Removable Mullions: Securing Pairs of Doors with Rim Exit Devices

Mullion vs. Astragal vs. Flush Bolts

Pairs of doors can be secured in a few different ways. Lori Greene at I Dig Hardware draws the distinction: an astragal "should not be confused with a mullion, which sits between (fixed) or behind (removable) the meeting stiles of a pair."

Why Specify a Removable Mullion

Greene explains that hardware removable mullions "are popular because they allow rim panic hardware to be used on pairs of doors, rather than vertical rod panic hardware." She notes that vertical rod and cable devices "can be more challenging to keep adjusted properly," that sill tolerances need to be fairly tight, and that such doors "require periodic attention." By contrast, "the rim x rim x removable mullion application is typically easier to maintain, and the panic hardware may latch more reliably."

With a mullion, each leaf carries its own rim device. See panic hardware for how rim, mortise, and vertical rod exit devices differ.

Hardware Mullions vs. Hollow Metal Mullions

Greene describes two main types:

• Hardware removable mullion (more common). Sits behind the meeting stiles and lets two rim panic devices latch into it.
• Hollow metal removable mullion (less common). Made from frame material with brackets for removal. It sits between the doors, not behind them. That lets each door carry its own lockset or latchset, for example on a mechanical room whose inactive leaf is opened only to move equipment.

Construction and Keyed Removal

As one manufacturer example, Von Duprin's auxiliary hardware catalog describes its removable steel mullions as 2 inches wide and 3 inches deep with a 1/8 inch wall, shipped prepared for strikes. Aluminum versions are also listed. The top fitting attaches to the frame head and is covered by a fitting cover.

Keyed removable (KR) mullions make authorized removal faster. Von Duprin says a KR mullion is released "by a single operation of the mortise cylinder," and the unit "will self lock when re-installed, without use of the cylinder key." It takes a standard 1-1/4 inch mortise cylinder, sold separately. See rim and mortise cylinders.

Von Duprin also lists accessories such as a two-piece interlocking stabilizer set and an angle plate for narrow transom frames.

Fire-Rated Pairs

Pairs of fire doors raise extra questions, and the answer always comes from the door and hardware listings, the adopted code, and the AHJ. High-level points from the sources:

• Labeled mullions exist. Von Duprin lists a UL fire-labeled mullion for openings up to 3 hours used with its fire exit rim devices, and notes that this model "is not easily removed due to special fittings." A keyed-removable version is listed as fire rated the same, with the note that "only qualifying applications will be provided with UL Label."
• Manual flush bolts are restricted. Greene notes that because manual flush bolts "do not provide a positive latch and could be left in the retracted position, they are not typically allowed for use on fire doors." The exception is rooms not normally occupied, if the AHJ allows it.
• Automatic and constant-latching flush bolts. "Either type may be used on fire door assemblies," according to Greene. Both are available as "LBB" (less bottom bolt) sets with a fire pin.
• Closing sequence. Where a pair needs the inactive leaf to close first, a coordinator handles the sequencing. Greene notes coordinators are only used on pairs and exist so the leaves "close in the proper sequence – the inactive leaf before the active leaf."
• Astragals. Greene notes that some door listings, typically for 3-hour doors, require an overlapping astragal with a 3/4 inch projection. That can conflict with independent operation of each leaf, so the whole opening has to be coordinated.

See fire-rated door hardware for the wider rules on labeled openings.

Access Control on Mullion Pairs

Mullion pairs are often part of access-controlled openings, using technologies such as electric latch retraction or electric strikes. This draft does not detail specific combinations. Compatibility depends on the particular mullion, strike, and exit device, so the manufacturers' data and the listings govern.

Related Topics

• Panic hardware: rim, mortise, and vertical rod exit devices
• Fire-rated door hardware: labeled openings
• Rim and mortise cylinders: the cylinder in a KR mullion
• Electric latch retraction: remote dogging and unlatching
• Electric strikes: strike-based access control
• Strike plates: where latches land
• Door closers: closing pairs reliably
• Door handing: active and inactive leaves

Sources