A key part of fire safety measures is to pressurize areas used as escape (namely stairwells) to prevent smoke entering the area. The design of these systems is complicated, because as people enter and leave the stairwell during an emergency there is an intermittent loss of effective pressurization. The system supply fan should have sufficient capacity to provide effective pressurization and prevent smoke entering the stairwell. Balancing this there should also be a method of preventing over-pressurization, which makes doors difficult to open.
Research clearly shows that when a fire occurs in a multi-compartmented building, the smoke from the fire is a far greater hazard to the occupants than the fire itself. Smoke can kill by asphyxiation or poisoning well before the temperature of the fire or smoke causes injury. Smoke also obscures vision, preventing occupants from finding safe escape routes and hindering the fire brigade search and rescue operation.
Safety factors for HVAC systems allow for uncertainties in the final design, construction and use of the building, but should be used reasonably. Greatly over-sized equipment operates less efficiently and costs more than properly sized equipment. For example, over-sized cooling systems may not dehumidify the air properly, resulting in cool but clammy spaces. It is unreasonable and expensive to assume a simultaneous worst-case scenario for all load components (occupancy, lighting, shading devices, weather) and then to apply the highest safety factors for sizing.