Hoistway Smoke Venting and Pressurization Screen

ScopeThe two approaches solve the same problem in opposite directions. VENTING accepts that smoke enters the shaft and gives it somewhere to go, through an opening at the top sized as a fraction of the hoistway plan area -- a 10 by 9 ft shaft at 3.5% is about 3.2 sq ft. PRESSURIZATION supplies air to the shaft to keep smoke out in the first place. Energy codes disliked permanent open vents at the top of every hoistway and smoke control practice preferred keeping the shaft clean, so pressurization became the common answer -- but which is REQUIRED depends entirely on the adopted code and the AHJ, and buildings exist with both. THE CONSTRAINT THAT BOUNDS PRESSURIZATION IS THE DOOR. Too little pressure and smoke migrates in; too much and the difference across the hoistway and stairwell doors makes them hard to open, which fails the egress force limits. A 36 by 84 in door is 21 sq ft, so at 0.10 in wc the pressure puts 10.9 lbf on the door; felt at a knob 3 in from the latch edge that is about 6.0 lbf (the NFPA 92 lever factor W / (2 (W - d))), which is tolerable; at 0.25 in wc it is 14.9 lbf at the knob, and on top of the closer's force that approaches the limit. With a 6 lbf closer and a 30 lbf limit, Klote's table allows 0.40 in wc on a 36 in by 7 ft door. The band has a ceiling set by the doors, not by the smoke control objective. And underneath it is the shaft's own stack effect, which in a tall building develops a pressure of the same order as the whole pressurization band, varies with the season, and REVERSES between summer and winter -- so the fan is not working against a still shaft, and a system commissioned in one season can behave quite differently in the other. A screen with supporting arithmetic, not a design. Whether a hoistway must be vented, may be pressurized, or requires neither is set by the adopted building and fire codes and by the AHJ, and the requirements have changed between code editions in ways that make older buildings and newer ones different. Pressurization design requires leakage areas for the shaft and the building, stack and wind effects across the seasons, the behaviour with doors open, the interaction with stairwell pressurization and the building's HVAC, and a commissioning test. Elevators used for occupant evacuation or firefighter operations carry additional requirements. The adopted building and fire codes, NFPA 92, ASME A17.1 where elevator operation is affected, a smoke control engineer, and the AHJ govern.

Formula or tablevent area = hoistway plan area x vent fraction, not less than 3 sq ft per car (the legacy IBC 3004.3 rule); opening force at the knob = closer force + door area x pressure difference x 5.2 lbf per sq ft per in wc x W / (2 (W - d)), the NFPA 92 relation with d the knob distance from the latch edge; supply air = about 2,610 cfm per sq ft of leakage x the square root of the pressure difference in in wc.

Source and editionThe historic vent-area fraction and the pressurization orifice relation, by name, with NFPA 92 and the adopted building code named as governing which approach applies. A screen with supporting arithmetic, not a smoke control design.

Free accessArea and orifice arithmetic on the user's own shaft dimensions and an entered leakage area; no code table is reproduced.

What governsEstimate. AHJ and licensed professional govern.

API field nameshoistway_plan_area_sqft, vent_fraction_pct, door_width_in, door_height_in, pressure_diff_inwc, door_force_limit_lbf, leakage_area_sqft, vent_area_sqft, door_area_sqft, door_force_added_lbf, max_pressure_inwc, supply_airflow_cfm

• Which approach applies is a code question vent, pressurize, or neither is set by the adopted edition and the authority having jurisdictionadopted building and fire code
• The door sets the ceiling pressure difference across the door must stay inside the egress opening-force limitadopted building code
• Stack effect is larger than the band and it reverses with the season, so commissioning in one season proves little about the otherNFPA 92