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FEATURES
1. A motorized zone mixing damper and space temperature
PID control loop for each zone provides zone control.
2. A single coil or group of coils in the hot deck furnishes
heat for the entire system.
3. The hot deck temperature is reset by the zone with greatest
load which provides efficient control when used in
conjunction with a cold deck coil/control.
4. A reasonably constant volume of air is delivered to each
room or zone.
5. A dynamic sequence-of-operation graphic display. This
not only clearly explains the sequence but also allows
easy program modification.
NOTE: Except for low-leakage OA dampers, measurable
leakage usually exists through "closed" dampers.
Given this leakage, as the zone heat demand increases
from zero to 15%, the hot deck temperature setpoint
is raised from the mixed air temperature to 27 C before
any zone damper movement. Zone heating demand
from 15 to 50% then modulates the zone dampers from
zero to 100% open to the 27 C hot deck. Further zone
heating demand from 50 to 100% raises the hot deck
temperature setpoint from 27 C to the upper limit of
41 C. A major objective of this strategy is to minimize
heat leakage to zones requiring no heat.
CONDITIONS FOR SUCCESSFUL OPERATION
1. A hot water coil on units having full flow at all times
provides uniform hot deck temperatures. This can be
accomplished with a three-way valve and coil pump.
Resetting hot water temperature also helps.
2. All zones are connected to load the reset program to
satisfy total load requirements. In large systems good
practice dictates connecting only selected diverse zone
loads. Zones that may be allowed to go out of control
(storage rooms, etc.) should not be connected to the load
reset program.
3. Each zone duct should have a balancing damper following
the mixing dampers to ensure design volume to each zone.
NOTE: See the Microprocessor-Based/DDC
Fundamentals section for a description of the
load reset program.
LIMITATIONS
1. If only selected zones are connected to the load reset
program, the load requirements of unconnected zones
may not be satisfied.
2. Reduced steam flow may cause temperature gradients
across the face of the coil.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
Anytime the supply fan runs, heating control shall be enabled.
Each zone mixing damper shall be modulated to maintain
zone space temperature setpoint. The hot water valve shall be
modulated to maintain the hot deck temperature setpoint.
The hot deck temperature setpoint shall be reset from the
MA temperature to 27 C as the heating demand from the zone
with the greatest heating demand varies from 0 to 15%.
Zone mixing dampers shall modulate from 0 to 100% open
to the hot deck as their respective zone demands for heating
vary from 15 to 55%.
The hot deck temperature setpoint shall be reset from 27 C
to 41 C as the heating demand from the zone with the greatest
heating demand varies from 55 to 100%.
PSYCHROMETRIC ASPECTS
In the following chart it is assumed that:
1. MA temperature is 13 C DB.
2. The zone with the greatest heating load requires a hot
deck temperature of 32 C DB.
3. The zone depicted requires 24 C DB SA to meet its load.
MA
13 C DB
Item
No.
Explanation
1
Heating of MA to the hot deck temperature
occurs along a line of constant moisture
content. Individual zones obtain any needed SA
temperature along this line with the coldest
zone controlling the hot deck temperature and
getting 100 percent hot deck air.
227
1
HOT DECK
32 C DB
24 C DB
SA FROM MIXING
DAMPER TO ZONE
C3241
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