SECTION 230993 - SEQUENCE OF OPERATIONS FOR HVAC CONTROLS
PART 1 - GENERAL
1.1
RELATED DOCUMENTS
A.
1.2
Drawings and general provisions of the Contract, including General and Supplementary
Conditions and Division 01 Specification Sections, apply to this Section.
SUMMARY
A.
This Section includes control sequences for HVAC systems, subsystems, and equipment.
B.
Related Sections include the following:
1.
1.3
Section 230900 "Instrumentation and Control for HVAC" for control equipment and
devices and for submittal requirements.
DEFINITIONS
A.
VAV: Variable air volume.
B.
RTU: Roof Top Unit
1.4
TERMINAL UNIT OPERATING SEQUENCE
A.
Cabinet Unit Heater, Electric: Room thermostat cycles fan and sequences stages of heating.
B.
Variable Air Volume/Constant Volume Boxes:
1.
An electronic temperature element shall monitor space temperature and control the box as
follows. The primary supply air damper is controlled within user defined separate
heating and cooling maximum and minimum supply air volume settings.
2.
During full cooling the primary air damper will be full open. As the cooling load
decreases, the primary air damper shall throttle back until it reaches its minimum
position. A call for cooling reverses the sequence.
3.
Upon a further drop in the space temperature, the electronic temperature sensor through
the box controller will energize the electric resistance heater in stages. During morning
warm-up and daytime warm-up modes, the electric heating coil shall be locked out.
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1.5
VENTILATION SEQUENCES
A.
1.6
Exhaust Fan: Space temperature sensor cycles fan. Fan to be interlocked with respective
motorized dampers at intake louvers.
PACKAGED OUTDOOR ROOFTOP UNITS (variable air volume)
A.
RTU-1: This unit is variable air volume, DX cooling, electric heating, with variable outside air
for normal occupied mode and allow for modulating outside air for economizer (free cooling)
mode. These packaged units consist of outside air dampers, filters, electric resistance heat
exchangers, DX evaporative cooling coils, refrigerant compressors, condenser section and
supply fan with a variable frequency drive.
B.
Start/Stop: The supply fan operation shall generally run continuously. When the fan is running,
the OA dampers shall be open to the minimum position (manually set). Whenever the fan is
off, all dampers shall assume their failed positions.
C.
Fan Speed: Variable frequency drives provide supply fan motor speed modulation. The drive
will accelerate or decelerate as required to maintain the supply static pressure setpoint. When
subjected to high ambient return conditions the VFD shall reduce its output frequency to
maintain operation. Bypass control is provided to maintain full nominal airflow in the event of
drive failure.
D.
Outside Air Control: As the return air CO2 concentration increases above the CO2 setpoint, the
Ventilation Control Module shall modify the minimum outdoor air CFM setpoint to increase the
amount of fresh air entering the unit. The setpoint shall be adjusted upward until the CO2
Maximum Reset value is reached. The maximum effective (reset) setpoint value for fresh air
entering the unit is limited to the maximum rated airflow for the unit. As the CO2 concentration
decreases, the effective (reset) setpoint value shall be adjusted downward toward the minimum
outdoor air CFM setpoint. Locate sensor in return duct accessible by maintenance personnel .
E.
Supply Air Pressure Control: A pressure transducer measures duct static pressure and the VFD
shall modulate to maintain the supply air static pressure within an adjustable user-defined range.
F.
Supply Air Temperature: During occupied cooling mode of operation, the economizer and
primary cooling are used to control the supply air temperature. The supply air temperature
setpoint is user-defined at the unit mounted VAV setpoint potentiometer. If the enthalpy of the
outside air is appropriate to use “free cooling”, the economizer will be used first to attempt to
satisfy the supply setpoint. A call for cooling will modulate the fresh air dampers open. The
rate of economizer modulation is based on deviation of the discharge temperature from setpoint,
the further away from setpoint, the faster the fresh air damper will open. The economizer is
only allowed to function freely if ambient conditions are below the enthalpy control setting or
below the return air enthalpy. If outside air is not suitable for economizing the fresh air
dampers shall drive to the minimum open position. A field adjustable potentiometer shall
provide the input to establish the minimum damper position. At outdoor air conditions above
the enthalpy control setting, primary cooling only is used and the fresh air dampers shall remain
at minimum position.
1.
