C403.6.10
High
efficiency variable air volume (VAV) systems. For HVAC systems subject
to the requirements of Section C403.3.5 but utilizing Exception 2 of that
section, a high efficiency multiple-zone VAV system may be provided without a
separate parallel DOAS when the system is designed, installed, and configured
to comply with all of the following criteria (this exception shall not be used
as a substitution for a DOAS per Section C406.6):
1. Each VAV system must serve a minimum of
3,000 square feet (278.7 m2) and have a minimum of
five VAV zones.
2. The VAV systems
are provided with airside economizer per Section C403.5 without
exceptions.
3. A direct-digital
control (DDC) system is provided to control the VAV air handling units and
associated terminal units per Section C403.4.11 regardless of sizing thresholds
of Table C403.4.11.1.
4.
Multiple-zone VAV systems with a minimum outdoor air requirement of 2,500 cfm
(1180 L/s) or greater shall be equipped with a device capable of measuring
outdoor airflow intake under all load conditions. The system shall be capable
of increasing or reducing the outdoor airflow intake based on feedback from the
VAV terminal units as required by Section C403.6.5, without exceptions, and
Section C403.7.1 demand controlled ventilation.
5. Multiple-zone VAV systems with a minimum
outdoor air requirement of 2,500 cfm (1180 L/s) or greater shall be equipped
with a device capable of measuring supply airflow to the VAV terminal units
under all load conditions.
6. In
addition to meeting the zone isolation requirements of C403.2.1 a single VAV
air handling unit shall not serve more than 50,000 square feet (4645
m2) unless a single floor is greater than 50,000
square feet (4645 m2) in which case the air handler
is permitted to serve the entire floor.
7. The primary maximum cooling air for the
VAV terminal units serving interior cooling load driven zones shall be sized
for a supply air temperature that is a minimum of 5°F greater than the
supply air temperature for the exterior zones in cooling.
8. Air terminal units with a minimum primary
airflow setpoint of 50 percent or greater of the maximum primary airflow set
point shall be sized with an inlet velocity of no greater than 900 feet per
minute.
9. Allowable fan power
shall not exceed 90 percent of the allowable fan power budget as defined by
Section C403.8.1.1.
10. All fan
powered VAV terminal units (series or parallel) shall be provided with
electronically commutated motors. The DDC system shall be configured to vary
the speed of the motor as a function of the heating and cooling load in the
space. Minimum speed shall not be greater than 66 percent of design airflow
required for the greater of heating or cooling operation. Minimum speed shall
be used during periods of low heating and cooling operation and
ventilation-only operation.
EXCEPTION:
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For series fan powered terminal units where the volume
of primary air required to deliver the ventilation requirements at minimum
speed exceeds the air that would be delivered at the speed defined above, the
minimum speed setpoint shall be configured to exceed the value required to
provide the required ventilation air.
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11.
Fan-powered VAV terminal units shall only be permitted at perimeter zones with
an envelope heating load requirement. All other VAV terminal units shall be
single duct terminal units.
EXCEPTION:
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Fan powered VAV terminal units are allowed at interior
spaces with an occupant load greater than or equal to 25 people per 1000 square
feet of floor area (as established in Table 403.3.1.1 of the
International Mechanical Code) with demand control ventilation
in accordance with Section C403.7.1.
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12.
When in occupied heating or in occupied deadband between heating and cooling
all fan powered VAV terminal units shall be configured to reset the primary air
supply setpoint, based on the VAV air handling unit outdoor air vent fraction,
to the minimum ventilation airflow required per International
Mechanical Code.
13.
Spaces that are larger than 150 square feet (14 m
2)
and with an occupant load greater than or equal to 25 people per 1000 square
feet (93 m
2) of floor area (as established in Table
403.3.1.1 of the
International Mechanical Code) shall be
provided with all of the following features:
13.1. A dedicated VAV terminal unit capable
of controlling the space temperature and minimum ventilation shall be
provided.
13.2. Demand control
ventilation (DCV) shall be provided that utilizes a carbon dioxide sensor to
reset the ventilation setpoint of the VAV terminal unit from the design minimum
to design maximum ventilation rate as required by Chapter 4 of the
International Mechanical Code.
