ASHRAE Journal - November 2014 - 50

COLUMN ENGINEER'S NOTEBOOK
Steven T. Taylor

Select & Control Economizer
Dampers in VAV Systems
BY STEVEN T. TAYLOR, P.E., FELLOW ASHRAE

Last month's column1 discussed how to control return fans in VAV systems. This
month, we will discuss a closely related subject: how to select and control economizer
dampers in VAV systems.
Damper type, sizing, and control strategies vary
depending on the type of building pressure control system. The three common types are:
1. Relief Dampers (aka non-powered or barometric
relief) as shown in Figure 1;*,2
2. Relief Fans (aka powered exhaust fans) as shown in
Figure 2; and
3. Return Fans (aka return/relief fans) as shown with
Airflow Tracking in Figure 3A and with Direct Pressure
Control in Figure 3B.
Figures 1 to 3B also show three common options to control minimum ventilation outdoor air. The advantages
and disadvantages of these options, and others, will be
the subject of a future Engineer's Notebook column.

Control Logic
Conventional economizer control logic calls for the
return air damper to close as the outdoor air damper
opens. For systems with return fans, conventional logic
calls for the relief air damper to track the outdoor air
damper. This is shown in Figure 4. But this logic is not
optimum for multiple-zone VAV system applications.
For VAV systems using Relief Dampers (Figure 1) or
Relief Fans (Figure 2), pressure drop and supply fan
energy can be reduced by sequencing the outdoor air
and return air dampers, as shown in Figure 5. The pressure drop savings is readily apparent at the 50% outdoor
air signal: the two dampers are both wide open rather
than both half closed. This substantially reduces pressure drop through the mixing plenum, particularly
if the dampers have opposed blade action (discussed
*The figure shows direct pressure control with motorized relief dampers. Gravity type barometric
dampers are also an option. See Reference 2 for a comparison of these two options.
50

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further below). The damper sequencing shown in Figure
5 can be achieved by using separate control system
outputs to the two dampers (which may also be needed
for minimum outdoor air control, to be discussed in a
future column) or by sequencing the active range of the
actuators, e.g., 0 to 5 VDC for one and 5 to 10 VDC for the
other.
Conventional logic is also not optimum for systems
with Return Fans (Figure 3A and Figure 3B). Not only does
it increase pressure drop and supply fan energy, it can
cause reverse outdoor airflow through the relief damper
when Airflow Tracking control is used.3 This can result
in excess outdoor air intake, depending on minimum
ventilation control logic, and also may cause contaminated air to be drawn in through the relief outlet since
its location may not meet the code and Standard 62.1
separation distances to pollutant sources required of
outdoor air intakes.
A better economizer control sequence for systems with
Return Fans and Airflow Tracking control is shown in
Figure 6 and summarized as follows:
* Simply open the outdoor air damper fully. This is
true even when the economizer is disabled, i.e., open
the outdoor air damper whenever the air handler is on
in occupied mode. (It is closed when the air handler is
off or when operating in non-occupancy modes such as
night setback, warm-up, etc.)
* Modulate the return air damper off the supply air
temperature loop in sequence with the mechanical cooling per standard economizer control logic.
Steven T. Taylor, P.E., is a principal of Taylor Engineering in Alameda, Calif. He is a member of SSPC 90.1 and chair of TC 4.3, Ventilation Requirements and Infiltration.
ASHRAE Journal - November 2014

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