NFPA 496 Building Pressurization Unit

A wide array of sizes, mounting options, easily operated controls with a wide array of options make Bebco Whisper Drive™ Units your best choice for building pressurization!


Bebco Model WDP Wall-Mount and RDP Remote-Mount Whisper Drive™ NFPA 496 Building HVAC Pressurization Units are the world’s finest stand-alone building pressurizers! These self-contained units are intended for control rooms or buildings that may or may not contain an internal source of flammable vapors or gases, since they can also operate as emergency ventilation units! Four different sizes are suitable for rooms or buildings with opening(s) ranging up to 75 square feet and floor areas up to12,000 square feet, but they can work in tandem for larger spaces. They also work well with HVAC units, by automatically minimizing the air necessary to accomplish effective pressurization.

In addition, the fully seal-welded housings of these HVAC pressurization units, coupled with a variable speed drive and extremely flexible control options, allow these units to pressurize buildings and rooms easily while maintaining the lowest possible noise levels, often well below 70 decibels!

To ensure ultra-quiet operation, these HVAC pressurization units feature primary and secondary axial fans to pressurize rooms or buildings The primary fan is controlled by a variable speed drive to quietly maintain a safe (0.25 ”) pressure when all openings (typically doorways) are sealed. The secondary fan is controlled by an optional delay timer, which engages the fan only upon loss of safe pressure when doorways are opened, to generate a minimum air velocity of sixty (60) feet per minute through the open doorways. As an added safety benefit, the secondary fan also functions as a redundant fan to maintain building pressure, if the primary fan fails.

When combined with an intake stack drawing fresh air from an unclassified area, these units reduce the hazardous (classified) area rating within your room or building, as UL Classified, from Class I, Division 1 or 2 Groups C and D, to unclassified, in accordance with NFPA 496.


After basic calibration is performed to set a preferred room pressure, an alarm set point and preferred time delays for pressure loss alarm and secondary fan activation, Unit operation is very easy and status indicators keep occupants informed of unit performance at all times. The HVAC pressurization unit is started by closing all doors, placing the secondary fan switch in the “automatic” position, and placing the primary fan switch in the “on” position. The primary fan then engages to regulate air flow and maintain a safe pressure. When a door is opened, the primary fan will increase velocity, in an attempt to hold pressure in the room or building, and the unit will activate the pressure loss alarm and secondary fan activation delay timers if pressure falls below the alarm set point. As the door is closed, and room or building pressure rises above the alarm set point, the secondary fan disengages, the alarm is deactivated and the primary fan throttles back automatically. In essence, once a Bebco Whisper Drive™ Unit is running, it does everything required to maintain building pressure or velocities through an open doorway automatically, keeping you in complete accordance with NFPA 496 at all times. You then have the abilities to override the alarm, (which causes that indicator to flash as a reminder) and manually activate the secondary fan as needed to enable tasks such as moving equipment in or out of the room or building safely without annoying alarms!