Positive Pressure

by Christian L. Mordh, AHC, the Kirsten Group


intumescent seals are now required on some doors


Is that what you feel when you’ve packed five hours of work into a two-hour time slot, or when you’ve eaten too many beans? Actually it is a term related to a new testing method for fire-rated doors. Previous door and frame testing was done under negative pressure, which did not represent the conditions a real fire.

To understand the reason for the new rules, imagine what happens to a door during a fire. Heat and gases are trying to squeeze through gaps between the door and the door frame; at the same time, cool air is being drawn into the fire at the lower part of the door. In other words, the door is subject to both positive and negative pressure. Steel doors were able to pass the new tests without difficulty, but wood doors fail unless they are modified.

Normal resilient seals do not work, but intumescent products, which expand at elevated temperatures, will effectively seal the gaps around a wood door. These products are similar to the little black pellets called “snakes” that you may have lit as a child.

Surface application of the intumescent product is the most cost-effective method, but it does have drawbacks. In an exposed condition, it is subject to wear and tear, and to vandalism. By the time it is needed, it may no longer be present in sufficient quantity to seal the door. It is also unattractive. A more durable, yet costly application is to conceal the intumescent material in the edge of the door, or into the edge of the doorframe.

It is important to note that twenty-minute labeled doors require a smoke seal in addition to the intumesence material. The smoke seal holds out the cold smoke prior to the expansion of the intumescent material.

Intumescent materials are classified in two types, soft puff and hard puff. Soft puff begins expanding at relatively low temperatures, 250 degrees (121 degrees C). It expands gradually with low force and forms a firm char. It has a reservoir for continued expansion. Hard puff begins expanding at higher temperatures, 450 degrees (232 degrees C). It expands with a strong force. It has a high expansion capacity, dissipating char, but no reservoir for additional expansion.

New test requirements also affect hardware. For door closers to meet the new test requirements, flame-retardant hydraulic oil must be used. The testing and certification of doors frames and hardware is now covered by UBC 7-2-1997.

Make sure your specifications comply with these new requirements.

Have questions? Call Chris for the answers at 612-550-9706, cmordh@worldnet.att.net.

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