Insights and expertise for doors and frames

Explaining Category A and B with Positive Pressure Wood Doors

Graham1Positive pressure was added to fire testing procedures in 1998 to more closely simulate real fire conditions. Before that there was no pressure type procedure in the testing so conclusions were drawn simply from flame and temperature. In real fire conditions as heat develops within the source area, pressure within an enclosed room begins to build relative to the pressure outside the room.  As heat rises, pressure follows suit and builds in that space.  This pressure creates strain on the top portion of doors as smoke, gases, and flame are forced through any openings in the assembly.

The new positive pressure testing is referred to as UL10C as compared to the old UL10B testing method. When the new UL10C test requirement was established, the UL10B method became known as neutral pressure. With positive pressure now positioned in the furnace during fire tests, it was soon discovered that a door’s edges above the 40″ pressure plane were susceptible to flame, smoke and hot gases escaping which results in a failed test. Steel doors may have an advantage as the steel itself expands and seals the door against the frame. Wood doors do not expand when heated; instead wood door edges may burn away potentially resulting in an increased gap between the door and frame. This increased gap combined with the positive air pressure against the door may result in flame, smoke and hot gases escaping to the non fire side of the door, thus failing the UL10C test.  One solution is to fill the gap during the fire with an intumescent seal. When heated, the intumescent expands and fills the gap between the door and frame containing the elements in the source room and thus passing the fire test.

Within the wood door industry, there are two methods to install the required intumescent to the door opening.  The first is to apply the seal to the frame with either an adhesive backing fixed on the seal or by mechanical means. The second method incorporates the seal into the door, either with a layer of thin intumescent material concealed under the vertical and top horizontal edges, or imbedded in a groove within the door edges. Aesthetically, the second method of imbedding the seal into the door edge design is more appealing, but in most cases this method is more expensive. In the early days of UL10C, the terminology for the two seal methods was “frame applied” & “built into door”. It was determined very early in the process that a method of communicating when and where an edge seal is required was necessary. This communication was imperative for a couple reasons – (1) adding another edge seal to a door (when one is already incorporated into the door) may actually cause additional problems in the event of fire, and (2) AHJ’s (Authorities Having Jurisdiction) needed a method of confirming when a door did not require an edge seal to pass the test.   To address these issues, the certification agencies developed the Category system.  Category A doors are those doors that (1) do not require any edge seal material, or (2) doors that already incorporate an edge seal into the door construction.  Category B doors are those doors that are required to have a Category G listed edge seal installed on the opening at the jobsite.  (Category G Edge Seals are products that have been specifically tested and listed as an Edge Seal – not to be confused with products test and listed for use in Smoke and Draft Control locations.  Smoke and draft Control gaskets are listed under Category H.)   To further simplify the identification process in the field, it was determined that the appropriate Category (A or B) would be stated on the door label.

Specifications from the architect will designate how the seals are to be applied in the opening and this information should be used when ordering the wood doors.   For wood doors with 20, 45, 60 & 90 minute ratings, the rule is simple.  If the requirement is for Category B, the frames must have the intumescent seal applied to the rabbet of the frame. For Category A, the intumescent seal must be built into the door, thus hidden from plain view.  Keep in mind any door opening that is also a rated smoke opening must have a seal set that meets the requirements of a Category H seal.

When it comes to 20 minute doors there is an exception to the rule for single swing 20 minute doors that are 4′ wide by 8′ high and smaller.   The exception is that if the opening also requires a smoke seal (Category H), an intumescent seal is no longer required for the door to meet UL10C positive pressure criteria, as long as the seal meets both Category G (edge sealing) & H (smoke & draft) requirements, for example a Pemko S88, you have met all requirements for a positive pressure opening.  This exception to the rule only applies to 20 minute rated fire doors.

The confusion relates to what is this door called:  is it a Category A door that requires no additional seal to achieve the positive pressure rating, or is it a Category B that requires an additional seal for the door to meet UL10C requirement?  My suggestion is to know what all is required of the opening.  If you know your single swing 20 minute doors also require smoke seals, then order the doors without built in seals and save money and resources.  If the opening does not require a smoke seal and the specification calls for built in seals, then provide doors with built in seals.  Remember, it’s okay to request further information from the architect.  Specialized product categories are always best to clearly define before the order goes in for production, and questions pertaining to the information in this blog will reassure your contractor that they have a competent supplier whom they can rely on.

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