Sometimes a question crosses my desk that surprises me – it seems like after 6+ years of writing about doors and hardware every day I would have heard them all, but this was a first. I was asked to find out where in the codes it states that a removable mullion is acceptable to use on a fire door assembly; an AHJ had questioned the application because when the mullion is removed, the doors will no longer be positive-latching as required by the model codes and NFPA 80 – Standard for Fire Doors and Other Opening Protectives.
If you’re not familiar with a removable mullion that is made by manufacturers of panic hardware and fire exit hardware (like Von Duprin and Falcon), it is designed to sit behind the pair of doors, and the rim exit devices latch into it. When the full width of the pair is needed in order to move something large through the opening, the mullion can be removed – either by turning a key in a cylinder or by removing screws. There is an article and video about removable mullions here.
Although there are many removable mullions which have been tested and listed for use on a fire door assembly (not all removable mullions are certified for this purpose), I have not found anything in the codes or standards that specifically addresses them. Removable mullions are quite common, so a code requirement related to their use on a fire door assembly must have slipped through the cracks.
NFPA 80 has a definition for “mullion” but no specific reference to removable mullions:
3.3.86 Mullion. A fixed or removable vertical member set in a double door opening that allows both leaves to be active or set between a door and a side light or a separate, framed, glazed area.
One paragraph in NFPA 80 can help us to understand the intent of the standard with regard to temporary conditions, but is relative to removable transom panels, not removable mullions:
126.96.36.199 Removable transom panels shall be permitted to allow for movement of materials or equipment through the opening.