Adoption of Positive Pressure Codes First, some background. For more than a decade, there has been a consistent call from some fire protection experts to revise the fire door test standards, requiring them to be conducted with positive pressure. At the same time, the positive pressure test is practiced internationally through such codes as ISO 3008, BS 476, and JIS A 1311. Basically, during a fire test, pressure is developed in a furnace. Positive pressure forces air out of the furnace.
Negative pressure acts to pull air into the furnace.
In traditional fire door and window tests, the furnace pressure at the top of the assembly is maintained slightly negative relative to the ambient environment. This means that cool ambient air will tend to be drawn into the furnace through any cracks or openings. This keeps some areas cooler than they might otherwise get or it may cause combustibles to burn faster due to an increase in air supply. This causes a positive pressure to be created above these locations. There will still be negative pressure below these points, but the magnitude of the pressure difference will be smaller and, thus, any airflow into the furnace at the bottom of the assembly will be less than in neutral pressure tests.