C403.12
High
efficiency single-zone 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
single-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 or as a modification to the requirements for the
Standard Reference Design in accordance with Section
C407):
1. The single-zone VAV system
is provided with airside economizer in accordance with Section C403.3 without
exceptions.
2. A direct-digital
control (DDC) system is provided to control the system as a single zone in
accordance with Section C403.4.11 regardless of sizing thresholds of Table
C403.4.11.1.
3. Single-zone VAV
systems with a minimum outdoor air requirement of 1,000 cfm (472 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 Section C403.7.1, Demand
controlled ventilation.
4.
Allowable fan power shall not exceed 90 percent of the allowable fan power
budget as defined by Section C403.8.1.1.
5. Each single-zone VAV system shall be
designed to vary the supply fan airflow as a function of heating and cooling
load and minimum fan speed shall not be more than the greater of:
5.1. 30 percent of peak design airflow;
or
5.2. The required ventilation
flow assuming no occupants.
6. 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:
6.1. Demand control ventilation (DCV) shall
be provided that utilizes a carbon dioxide sensor to reset the ventilation
setpoint of the single-zone VAV system from the design minimum to design
maximum ventilation rate as required by Chapter 4 of the International
Mechanical Code.
6.2.
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.
7.
Single-zone VAV systems shall comply with one of the following options:
7.1. Single-zone VAV air handling units with
a hydronic heating coil 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
for peak anticipated heating load conditions.
7.2. Single-zone VAV air handing units with a
chilled water coil 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.
7.3. Single-zone VAV air handling units with
DX cooling, heat pump heating or gas-fired furnace shall comply with the
following requirements as applicable:
7.3.1.
Have a DX cooling coil with cooling part load efficiency that is a minimum of
15 percent higher than the minimum SEER or IEER listed in Tables C403.3.2(1),
C403.3.2(2), and C403.3.2 (14).
7.3.2. Have a gas-fired furnace with a
thermal efficiency, Et, of not less than 90 percent or
heat pump with a minimum heating HSPF or COP efficiency that are a minimum of
10 percent higher than the minimum heating efficiency in Tables C403.3.2(1),
C403.3.2(2), and C403.3.2 (14).
7.3.3. Heating coils or burner output shall
be modulating or have a minimum of 2 stages with the first stage being less
than 50 percent of total heating capacity. Cooling coils shall be modulating or
have a minimum of 2 stages with the first stage being less than 50 percent of
the total cooling capacity.
8. The DDC system shall include a fault
detection and diagnostics (FDD) system complying with the following:
8.1. The following temperature sensors shall
be permanently installed to monitor system operation:
8.1.1. Outside air.
8.1.2. Supply air.
8.1.3. Return air.
8.2. 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).
8.3. The
single-zone VAV air handling unit controller shall be configured to provide
system status by indicating the following:
8.3.1. Free cooling available.
8.3.2. Economizer enabled.
8.3.3. Compressor enabled.
8.3.4. Heating enabled.
8.3.5. Mixed air low limit cycle
active.
8.3.6. The current value of
each sensor.
8.4. The
single-zone 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.
8.5. The single-zone 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.
8.6. The FDD system shall be configured to
detect the following faults:
8.6.1. Air
temperature sensor failure/fault.
8.6.2. Not economizing when the unit should
be economizing.
8.6.3. Economizing
when the unit should not be economizing.
8.6.4. Outdoor air or return air damper not
modulating.
8.6.5. Excess outdoor
air.