13.3. Occupancy sensors shall be provided
that are configured to reduce the minimum ventilation rate to zero and setback
room temperature setpoints by a minimum of 5°F, for both cooling and
heating, when the space is unoccupied.
14. Dedicated data centers, computer rooms,
electronic equipment rooms, telecom rooms, or other similar spaces with cooling
loads greater than 5 watts/sf shall be provided with separate cooling systems
to allow the VAV air handlers to turn off during unoccupied hours in the office
space and to allow the supply air temperature reset to occur.
EXCEPTION:
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The VAV air handling unit and VAV terminal units may be
used for secondary backup cooling when there is a failure of the primary HVAC
system.
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Additionally, computer rooms, electronic equipment rooms,
telecom rooms, or other similar spaces shall be provided with airside
economizer in accordance with Section 403.5 without using the exceptions to
Section C403.5.
EXCEPTION:
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Heat recovery per Exception 9 of Section C403.5 may be
in lieu of airside economizer for the separate, independent HVAC system.
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15.
HVAC system central heating or cooling plant will include a minimum of one of
the following options:
15.1. VAV terminal
units with hydronic heating coils connected to systems with hot water
generation equipment limited to the following types of equipment: Gas-fired
hydronic boilers with a thermal efficiency, Et, of not
less than 92 percent, air-to-water heat pumps or heat recovery chillers.
Hydronic heating coils shall be sized for a maximum entering hot water
temperature of 120°F (48.9°C) for peak anticipated heating load
conditions.
15.2. Chilled water VAV
air handing units connected to systems with chilled water generation equipment
with IPLV values more than 25 percent higher than the minimum part load
efficiencies listed in Table C403.3.2(3), in the appropriate size category,
using the same test procedures. Equipment shall be listed in the appropriate
certification program to qualify. The smallest chiller or compressor in the
central plant shall not exceed 20 percent of the total central plant cooling
capacity or the chilled water system shall include thermal storage sized for a
minimum of 20 percent of the total central cooling plant
capacity.
16. The DDC
system shall include a fault detection and diagnostics (FDD) system complying
with the following:
16.1. The following
temperature sensors shall be permanently installed to monitor system operation:
16.1.1. Outside air.
16.1.2. Supply air.
16.1.3. Return air.
16.2. Temperature sensors shall have an
accuracy of +-2°F (1.1°C) over the range of 40°F to 80°F
(4°C to 26.7°C).
16.3. The
VAV air handling unit controller shall be configured to provide system status
by indicating the following:
16.3.1. Free
cooling available.
16.3.2.
Economizer enabled.
16.3.3.
Compressor enabled.
16.3.4. Heating
enabled.
16.3.5. Mixed air low
limit cycle active.
16.3.6. The
current value of each sensor.
16.4. The VAV air handling unit controller
shall be capable of manually initiating each operating mode so that the
operation of compressors, economizers, fans and the heating system can be
independently tested and verified.
16.5. The VAV air handling unit shall be
configured to report faults to a fault management application able to be
accessed by day-to-day operating or service personnel or annunciated locally on
zone thermostats.
16.6. The VAV
terminal unit shall be configured to report if the VAV inlet valve has failed
by performing the following diagnostic check at a maximum interval of once a
month:
16.6.1. Command VAV terminal unit
primary air inlet valve closed and verify that primary airflow goes to
zero.
16.6.2. Command VAV terminal
unit primary air inlet valve to design airflow and verify that unit is
controlling to within 10 percent of design airflow.
16.7. The VAV terminal unit shall be
configured to report and trend when the zone is driving the following VAV air
handling unit reset sequences. The building operator shall have the capability
to exclude zones used in the reset sequences from the DDC control system
graphical user interface:
16.7.1. Supply air
temperature setpoint reset to lowest supply air temperature setpoint for
cooling operation.
16.7.2. Supply
air duct static pressure setpoint reset for the highest duct static pressure
setpoint allowable.
16.8. The FDD system shall be configured to
detect the following faults:
16.8.1. Air
temperature sensor failure/fault.
16.8.2. Not economizing when the unit should
be economizing.
16.8.3. Economizing
when the unit should not be economizing.
16.8.4. Outdoor air or return air damper not
modulating.
16.8.5. Excess outdoor
air.
16.8.6. VAV terminal unit
primary air valve failure.