Return Damper Control For Vav Systems - Honeywell AUTOMATIC CONTROL Engineering Manual

RETURN DAMPER CONTROL FOR VAV SYSTEMS

In systems having a return fan, when the mixed air control
cycle is not operating, the outdoor air (or maximum outdoor
air) and relief dampers are closed and the return damper remains
fully open. When the mixed air control cycle is operating, the
return damper modulates closed and outdoor air (or maximum
outdoor air) and relief dampers modulate open. The return
damper should be sized for twice the pressure drop of the
outdoor air (or maximum outdoor air) and relief dampers. This
prevents the possibility of drawing outdoor air through the relief
damper (Fig. 34).
RELIEF
RA
DAMPER
OUTDOOR
AIR
OA
DAMPER
Fig. 34. Return Air Damper Mixed Air Control Cycle.
In systems not having a return air fan, the return damper
controls minimum outdoor airflow or building pressurization.
Airflow tracking or direct building static pressure control can
be used to control the return damper, depending on which
parameter is most important. See Figure 35 for airflow tracking
and Figure 36 for direct building static pressure control.
OA
AIRFLOW
CONTROLLER
A. FLOW MEASURING STATION IN OUTDOOR AIR DUCT
FLOW
MEASURING
STATION
OA
B. FLOW MEASURING STATIONS IN SUPPLY AND RETURN DUCTS
Fig. 35. Return Damper Control Using Airflow Tracking.
RETURN
FAN
RETURN
AIR
DAMPER
SUPPLY
FAN
MIXED AIR
TEMPERATURE
CONTROLLER
C2629
RETURN
AIR
DAMPER
SUPPLY
FLOW
FAN
MEASURING
STATION
AIRFLOW
CONTROLLER
C2630
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
OA
STATIC PRESSURE
CONTROLLER
Fig. 36. Return Damper Control Using
Direct Building Static Pressure.
If a mixed air control cycle is required, a relief fan may be
required. In airflow tracking (Fig. 37), the mixed air controller
opens the outdoor air damper above minimum and the relief
fan tracks the outdoor air. In direct building static pressure
control (Fig. 38) the mixed air controller opens the outdoor air
and relief dampers and closes the return damper, causing the
relief fan to eliminate excessive pressure.
RELIEF
FAN
RA
AIRFLOW
CONTROLLERS
SELECTOR
OA
Fig. 37. Mixed Air Control Cycle with Relief
Fan Control Using Airflow Tracking.
BUILDING OUTDOOR
SPACE STATIC
STATIC PRESSURE
PRESSURE
RA
PRESSURE
CONTROLLERS
SELECTOR
Fig. 38. Mixed Air Control Cycle with Relief Fan Control
Using Direct Building Static Pressure.
285
ENGINEERING MANUAL OF AUTOMATION CONTROL
SUPPLY
FAN
BUILDING SPACE
STATIC PRESSURE
OUTDOOR STATIC
PRESSURE
C2631
FLOW
MEASURING
STATION
SUPPLY
FAN
FLOW
MEASURING
STATION
MIXED AIR
TEMPERATURE
CONTROLLER
C2632
RELIEF
FAN
STATIC
SUPPLY
FAN
OA
MIXED AIR
TEMPERATURE
CONTROLLER
C2633