C403.6 Requirements for Mechanical Systems Serving Multiple Zones
Supply air systems serving multiple zones shall be variable air volume (VAV) systems that have zone controls configured to reduce the volume of air that is reheated, recooled or mixed in each zone to one of the following:
- Twenty percent of the zone design peak supply for systems with DDC and 30 percent for other systems.
- 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 and 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.
- The outdoor airflow rate required to meet the minimum ventilation requirements of Chapter 4 of the International Mechanical Code.
- 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.
- The airflow rate required to comply with applicable codes or accreditation standards such as pressure relationships or minimum air change rates.
Exception: The following individual zones or entire air distribution systems are exempted from the requirement for VAV control:
- 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, including condenser heat, or site-solar energy source.
- 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.
Multiple-zone HVAC systems shall include controls that 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 not less than 25 percent of the difference between the design supply-air temperature and the design room air temperature.
- Systems that prevent reheating, recooling or mixing of heated and cooled supply air.
- Seventy-five percent of the energy for reheating is from site-recovered or site-solar energy sources.
- Zones with peak supply air quantities of 300 cfm (142 L/s) or less.
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 (Ev) as defined by the International Mechanical Code.
- 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.
- Systems where total design exhaust airflow is more than 70 percent of total design outdoor air intake flow requirements.
Parallel-flow fan-powered VAV air terminals shall have automatic controls configured to:
- Turn off the terminal fan except when space heating is required or where required for ventilation.
- Turn on the terminal fan as the first stage of heating before the heating coil is activated.
- 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.
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 case, the setpoint 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: