C403.6
Requirements for mechanical systems serving multiple zones.
Sections C403.6.1 through C403.6.10 shall apply to mechanical systems serving
multiple zones.
C403.6.1
Variable air
volume (VAV) and multiple zone systems. Supply air systems serving
multiple zones shall be VAV systems that have zone controls configured to
reduce the volume of air that is reheated, re-cooled or mixed in each
zone to one of the following:
1. Twenty percent of the zone design peak
supply for systems with direct digital control (DDC) and 30
percent of the maximum supply air for other systems.
2. Systems with DDC where items 2.1 through
2.3 apply.
2.1. The airflow rate in the
deadband between heating and cooling does not exceed 20 percent of the zone
design peak supply rate or higher allowed rates under Items 3, 4, or 5 of this
section.
2.2. The first stage of
heating modulates the zone supply air temperature setpoint up to a maximum
setpoint while the airflow is maintained at the deadband flow rate.
2.3. The second stage of heating modulates
the airflow rate from the deadband flow rate up to the heating maximum flow
rate that is less than 50 percent of the zone design peak supply
rate.
3. The outdoor
airflow rate required to meet the minimum ventilation requirements of Chapter 4
of the International Mechanical Code.
4. Any higher rate that can be demonstrated
to reduce overall system annual energy use by offsetting reheat/recool energy
losses through a reduction in outdoor air intake for the system, as
approved by the code official.
5. The airflow rates to comply with
applicable codes or accreditation standards such as pressure relationships or
minimum air change rates.
EXCEPTION:
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The following individual zones or
entire air distribution systems are exempted from the requirement for VAV
control:
1. Zones or supply air systems where
not less than 75 percent of the energy for reheating or for providing warm air
in mixing systems is provided from a site-recovered source, including condenser
heat.
2. Systems that prevent reheating, recooling, mixing or
simultaneous supply of air that has been previously cooled, either mechanically
or through the use of economizer systems, and air that has been previously
mechanically heated.
3. Ventilation systems complying with Section C403.3.5,
DOAS, with ventilation rates comply with Section C403.2.2.
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C403.6.2
Single duct variable air
volume (VAV) systems, terminal devices. Single duct VAV systems shall
use terminal devices capable of and configured to reduce the supply of primary
supply air before reheating or recooling takes place.
C403.6.3
Dual duct and mixing VAV
systems, terminal devices. Systems that have one warm air duct and one
cool air duct shall use terminal devices which are capable of and configured to
reduce the flow from one duct to a minimum before mixing of air from the other
duct takes place.
C403.6.4
Supply-air temperature reset controls. Multiple
zone
HVAC systems shall include controls that are capable of and
configured to automatically reset the supply-air temperature in response to
representative building loads, or to outdoor air temperature. The controls
shall be configured to reset the supply air temperature at least 25 percent of
the difference between the design supply-air temperature and the design room
air temperature. Controls that adjust the reset based on zone humidity are
allowed. HVAC zones that are expected to experience relatively constant loads
shall have maximum airflow designed to accommodate the fully reset supply air
temperature.
EXCEPTIONS:
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1. Systems that prevent reheating, recooling or mixing
of heated and cooled supply air.
2. Seventy-five percent of the energy for reheating is
from a site-recovered source.
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C403.6.5
Multiple-zone VAV system
ventilation optimization control. Multiple-zone VAV systems with direct
digital control of individual zone boxes reporting to a central control panel
shall have automatic controls configured to reduce outdoor air intake flow
below design rates in response to changes in system ventilation efficiency
(E
v) as defined by the
International Mechanical
Code.
EXCEPTIONS:
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1. VAV systems with zonal transfer fans that
recirculate air from other zones without directly mixing it with outdoor air,
dual-duct dual-fan VAV systems, and VAV systems with fan-powered terminal
units.
2. Systems where total design exhaust airflow is more
than 70 percent of total design outdoor air intake flow requirements.
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C403.6.6
Parallel-flow fan-powered VAV
air terminal control. Parallel-flow fan-powered VAV air terminals shall
have automatic controls configured to:
1. Turn
off the terminal fan except when space heating is required or where required
for ventilation.
2. Turn on the
terminal fan as the first stage of heating before the heating coil is
activated.
3. During heating for
warmup or setback temperature control, either:
3.1. Operate the terminal fan and heating
coil without primary air.
3.2.
Reverse the terminal damper logic and provide heating from the central air
handler by primary air.
C403.6.7
Reserved.
C403.6.8
Set points for direct digital
control. For systems with direct digital control of individual
zones reporting to the central control panel, the static
pressure setpoint shall be reset based on the
zone requiring
the most pressure. In such cases, the set point is reset lower until one zone
damper is nearly wide open. The direct digital controls shall be capable of
monitoring zone damper positions or shall have an alternative method of
indicating the need for static pressure that is configured to provide all of
the following:
1. Automatically detecting any
zone that excessively drives the reset logic.
2. Generating an alarm to the system
operational location.
3. Allowing
an operator to readily remove one or more zones from the reset
algorithm.
C403.6.9
Static pressure sensor location. Static pressure sensors used to
control VAV fans shall be located such that the controller setpoint is no
greater than 1.2 inches w.c. (299 Pa). Where this results in one or more
sensors being located downstream of major duct splits, not less than one sensor
shall be located on each major branch to ensure that static pressure can be
maintained in each branch.
EXCEPTION:
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Systems complying with Section C403.6.8.
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