85
Economize r
The Economizer dampers are used to provide free cooling
and Indoor Air Quality, if optional CO
2
sensor is installed,
when the out side conditions are suitable.
The following conditions must be true for economizer
operation:
S Indoor Fan has been on for at least 30 seconds.
S Enthalpy is Low if the Enthalpy input i s enabled.
S SAT reading is available.
S OAT reading is available.
S SPT reading is available.
S OAT <= High OAT economizer lockout configura tion
(default = 75).
S OAT <= SPT
If any of the mentioned conditions are not true, the
economizer will be set to its configured minimum
position. The minimum damper position can be
overridden by the IAQ routine described later in this
section.
If the above conditions are true, the Economizer Control
Master Loop will calculate a damper position value based
on the following calculation:
Damper Position = minimum position + PID (SPT -- econ
setpoint). Econ setpoint is half way between the effective
cool and heat setpoints. If the SAT drops below the
cooling low supply air setpoint (+ 5_F), the ec onomizer
will ramp down to minimum position.
Power
Exhaust
If RTU --MP is also controlling an exhaust fan, it can be
enabled based on damper position or by occupancy. If
configured for continuous occupied operation, it will be
energized whenever the controller is in the occupied mode
and disabled when in the unoccupi ed mode. If configured
for damper position c ontrol, it will be energized whenever
the economizer exceeds the power exhaust setpoint and
disabled whe n the e conomizer drops below the setpoint by
a fixed hysteresis of 10%.
Heating
The heating outputs are controlled by the Heating Control
PID Loop and Heating Stages Capacity algorithm. They
will be used to calculate the desired number of stages
needed to satisfy the space by comparing the SPT to the
Occupie d Heat Setpoint plus the T56 slider offset when
occupi ed and t he Unoccupi ed Heat Setpoint plus the T56
slider offset if unoccupied. The following conditions must
be true in order for t his algorithm to run:
S Indoor Fan has been ON for at least 30 seconds.
S Cool mode is not active and the time guard between
modes equals ze ro.
S If occupied and SPT <(occupied heat setpoint plus T56
slider offset)
S SPT reading is available
S If it is unoc cupied and the SPT < (unoc cupied heat
setpoint plus T56 slider offset). The indoor fan will be
turned on by the staging algorithm.
S OAT < High OAT lockout tempera ture.
If all of the above conditions are met, the heating outputs
will be energized as required, otherwise the y will be
de --energized. If the SAT begins to exceed the high supply
air setpoint, a ramping function will cause the Heat Stages
Capac ity algorithm to decrease the number of stages until
the SAT has dropped below the setpoint.
There is a fixed one minute minimum on time and a one
minute off time for each heat output. Heat staging has a 3
minute stage up and 30 second stage down delay.
Indoor Air
Quality
If the optional indoor air quality sensor is installed, the
RTU --MP will maintain indoor air quality within the space
at the user configured differential setpoint. The setpoint is
the difference between the indoor air quality and an
optional outdoor air quality sensor. If the outdoor air
quality is not present then a fixed value of 400ppm is
used. The following conditions must be true in order for
this algorithm to run:
S The mode is occupied.
S Indoor Fan has been ON for at least 30 seconds.
S Indoor Air Quality sensor has a valid reading
As air quality within the space changes, the minimum
position of t he economizer damper will be changed t hus
allowing more or less outdoor air into the space depending
on the relationship of the indoor air quality to the
differential setpoint. If all the above conditions are true,
the IAQ algorithm will run and calculates an IAQ
minimum position value using a PID loop. The IAQ
minimum damper position is then compared against the
user configured economizer minimum position and the
greatest value becomes the final minimum damper
position of the economizer output.
If the calculated IAQ minimum position is greater than
the IAQ maximum damper position configuration then it
will be clamped to the configured value.
Demand
Limit
If the RTU--MP receives a level 1 (one degree offset), 2
(two degree offset), or a 3 (4 degree offset) to the BACnet
demand limit variable, the controller will expand the
heat ing and cooling setpoi nts by the configured demand
limit setpoint value and remain in effect until the BACnet
demand limit variable receives a 0 value.
FASTENER TORQUE VALUES
See Tabl e 33 for torque values.
48TC
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