Supply air setpoint reset shall be based on zone temperature. Zone reset is applied to the
zone in a building that tends to overcool or overheat. The supply air temperature setpoint
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is adjusted based on the temperature of the critical zone(s). These are user adjusted
parameters. Logic for zone reset control is provided by the zone sensor. The amount of
reset applied is dependent upon how far the zone is below the supply air reset setpoint.
The amount is zero where they are equal and increases linearly toward the value set at the
reset amount potentiometer.
G.
1.7
2.
Unoccupied zone heating and cooling, during this mode, the unit is operated as a constant
volume unit. The VAV boxes are driven full open and the unit shall control the zone
temperature to the unoccupied zone cooling and heating setpoints.
3.
During occupied daytime warm-up mode, if the zone temperature falls to a temperature
three degrees below the morning warm-up setpoint, daytime warm-up shall be initiated.
The system shall change to constant volume heating (full air flow) the heating algorithm
is in control until the morning warm-up setpoint is reached. The unit is then returned to
VAV cooling mode.
4.
The morning warm-up (MWU) control shall be activated whenever the unit switches
from unoccupied to occupied and the zone temperature is at least 1.5 degrees below the
MWU setpoint. When MWU is activated the VAV box output will be energized for at
least 6 minutes to drive all boxes open, then full heat is energized. When MWU is
activated the economizer damper is driven fully closed. When the zone temperature
meets or exceeds the MWU setpoint minus 1.5 degrees, the heat will be turned down.
When the zone temperature meets or exceeds the MWU set point then MWU will be
terminated and the unit will switch over to VAV cooling.
Safeties/Alarms:
1.
Low temperature shutdown: A unit supplied low temperature detection thermostat,
manual reset, located downstream of the heat exchanger, will be hardwired to the fan
circuit. Upon detection of a low temperature condition below 35F (adj.) the supply fan
will shut down, the outside air dampers will close and a low temperature alarm will be
activated.
2.
Loss of Air Flow: When the pressure differential across the supply fan indicates a no air
flow condition and the fan is commanded "ON" a fan failure alarm will be indicated at
the remote control panel.
3.
Fire/Smoke Operation: When the duct mounted smoke detectors alarm, they will send a
signal to the local fire alarm panel, which will signal an “alarm condition”, the signal
from the fire alarm panel will shut down the supply air fan.
PACKAGED OUTDOOR ROOFTOP UNITS (constant volume)
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A.
RTU-2: This unit is a constant volume, DX cooling unit with electric resistance heating, CO2
based modulating minimum outside air for normal occupied mode and separate modulating
outside air for economizer (free cooling) mode. The packaged unit consists of outside air
dampers, filters, natural gas-fired heat exchanger, DX evaporative coil, refrigerant
compressors, condenser section and supply fan.
B.
Start/Stop: The supply fan operation shall generally run continuously, but can be controlled
from the building automation system through the space sensor. When the fan is running, the
OA damper shall be open to the minimum position (manually set). Whenever the fan is off,
all dampers shall assume their failed positions.
C.
Fan Speed: The supply fan speed shall be constant to maintain the supply air flow.
D.
Outside Air Control: As the return air CO2 concentration increases above the CO2 setpoint,
the Ventilation Control Module shall modify the minimum outdoor air CFM setpoint to
increase the amount of fresh air entering the unit. The setpoint shall be adjusted upward until
the CO2 Maximum Reset value is reached. The maximum effective (reset) setpoint value for
fresh air entering the unit is limited to the maximum rated airflow for the unit. As the CO2
concentration decreases, the effective (reset) setpoint value shall be adjusted downward
toward the minimum outdoor air CFM setpoint. Locate sensor in return duct accessible by
maintenance personnel .
E.
Supply Air Temperature: A space temperature sensor shall monitor the room air temperature
and send an input signal to the unitary controls. The output signal from the unitary controls
shall modulate the refrigeration compressors in the cooling mode and the heat exchanger gas
valve in the heating mode to maintain the room temperature set-point (adjustable).
F.
Economizer: The enthalpy controlled economizer shall modulate the economizer outside air
damper, return air damper and relief air damper along with the DX system refrigerant
compressors in sequence to maintain the desired discharge air temperature (adjustable). The
outside, return, and relief air dampers shall modulate to maintain the preset mixed air
temperature as sensed by the mixed air temperature sensor. Mixed air temperature set point
shall be reset up as outside air temperature goes down.
1. The outside air dampers are split into two sections. One section is used to satisfy the
minimum O.A. requirements and shall be opened any time the fan is running. The other
outside air intake damper section shall be used as a modulating damper for economizer
control.
