Basic Documentation

Laboratories
In a VAV laboratory the Room Controller must modulate the Room General Exhaust to
ensure that there is sufficient total room exhaust to meet the required minimum room
ventilation rate (ACH). The ROOM CONTROLLER must also modulate the Room General
Exhaust to ensure that there is sufficient total room exhaust for the amount of supply airflow
necessary to maintain the desired room temperature.
To maintain the required room static pressure, the room controller modulates the total room
supply airflow to maintain the desired airflow tracking offset between the total room supply
and total room exhaust airflows. Whenever the room's total exhaust airflow changes, the
room controller adjusts the room supply airflow to maintain the airflow tracking offset.
With a VAV ventilation system there may be times when the room will require more supply
airflow to cool the room in order to maintain the desired room temperature. In such instances,
the room controller increases the amount of supply airflow to maintain the desired room
temperature. In addition, the room controller also increases the room general exhaust so that
a corresponding amount of room air is exhausted to maintain the airflow tracking offset.
Summarized, the many functions of the room controller include:
• Maintaining the proper amount of room general exhaust to ensure that the required
minimum room ventilation rate
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is always maintained.
• Maintaining the proper amount of room supply airflow needed to maintain a constant
airflow tracking offset between the total room exhaust and room supply airflows.
• Increasing the supply airflow when needed to maintain the proper room temperature.
Simultaneously increasing the room general exhaust to maintain the required
constant airflow tracking offset.
Airflow Tracking Control Considerations
As stated previously, the specific airflow tracking offset necessary to maintain a certain room
static pressure value is dependent upon the room's total leakage area. However, it is not
possible to know what a particular room’s total leakage area will be before construction is
completed. When specifying the room control scenario, a ventilation system designer must
ensure that the proper room airflow tracking offset will be maintained. To address this need,
two options are suggested:
1. A specific room airflow tracking offset may be estimated by the designer based on past
experience or as a practical maximum limit. This results in a final room differential
pressure that may or may not be within the desired pressure range
2. A specific room static pressure value may be specified. This requires some trial and error
effort during the test and balancing process to determine the necessary airflow tracking
offset. This means that the room control system must be set up to maintain the required
airflow tracking offset.
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In negatively pressurized rooms the ACH rate is determined by the total room exhaust airflow. In positively pressurized rooms
the ACH rate is determined by total room supply airflow.
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