floor surface and the bottom of the door?
 
That is what he is alluding too, at what dimension is the gap too big to affect the fire rating?
Or, can the door swing over dissimilar flooring materials, carpet to vct without a threshold?
 
NFPA 80 states 3/4-inch maximum, unless the door assembly has been tested with a gap greater than 3/4-inch.
 
The bottom of the door is at negative pressure during a fire, so air will be pulled in rather than flames & smoke escaping.
 
That’s not how it was in Backdraft??

I am thinking pressure may depend on the stage of the fire???
 
As a fire burns and heat rises, it draws combustion air from the lower levels, and door bottom clearances is an area where that air is drawn through. As the air is sucked through the bottom of the door in puts an inward pressure on the door (inward toward the fire). Hot air and gases build up in the upper portion of the space creating an outward pressure on the upper surface of the door. These two forces will attempt to warp the door. This is why fire doors are tested with a neutral plane at 40 inches above the floor per UL10C and NFPA 252.

If smoke begins to be drawn back through gaps at the upper portion of the door, then the fire is starving for oxygen and hot combustible gases begin to build up inside the space (i.e., backdraft). If oxygen is suddenly introduced into the space (e.g., a window breaks), then the hot combustible gases may rapidly re-ignite (explosive-like) similar to a properly balanced fuel/air mixture.
 
Yes thanks! I have a hard copy of NFPA 80 buried some place. Sometimes finding stuff I save is like looking for Jimmy Hoffa! 3/4 is what I remembered. Just ran across 6 doors that have been installed @ 11/4" off the floor. That is why I raised the question.
 
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