During what circumstances is a firestop critical for a type "B" vent installation?

A firestop is essential when a type "B" vent passes through a ceiling or floor because it serves to maintain the integrity of fire-rated assemblies. When venting through these structural components, it is crucial to prevent the spread of fire and smoke from one area of a building to another. Firestops are designed to fill the gaps around the vent where it penetrates through ceilings or floors, thus reducing the risk of fire spreading and allowing for proper ventilation without compromising safety.

In such installations, the firestop creates a barrier that ensures materials around the vent do not catch fire and are not exposed to direct heat. This is vital as type "B" vents are specifically used for the venting of gas appliances, which can produce significant heat. By effectively sealing off these vulnerabilities, firestops help to protect both the building structure and the occupants.

Given this context, the other situations, such as being installed outdoors or indoors or being connected to an appliance, do not carry the same critical necessity for a firestop as they do not involve the same risk of fire passage through structural barriers.