G.
Safeties/Alarms:
1. Low temperature shutdown: A unit supplied low temperature detection thermostat, manual
reset, located downstream of the heat exchanger, will be hardwired to the fan circuit. Upon
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detection of a low temperature condition below 35F (adj.) the supply fan will shut down, the
outside air dampers will close and a low temperature alarm will be activated.
2. Loss of Air Flow: When the pressure differential across the supply fan indicates a no air flow
condition and the fan is commanded "ON" a fan failure alarm will be indicated at the remote
control panel.
3. Fire/Smoke Operation: When the duct mounted smoke detectors alarm, they will send a
signal to the local fire alarm panel, which will signal an “alarm condition”, the signal from
the fire alarm panel will shut down the supply air fan.
H. Off Mode
1. Occurs in the unoccupied mode.
2. Supply fan is off and the outside air damper is closed.
1.8
SPLIT SYSTEMS WITH AIR HANDLING UNITS (constant volume)
A.
AHU-1 : This unit is constant volume, DX cooling unit with electric heating,. The split system
consists of outside air dampers, filters, electric heating coil, DX evaporative coil, refrigerant
compressors, condenser section and supply fan.
B.
Start/Stop: The supply fan operation shall generally run continuously, but can be controlled
from the building automation system through the space sensor. When the fan is running, the
OA damper shall be open to the minimum position (controlled via CO2 sensort). Whenever the
fan is off, all dampers shall assume their failed positions.
C.
Fan Speed: The supply fan speed shall be constant to maintain the supply air flow.
D.
Supply Air Temperature: A space temperature sensor shall monitor the room air temperature
and send an input signal to the unitary controls. The output signal from the unitary controls
shall modulate the refrigeration compressors in the cooling mode and stage the electric heating
coil in the heating mode to maintain the room temperature set-point (adjustable).
E.
As the return air CO2 concentration increases above the CO2 setpoint, the Ventilation Control
Module shall modify the minimum outdoor air CFM setpoint to increase the amount of fresh air
entering the unit. The setpoint shall be adjusted upward until the CO2 Maximum Reset value is
reached. The maximum effective (reset) setpoint value for fresh air entering the unit is limited
to the maximum rated airflow for the unit. As the CO2 concentration decreases, the effective
(reset) setpoint value shall be adjusted downward toward the minimum outdoor air CFM setpoint. Locate sensor in return duct accessible by maintenance personnel .
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F.
G.
1.9
Safeties/Alarms:
1.
Low temperature shutdown: A unit supplied low temperature detection thermostat, manual reset, located downstream of the heat exchanger, will be hardwired to the fan circuit.
Upon detection of a low temperature condition below 35F (adj.) the supply fan will shut
down, the outside air dampers will close and a low temperature alarm will be activated.
2.
Loss of Air Flow: When the pressure differential across the supply fan indicates a no air
flow condition and the fan is commanded "ON" a fan failure alarm will be indicated at
the remote control panel.
3.
Fire/Smoke Operation: When the duct mounted smoke detectors alarm, they will send a
signal to the local fire alarm panel, which will signal an “alarm condition”, the signal
from the fire alarm panel will shut down the supply air fan.
Off Mode
1.
Occurs in the unoccupied mode.
2.
Supply fan is off and the outside air damper is closed.
MINI-SPLIT SYSTEMS
A.
This unit is a constant volume cooling only unit with remote controls.
B.
Start/Stop: The supply fan and transfer air fan operation shall be interlock to operate
simultaneously. The operation of the supply fan shall be determined by a fan switch on the
thermostat that allows either manual or automatic control. When the fan switch is in the
manual “on” position, the supply fan and transfer air fan shall run continuously. When the fan
switch is in the automatic position, the supply fan and transfer air fan shall cycle on and off
with the mini-split fan coil unit to maintain temperature set point.
C.
Cooling Mode: Upon a space temperature greater than the room set point temperature, the
mini-split system refrigeration compressor shall cycle on and off to maintain the set point
temperature. Upon a drop in room temperature below the set point temperature, the
supplemental electric resistance heat will be energized to warm the space. The supplemental
heaters have integral thermostat controls.
PART 2 - PRODUCTS (Not Applicable)
PART 3 - EXECUTION (Not Applicable)
END
OF
SEQUENCE OF OPERATIONS FOR
HVAC CONTROLS
SECTION
230993
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