Airflow Tracking Control; Duct Static Control; Relief Fan Control For Vav Systems - Honeywell AUTOMATIC CONTROL Engineering Manual

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
RETURN
FAN
CONTROLLER
SUPPLY
FAN
Fig. 29. Airflow Tracking Control for Return Fan.
Minimum outdoor airflow should be maintained at a constant
level, independent of exhaust fan airflow and changes in
building infiltration/exfiltration. Building space pressurization
varies only when building infiltration/exfiltration changes.
Exhaust fan airflow must reset the return fan airflow for building
pressurization to be independent of exhaust fans. In a control
sequence where the outdoor air damper is closed, the differential
between supply airflow and return airflow must be reset to zero
and all exhaust fans should be turned off.
Refer to the Air Handling System Control Applications
section for an example of a VAV AHU WITH RETURN FAN
AND FLOW TRACKING CONTROL.

Duct Static Control

Duct static control is similar to supply fan duct static high-
limit control, except return duct static pressure is negative. If
individual space returns are damper controlled, return fan
control must use this technique (Fig. 30). Duct static control is
relatively simple, but individual space return controls make the
entire system complex.
RETURN
FAN
CONTROLLER
Fig. 30. Duct Static Control.
Minimum building outdoor air is the difference between
supply total airflow and return total plus the exhaust fan total
airflow. If controlling the space returns from airflow tracking
(using dampers), the exhaust fan volume must be included in
ENGINEERING MANUAL OF AUTOMATION CONTROL
FLOW
MEASURING
STATIONS
C2624
DUCT STATIC
PRESSURE SENSOR
STATIC PRESSURE
REFERENCE
C2625
284
the tracking control system for constant building and space
pressurization. If controlling the space returns by space
pressurization, building and space pressurization remain
constant regardless of exhaust fan operation.

RELIEF FAN CONTROL FOR VAV SYSTEMS

Relief fans are exhaust fans for the central air handling
system. They relieve excessive building pressurization and
provide return air removal for economizer cycles.
In Figure 31, a relief damper is located after the relief fan
and is controlled to open fully whenever the relief fan operates.
Direct building static pressure or airflow tracking controls the
relief fan. In direct building static pressure control (Fig. 32),
the same guidelines apply as for return fan control. During
minimum ventilation cycles and when the outdoor air damper
is closed, the relief fan should be turned off and the relief damper
closed. In airflow tracking (Fig. 33), flow measuring stations
should be located in relief and outdoor air ducts, not in supply
and return ducts as with return fans.
RELIEF
FAN
RA
RELIEF
DAMPER
OA
Fig. 31. Relief Damper Location.
RELIEF
FAN
STATIC PRESSURE
CONTROLLER
Fig. 32. Direct Building Static Control for Relief Fan.
RELIEF
FAN
RA
AIRFLOW
CONTROLLER
OA
Fig. 33. Airflow Tracking Control for Relief Fan.
EXHAUST
FAN
BUILDING
SUPPLY
SPACE
FAN
C2626
BUILDING SPACE
STATIC PRESSURE
OUTDOOR STATIC
PRESSURE
C2627
FLOW
MEASURING
STATIONS
C2628