Return Fan Control For Vav Systems; Open Loop Control; Direct Building Static Control - Honeywell AUTOMATIC CONTROL Engineering Manual

RETURN FAN CONTROL FOR VAV SYSTEMS

Return fan operation influences building (space)
pressurization and minimum outdoor air. There are four
techniques to control the return fan: open loop, direct building
static, airflow tracking, and duct static.

Open Loop Control

Open loop control (Fig. 27) modulates the return fan without
any feedback. This type of control presumes a fixed relationship
between the supply and return fans, controls the return fan in
tandem with the supply fan, and changes the output of the return
fan without measuring the result. Open loop control requires
similar supply and return fan operating characteristics.
Therefore, a careful analysis of the supply and return fan
operating curves should be done before selecting this technique.
Also, accurate balancing is essential to ensure proper adjustment
at maximum and minimum operating points. Mechanical
linkage adjustments or other means are used to adjust the
differential between the two fans for desired flows and to
minimize tracking errors at other operating points. With digital
control, software can be used to align the fan loading
relationships, to vary exhaust effects, and to offset dirty filter
effects to minimize flow mismatches.
RETURN
AIR TERMINAL
FAN
UNITS
SUPPLY
FAN
CONTROLLER
Fig. 27. Open Loop Control.
Open loop control is often acceptable on small systems more
tolerant of minimum outdoor air and building pressurization
variations. Since open loop control does not sense or control
return airflow, changes caused by the return side dampering
and exhaust fan exfiltration change the minimum airflow and
building pressurization. Systems with low airflow turndowns
also are more suitable for open loop control. As a rule,
turndowns should not exceed 50 percent.
In a control sequence where the outdoor air damper is closed
(e.g., night cycle or morning warm-up), open loop control
should not be used. Excessive negative pressurization will occur
in the duct between the return and supply fans.
DUCT STATIC
PRESSURE SENSOR
STATIC PRESSURE
C2622
REFERENCE
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS

Direct Building Static Control

In direct building static control, the return fan responds
directly to the building space static pressure referenced to the
static pressure outside of the building (Fig. 28). The location
of the building space static pressure sensor should be away
from doors opening to the outside, elevator lobbies, and some
confined areas. Usually a hallway on an upper floor is suitable.
The outdoor static pressure sensor must avoid wind effects and
be at least 15 feet above the building to avoid static pressure
effects caused by wind. Due to stack effect, this technique
should not be used in tall buildings with open spaces or atriums,
unless the building has been partitioned vertically. This
technique should use proportional plus integral control with a
wide throttling range for stable, accurate, and responsive
operation.
RETURN
FAN
CONTROLLER
Fig. 28. Direct Building Static Control.
Since building static is controlled directly, the pressure
remains constant even when exhaust fan airflow changes.
Minimum outdoor airflow varies with changes in exhaust fan
airflow and building infiltration/exfiltration. In a control
sequence where the outdoor air damper is closed, the building
static must be reset to zero and all exhaust fans should be turned
off.
Airflow Tracking Control
In airflow tracking (Fig. 29) control, the return fan airflow is
reset based on the relationship between supply and exhaust fan
airflows. That relationship is usually a fixed difference between
the supply total airflow and return plus exhaust total airflow,
but it can also be a percentage of supply total airflow. When
duct layout prevents measuring of total airflow from one flow
station, measurements in multiple ducts are totaled. This
technique is usually higher cost, especially when multiple flow
stations are required. Airflow tracking control and direct
building static control are preferable to open loop control.
Proportional plus integral control is necessary for accurate
operation.
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ENGINEERING MANUAL OF AUTOMATION CONTROL
BUILDING SPACE
STATIC PRESSURE
OUTDOOR STATIC
PRESSURE
C2623