MEANS TO MINIMIZE STACK EFFECT

Fortunately there are steps that can and should be taken in the design process to minimize the potential problems that will develop through stack effect. The necessary steps must be taken by both the architect and the HVAC design engineer. The steps that can be taken involve minimizing the air leakage into or out of the building. While it is not possible to completely seal any building, through consideration of the normal points at which outside air can and does enter and move vertically through the building, the problem can be mitigated.

The points at which outside air will infiltrate a building include the entry doors to the building as well as doors that open to truck docks, any outside air intake or exhaust louvers that are provided in the building, overhangs in the construction with light fix-tures that are located immediately above the ground level and are not properly sealed against leakage or provided with heat, and any possible small fissures in the exterior wall itself. Internally, the building will allow the passage of air through the fire stairs, elevator shafts, mechanical shafts for ducts and piping, and any other vertical penetra-tions that exist at the edge of the floor slab at the exterior wall or for pipes. All these are candidates for careful review to ensure, to the degree possible, that a tight exterior wall is constructed, closure of all shafts is provided, and the sealing of all penetrations is pro-vided. Vestibules or air locks can be provided for loading docks, with good door seals on the doors to and from the loading dock.

The entry doors for tall buildings in cold climates should always be revolving doors. Doors of this type are important in that they are balanced with equal pressure in opposite directions on the panels on either side of the central pivot, making their opera-tion relatively simple with no special effort being required to have them turn. They also

Chapter 2—Stack Effect | 25

provide closure at all times due to the gasketing that is included in a quality revolving door.

Two-door vestibules with adequate heat will work for the loading dock, assuming the doors are properly spaced to allow them to be operated independently, with one of the two doors to the vestibule always being closed, and sufficient heat is provided in the space between the doors. If properly spaced, the simultaneous opening of both doors on either side of the vestibule can be controlled. However, two-door vestibules have proven inadequate for personnel entry since, with large numbers of people entering the building at various times, both doors will be open simultaneously and significant quan-tities of air can enter the building. In projects where two-door vestibules have been tried in cold climates for tall buildings, there have inevitably been problems. It is strongly recommended that revolving doors be used at all points of personnel entry.

To control possible airflow into the elevator shaft, the inclusion of doors at the entry to the elevator banks should be considered. This creates an elevator vestibule on each floor that will minimize the flow through elevator doors that open on any given floor.

It is possible, and will have beneficial effect, if stairs are interrupted with doors with good seals, to minimize the flow of air vertically through the buildings. This is particularly true of fire stairs that would run the height of the building. These entry doors to fire stairs should be provided with good door and sill gaskets. As is discussed in chapter 9, “Vertical Transportation,” the elevator shafts are a problem because an air opening may be required at the top of the elevator shaft. All shafts, however, can be sealed in their vertical faces to minimize airflow into the shaft that would travel verti-cally in the shaft to the openings at its top.

The last key item is to ensure a tight exterior wall through specification, proper test-ing, and the hiring of a proper contractor to erect the wall.

All of the above discussions involve the architect and the trades for which the archi-tect is responsible. The HVAC designer must include mechanical air-conditioning and ventilating systems that supply more outside air than they exhaust. This is true of all systems where, to ensure pressurization, a full air balance should be used for the entire building with a minimum of five percent more outside air than the combination of spill and exhaust air being provided at all operating conditions. In addition, it is good design, and often required by code for smoke control reasons, to have a separate system for the entrance lobby. Although not always required, this system, if provided, can be designed to operate in extreme winter outside air conditions with 100 percent outside air. Under these circumstances, this air will be used to pressurize the building lobby, which is a point of extreme vulnerability in the overall efforts to minimize the harmful impact of stack effect.

Chapter 3