ASHRAE Journal - October 2014 - 58

air pressure attributed to wind gusts from exciting the
control systems, digital filters should be configured to reject
input spikes and/or provide a moving average of the building's differential pressure signal.
In other words, the building pressure control loop
must be very slow relative to the return fan control loop,
and the building pressure control point should not be
the realtime DP reading but instead a moving average
of the DP reading to prevent spikes from causing relief
damper control instability.

Discussion and Conclusions
Airflow Tracking and Direct Pressure Control have
both advantages and disadvantages:
First costs. Airflow tracking control costs are higher
than Direct pressure control if return and/or supply airflow sensors are installed. But if Option a) (sum of zone
airflow) or b) (speed tracking) described earlier are used
for return airflow measurement and Option a) is used
for supply airflow measurement, then Airflow Tracking
costs are lower.

Energy costs. Fan energy is similar for the two control
schemes unless the building has some other intermittent
relief exhaust paths, such as operable windows or doors.
If so, then Direct Pressure Control logic will sense the
lower building pressure and reduce the amount of relief
airflow at the air handler, reducing return fan energy use.
Because Airflow Tracking logic only indirectly controls
building pressure, it will relieve the same amount of air at
the air handler regardless of other relief paths.
Control stability and reliability. Airflow tracking
logic relies on airflow measurements that are seldom
accurate, but fortunately extreme accuracy is not
needed for building pressure control. The control logic
is inherently reasonably stable provided dampers are
properly selected and controlled (which will be the
subject of my next column in November 2014). Direct
pressure control relies on a representative and stable
building pressure signal, which requires care in sensor
port placement and type. And, as noted in Guideline
16, control can be unstable if the building pressure and
return fan discharge pressure control loops are not
properly tuned.
So both Airflow Tracking and Direct Pressure Control
are valid building pressure control schemes for VAV systems with return fans and outdoor air economizers. But
Direct Pressure Control is favored by both Guideline 16
and the master control sequences developed by RP-1455,
and by the author,** because it is a more direct approach
to building pressure control.

References
1. Taylor, S. 2000. "Comparing economizer relief systems."
ASHRAE Journal (9).
2. Kettler, J. 2004. "Return fans or relief fans; how to choose?"
ASHRAE Journal (4).
3. Avery, G. 1995. "Building pressure controls to comply with the
americans with disabilities act and ASHRAE Standard 62-1989."
ASHRAE Transactions 101(2).
4. ASHRAE Standard 62.1-2013, Ventilation for Acceptable Indoor Air
Quality.
5. Dickerhoff, D., J. Stein. 2007. "Stability and Accuracy of VAV
Terminal Units at Low Flow." Pacific Gas & Electric Co.
6. Lui R., et al. 2012. "ASHRAE 1353-RP, Stability and accuracy of
VAV box control at low flows." ASHRAE Transactions.
7. Guideline 16-2003, Selecting Outdoor, Return, and Relief Dampers
for Air-Side Economizer Systems."
8. Hydeman et al, Final Report ASHRAE RP-1455 Advanced Control Sequences for HVAC Systems, Phase I, Jan. 14, 2014
9. Taylor, S. 2007. "Increasing efficiency with VAV system static
pressure reset." ASHRAE Journal (6).
www.info.hotims.com/49809-5
58

ASHRAE JOURNAL

ashrae.org

O CT O B E R 2 0 1 4

**Full disclosure: The author was involved in the development of both Guideline 16 and RP-1455.



ASHRAE Journal - October 2014

Table of Contents for the Digital Edition of ASHRAE Journal - October 2014

Contents
ASHRAE Journal - October 2014 - Cover1
ASHRAE Journal - October 2014 - Cover2
ASHRAE Journal - October 2014 - 1
ASHRAE Journal - October 2014 - 2
ASHRAE Journal - October 2014 - Contents
ASHRAE Journal - October 2014 - 4
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ASHRAE Journal - October 2014 - HR1
ASHRAE Journal - October 2014 - HR2
ASHRAE Journal - October 2014 - HR3
ASHRAE Journal - October 2014 - HR4
ASHRAE Journal - October 2014 - HR5
ASHRAE Journal - October 2014 - HR6
ASHRAE Journal - October 2014 - HR7
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ASHRAE Journal - October 2014 - HR11
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ASHRAE Journal - October 2014 - HR13
ASHRAE Journal - October 2014 - HR14
ASHRAE Journal - October 2014 - HR15
ASHRAE Journal - October 2014 - HR16
ASHRAE Journal - October 2014 - HR17
ASHRAE Journal - October 2014 - HR18
ASHRAE Journal - October 2014 - HR19
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ASHRAE Journal - October 2014 - HR22
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ASHRAE Journal - October 2014 - HR24
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ASHRAE Journal - October 2014 - HR27
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ASHRAE Journal - October 2014 - Cover3
ASHRAE Journal - October 2014 - Cover4
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