C403.2.4 HVAC System Controls
The supply of heating and cooling energy to each zone shall be controlled by individual thermostatic controls capable of responding to temperature within the zone. Where humidification or dehumidification or both is provided, at least one humidity control device shall be provided for each humidity control system.
Exception: Independent perimeter systems that are designed to offset only building envelope heat losses, gains or both serving one or more perimeter zones also served by an interior system provided:
- The perimeter system includes at least one thermostatic control zone for each building exposure having exterior walls facing only one orientation (within +/-45 degrees) (0.8 rad) for more than 50 contiguous feet (15 240 mm); and
- The perimeter system heating and cooling supply is controlled by thermostats located within the zones served by the system.
Where used to control both heating and cooling, zone thermostatic controls shall be capable of providing a temperature range or deadband of at least 5°F (2.8°C) within which the supply of heating and cooling energy to the zone is capable of being shut off or reduced to a minimum.
Outdoor air intake and exhaust openings and stairway and shaft vents shall be provided with Class I motorized dampers. The dampers shall have an air leakage rate not greater than 4 cfm/ft2 (20.3 L/s • m2) of damper surface area at 1.0 inch water gauge (249 Pa) and shall be labeled by an approved agency when tested in accordance with AMCA 500D for such purpose.
Outdoor air intake and exhaust dampers shall be installed with automatic controls configured to close when the systems or spaces served are not in use or during unoccupied period warm-up and setback operation, unless the systems served require outdoor or exhaust air in accordance with the International Mechanical Code or the dampers are opened to provide intentional economizer cooling.
Stairway and shaft vent dampers shall be installed with automatic controls configured to open upon the activation of any fire alarm initiating device of the building's fire alarm system or the interruption of power to the damper.
Exception: Gravity (nonmotorized) dampers shall be permitted to be used as follows:
- In buildings less than three stories in height above grade plane.
- In buildings of any height located in Climate Zones 1, 2 or 3.
- Where the design exhaust capacity is not greater than 300 cfm (142 L/s).
Gravity (nonmotorized) dampers shall have an air leakage rate not greater than 20 cfm/ft2 (101.6 L/s • m2) where not less than 24 inches (610 mm) in either dimension and 40 cfm/ft2 (203.2 L/s • m2) where less than 24 inches (610 mm) in either dimension. The rate of air leakage shall be determined at 1.0 inch water gauge (249 Pa) when tested in accordance with AMCA 500D for such purpose. The dampers shall be labeled by an approved agency.
HVAC systems serving zones that are over 25,000 square feet (2323 m2) in floor area or that span more than one floor and are designed to operate or be occupied nonsimultaneously shall be divided into isolation areas. Each isolation area shall be equipped with isolation devices and controls configured to automatically shut off the supply of conditioned air and outdoor air to and exhaust air from the isolation area. Each isolation area shall be controlled independently by a device meeting the requirements of Section C403.2.4.2.2. Central systems and plants shall be provided with controls and devices that will allow system and equipment operation for any length of time while serving only the smallest isolation area served by the system or plant.
- Exhaust air and outdoor air connections to isolation areas where the fan system to which they connect is not greater than 5,000 cfm (2360 L/s).
- Exhaust airflow from a single isolation area of less than 10 percent of the design airflow of the exhaust system to which it connects.
- Isolation areas intended to operate continuously or intended to be inoperative only when all other isolation areas in a zone are inoperative.
Air-cooled unitary direct-expansion units listed in Tables C403.2.3(1) through C403.2.3(3) and variable refrigerant flow (VRF) units that are equipped with an economizer in accordance with Section C403.3 shall include a fault detection and diagnostics (FDD) system complying with the following:
- 1.1. Outside air.
- 1.2. Supply air.
- 1.3. Return air.
- 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).
- Refrigerant pressure sensors, where used, shall have an accuracy of ±3 percent of full scale.
- 4.1. Free cooling available.
- 4.2. Economizer enabled.
- 4.3. Compressor enabled.
- 4.4. Heating enabled.
- 4.5. Mixed air low limit cycle active.
- 4.6. The current value of each sensor.
- The 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.
- The unit shall be capable of reporting faults to a fault management application accessible by dayto-day operating or service personnel, or annunciated locally on zone thermostats.
- 7.1. Air temperature sensor failure/fault.
- 7.2. Not economizing when the unit should be economizing.
- 7.3. Economizing when the unit should not be economizing.
- 7.4. Damper not modulating.
- 7.5. Excess outdoor air.