One quick and easy inspection that should be regularly performed is verifying that the fire door gaps meet NFPA requirements. Through daily use, mistreatment, or even incorrect installation, these door gaps can be too large and non-compliant, putting building occupants at unnecessary risk. These gaps seem like a trivial issue on the surface, but during a fire they actually play a very important role.
Fire uses oxygen to burn, and a large fire in a closed room with begin to consume all the oxygen in that area, creating a vacuum of negative pressure near the base of the fire. The heat from the burning materials creates intense positive pressure near the ceiling of the room. This is where the passive fire protection, including fire door gaps, becomes important. With properly sealed and fire resistant gaps, the fire will not have a constant supply of fresh oxygen to burn, and will burn smaller or even die out on its own.
If the gaps are too big, the negative pressure will cause fresh oxygen to be sucked into the room with the fire, feeding it and allowing it to grow bigger and hotter. This will also make the positive pressure stronger, forcing the hot air and smoke out of the higher fire door gaps and into areas that are supposed to be safe from the fire. This hot air and smoke can actually be able to ignite combustibles, creating a fire on both sides of the closed door.
Fire doors are also required to have an active latch built in to the hardware, just to prevent them from being pushed open by the positive pressure created by the fire and the expanding gases created by it. This a change to fire door code, which used to only focus on the negative pressure and keeping oxygen from being able to feed the fire.