Basic Documentation

Room Pressurization Control Application Guide
Option 1, designing the ventilation system to maintain a specific room differential airflow, is
generally recommended for applications where a lower room differential pressure (perhaps
0.01 to 0.02 inches w.c.) is sufficient. Recall that the primary purpose of room pressurization
is to create the proper directional airflow to prevent or retard undesirable transfer of air.
Therefore, it is the airflow direction rather than an arbitrary static pressure
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that should be the
design goal of a laboratory ventilation system. This approach enables the room airflow
tracking control scenario to be set up without a lengthy trial and error process.
Option 2, choosing a specific room static pressure, is usually only necessary for applications
where higher level room static pressures are required and where cross contamination must
be strictly prevented. This presents a more difficult challenge for the testing and balancing
process, and is not recommended for most laboratory applications. In actual practice it
typically requires simultaneous adjusting the airflow in many rooms to achieve the correct
airflow balance. This is particularly the case when several rooms adjoin a common corridor.
Furthermore, it may not be possible to achieve a given static pressure relationship if a room’s
leakage area is too large or the adjacent space does not have sufficient excess supply
makeup air (such as a corridor).
For example, consider using Option 2 in a situation where 10 laboratory rooms adjoin a
common corridor. Also assume that the balancing process finds that an average airflow
tracking offset of 400 cfm is necessary to maintain a specific negative room pressurization
value. The corridor will, therefore, need to have a supply makeup airflow of 10 rooms × 400
cfm (or 4000 cfm) to attain the required room static pressure. However, the total amount of
room transfer air required (in this case 4,000 cfm) would not be known during the design
process, and might exceed the actual amount of corridor supply air that is actually available.
In contrast, following Option 1 would enable an airflow tracking offset to be initially chosen
and specified for each room. Then the total amount of excess corridor air would be known.
Even when a laboratory room door is fully opened and the doorway area increases the total
leakage area of a room, no change in the amount of room transfer airflow occurs. Although
the room static pressure will diminish, the desired airflow direction (into the negatively
pressurized room) will be maintained.
Finally, a specific airflow tracking value cannot ensure that a constant positive or negative
static pressure value will be maintained over the life of a building. As seasons pass and
building conditions change over time, the actual room static pressure level will also
undoubtedly vary somewhat from what it was initially. This necessitates periodically checking
(at least annually) and, perhaps, readjusting the differential tracking value as needed.
Although there will likely be some variation in the specific static pressure level when using
airflow tracking, it should again be noted that the primary goal of maintaining a room at a
negative (or positive) static pressure is to ensure that an undesirable transfer of air does not
occur. Airflow tracking will meet this goal by maintaining the proper directional airflow.
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Differential pressure should be used to ascertain if the airflow is in the right direction (into the room) and as a convenient
reference for periodic testing to ensure that the amount of differential airflow is consistent over time.
20 Siemens Building Technologies, Inc.