In normal operation, a VAV box modulates flow based on space temperature. In cooling mode, a properly configured box will modulate between minimum flow when there is little call for cooling, and maximum flow when there is a maximum call for cooling. Pneumatically controlled VAV boxes control flow with a device called a pneumatic volume regulator (PVR), which has inputs for flow and branch pressure, typically from a wall mounted zone thermostat. Typically, these devices had two settings, one for minimum flow and a second for maximum flow. The system designer typically specifies the maximum flow based on the cooling load for the zone and the minimum flow on either the minimum ventilation needs for the zone. If the box also has a reheat coil, then the minimum flow setting is what is required to heat the zone.
When we use a DDC controller for a VAV box, the basic control theory is unchanged. The function of the PVR is replaced by a DDC controller, which has a temperature input from a zone-mounted sensor and also measures box flow. The digital control is not limited to having just a minimum and maximum flow setting. Most DDC VAV controllers allow for settings for maximum flow as well as minimum flow for cooling and minimum flow for heating. How these settings are configured depends on the application of the box. Here are some general rules:
• Maximum flow / maximum cooling flow: The maximum box flow should be set based on the cooling loads for the zone. At maximum flow, the box will be delivering full cooling capacity to the zone.
• Minimum flow – cooling: The minimum cooling flow should be set based on the ventilation requirements for the zone based on the requirements of ASHRAE 62 or local codes. The minimum flow setting should be the same, regardless of whether the box is configured as cooling only, cooling with reheat, or fan powered.
• Minimum flow – heating: For boxes that are shut-off only, or fan-powered (either series or parallel), the heating minimum setting should be the same as the cooling minimum flow setting. Shut-off boxes do not have the capability to heat and should not provide any less air than is required for ventilation. Fan-powered boxes will bring in return air to provide the required airflow for proper heating of the zone. Shut-off boxes with reheat should have the heating minimum flow setting set at the level required for proper heating of the zone. This value is typically higher than the minimum cooling flow setpoint. In normal operation, the box will modulate from maximum flow down to minimum cooling flow as there is less call for cooling, and then it will open to the heating minimum flow and open the heating coil on a call for heat.ES