ASHRAE Journal - October 2014 - 56

RF DP Setpoint

Damper Position, Percent Open

close, causing control instabilities.
FIGURE 2 Direct pressure control (adapted from ASHRAE Guideline 16).
A better location is in an interior
P-2
space physically separated by walls
Return
from exterior doors and elevator
Fan
M
lobbies. For high rise buildings,
Return/Relief
Relief Air
Return Air
Air Plenum
multiple sensors can be installed
PL-1
Relief Damper
Speed
with the highest reading used for
D-1
Control
* The inside port of the DP
P-3
control.
[B]
[A]
sensor could simply be an open tube
Building
Atmosphere
MIN T-2
T-1
popped through a wall or ceiling,
(Outdoors)
Return Air
Supply
V-2
M Damper D-2
but since its purpose is not obviFan
M2
M
Mixed Air
Airflow
Supply Air
ous, it is not uncommon for these
Outdoor Air
Plenum
Sensor
Economizer OA PL-2
terminations to be shoved back into
Filter Cooling
Speed
Damper D-3
Coil
the wall or ceiling or spackled over
Control
P-1
by painters. So the port should be
connected to a plate pressure pickup
mounted in either a "permanent"
ceiling or wall. "Permanent" in this case means not likely
FIGURE 3 Relief damper control and return fan setpoint reset.
to be removed when tenant improvement upgrades are
RFSPmax
made in the future.
100
The ambient pressure port should be connected to
a pickup designed to stabilize readings by dampening
pressure fluctuations caused by wind gusts. These pickups, available from several manufacturers, can be fitted
Relief Air Damper
with a pneumatic signal dampener to further reduce
RF DP
Setpoint
spikes.
Return fan speed is controlled to maintain a posi0
tive pressure in the return/relief plenum to ensure no
Building Pressure Control Loop Output Signal
RFSPmin
backflow of outdoor air through the relief damper. To
minimize energy use, the plenum pressure setpoint
whatever the maximum is when in economizer mode)
should be reset as shown in Figure 3. The x-axis is the
and supplying 100% outdoor air.
output of the building pressure control loop that is
One potential disadvantage of Direct Pressure Control
maintaining building pressure at setpoint. The loop
logic is control stability. A footnote in Guideline 16 says:
output is mapped to first open the relief dampers
Due to the potential for interaction between the building
then to raise the return/relief plenum pressure setpressurization and return fan control loops, extra care
point from RFSPmin to RFSPmax. RFSPmin is the pressure required to deliver the design return air volume
must be taken in selecting the parameters that determine
across the return air damper when the supply air fan
the dynamic response of the digital filters and feedback
is at design airflow and supplying minimum outdoor
controllers. To prevent excessive control loop interaction,
air, but no less than 0.01 in. w.c. (±2.5 Pa). (It must be
the closed loop response time of the building pressurizapositive so outdoor air is not drawn backwards through
tion loop should not exceed one-fifth of the closed loop
the relief damper.) RFSPmax is the pressure required
response time of the return fan control loop. This can be
to relieve enough air to maintain building pressure at
accomplished by decreasing the gain of the building pressetpoint when the supply air fan is at design airflow (or
surization controller. To prevent fluctuations in outdoor
*For high rise buildings in very cold or very warm climates, controlling building pressure for many floors with one sensor and airflow control point may not be possible due to stack effect. DP sensing
and control on every floor or a range of a few floors, typically using modulating return air smoke dampers, will ensure more uniform building pressure. Relief dampers in this case should be controlled to
maintain return air shaft pressure with the setpoint reset based on return air smoke damper position, similar to DP setpoint reset for supply air fans.9
56

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