Vestibules: What We Need To Know Where The Indoors Meets The Outdoors
Every building has entrances and exits. The door line barrier, or vestibule, defines where the outdoor weather meets the indoor controlled climate. Traditionally, buildings in northern climates have always had an entrance vestibule with a heating device between the two sets of doors. Often, this can be a point of conflict, where the design engineer must provide a solution to make the indoor spaces comfortable to meet the client's expectations. The vestibule's central purpose is to stop the cold and wind from entering the occupied space of the building, thus allowing the interior space to remain warm and not subject to cold drafts. The variables that affect the design solution are:
- Occupant use of the interior space (lobby, waiting room, reception desk, corridor);
- Exposure to prevailing wind;
- Building pressurization;
- Power door activation unit operation in a cold environment;
- Snow or water on the floor;
- Fire sprinkler freeze protection;
- Traffic pattern; and
- Leaks from door cracks.
In northern climates, a heater is provided to temper the climate in the transition zone between the two sets of doors. Any air coming into the interior occupied space would be warmer than the cold, raw outside air. Other strategies have been used to achieve the tempering at the barriers. These include:
- Air curtains, without doors, in high passage count facilities or air curtains within the vestibule;
- Radiant ceiling panels and radiant floors;
- Cabinet unit heaters at the ceiling, in the walls, and under the floor;
- Heating capacity in the area adjacent to the vestibule; and
- Revolving doors.
Wind is often the reason for the enclosed vestibule concept. However, the "wind" is often cold replacement air rushing into the negatively pressured building. Before any barrier concept is designed, the negative pressure problem of the building must be resolved.
Checklist Before BeginningThe vestibule's principal goal is to achieve comfort in the occupied space. Over the years in the consulting engineering field, many attempts have been made to meet this simple goal. Consider the following checklist to arrive at the solution for the project under consideration:
- Verify the tightness of the door assemblies. Old doors have high crack leakage rates while modern doors are very tight with low crack leakage rates.
- Where are occupants relative to the doors?
- Where is the source of heat within the vestibule?
- Verify that the building pressure relationship between the indoors and outdoors is not negative.
- Will the facility have ADA power-accessible door operators? How long is the open cycle? Do both the interior and exterior door sets open from one activation point?
- Verify passage count and pattern: occasional, steady, or large rushes?
- How are snow and water handled? Verify location and length of floor grates and floor mats.
- Remember that warm air rises, cold air falls, and air velocity over 500 fpm is too drafty.
- Are the doors exposed to the prevailing winter wind direction?
The traditional heated entrance vestibule was developed in the days before air conditioned buildings. Some buildings were heated by hydronic systems and others by air systems. Little concern was paid to the pressure relationship between indoor and outdoor air.
However, air generally flows inward, thus bringing in the cold winter air. Engineers began to analyze the stack effect problems of negative-pressure buildings and discovered that if the top of the building was air tight, the in-flow of cold air was reduced. As air conditioning became the norm, engineers detected that fans could be used to slightly pressurize the facility, thus reducing the air in-flow. As IAQ became a popular issue, more outdoor air delivered within the building became available to maintain the building positive pressure.
As energy conservation became a hot topic, building doors and windows became tighter, consequently reducing cold air infiltration.
Building Pressure ControlThe initial design choices should focus on avoiding a negative pressure building. This is accomplished by reviewing the typical negative air paths shown in Figure 1. Consider the following topics:
- Penthouse or roof exhaust fans will exhaust up to four times the designed cfm in buildings over 10 stories high because of the boost of "stack effect" in the winter. The stack effect is the upward movement of air within a building due to the buoyant force of the warmer air relative to the cold outdoor air. Use variable-speed fans and airflow measuring to maintain constant airflow. In exhaust systems up to 3,000 cfm, consider a VAV box before the fan is set to control a constant cfm.
- Avoid natural ventilation cooling in elevator penthouses. Both the warm air rising and stack effect will activate the elevator machine room's cooling thermostat. Warm air now will exit the fan and the intake louver by the stack effect force. Use split system self-contained units to cool the elevator machine space.
- Fan tracking of supply and return/exhaust (or relief) fans should be based on pressure relationship to avoid over-exhausting the building, thus causing a negative building.
- Provide a normally closed damper in the elevator shaft vent opening.
Entrance Heat ChoicesFigure 2 shows choices for location of vestibule or entrance heaters. The primary heat source should serve the building occupancy. Therefore, the recommended first choice is to use Unit Type A. The high capacity heat is in the occupied portion of the building. The supply is from linear grilles in the ceiling, next to the doors. A duct branch is used to pressurize the vestibule. The supply source should be at the door height versus the often higher ceiling height of building lobbies.
Unit Type B is a typical wall unit. Use inverted airflow to get the heat low. Unit Type C is used with floor grilles when wall space is not available. Heat down to the low level helps dry the moisture and snow on the floor. Unit Type D is a ceiling mounted unit often directed by the project designer. In both C and D, use high duct static motors in order to move air through ductwork and downward discharge velocity to get the heat to the floor from the ceiling. Use a ceiling plenum return with a ceiling-mounted unit to create a heated environment for the vestibule fire sprinkler.
Unit Type E is an air curtain with heat. This device can be used inside or outside of vestibules. The unit produces noise, hot air on the head, and stops and starts with door openings.
Design ConsiderationsWith entrance heat choices and building pressure controls underway, begin the design thought process with the simple question: "Does the project require a double door vestibule?" With the advent of more weather-tight doors, improved closure devices, and more outdoor air available for pressurization, the need for the vestibule barrier is decreased.
If the building air supply system is designed for 24/7 operation; use air pressure to keep the wind out of the entrance. The selection of heating at the nonvestibule barrier should be selected for high heat output. The cabinet unit heaters, fancoil units with linear diffusers at or above the doors, are the appropriate heating source at the main building entrances. Emergency exits do not require the high capacity heat source. Radiant ceiling or floor panels are used to control moisture from rain and snow. They are also used when the vestibule is for "waiting."
Remember, the back door to the facility is also a climate barrier. It is often the loading dock with large overhead doors. The loading dock area should be considered as one large vestibule space. The inside doors should be closed when the outside doors are open. Heating options for this dock area are ceiling radiant and ceiling-hung unit heaters. The unit heater type of vertical or horizontal discharge is dependent upon the height of the space. Consider heated air curtain units at high passage or loading dock doors that need to be open during deliveries. Always consider an air curtain at service entrance doors to kitchen areas.
When considering a lobby with a reception desk, place yourself in the chair behind the desk without an electric heater. Then, think of the lobby entrance design that would be warm and comfortable to you. Remember that a variable-volume system is primarily a zone cooling system. With heating coils added, it becomes a constant volume heating system. However, the distribution must get the heat down to your level at the reception desk.
Sizing of the UnitsIn a vestibule, use the largest unit that will fit. For an air curtain type, use a grille width to match the doors and a 500-fpm discharge velocity at an 8-ft ceiling height. Design discharge temperature when blowing down (ducted or ceiling-mounted cabinet unit heater) should not exceed 95 degrees F to avoid the buoyancy factor that turns the airflow up before it reaches the floor. Design discharge temperatures from floor grilles or wall-mounted cabinet unit heaters should not exceed 120 degrees to avoid the feeling of extreme heat.
ConclusionDesign focus should be on occupant comfort. Provide a high capacity heat source in the occupied area of the building. Control building pressure. Avoid a negative building by solving stack effect air paths from the entrances. Maintain a 0.10 in. wc building positive pressure. Consider no vestibules in high-traffic passages. ES
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