AutomatedBuildings.com Column - VAV Box Flow Settings

Often we find projects where VAV box flow setpoints are not properly configured. In many cases this is due to changes made by operators, and in others it may come from issues in the original design. This month we would like to look at how to properly specify and set up the flow setpoints on a DDC VAV box.

Control Theory:

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 cooling 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.

DDC VAV:

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. Since it is a digital control it 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.

[an error occurred while processing this directive] 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 if 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 then 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 then 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, then will open to the heating minimum flow and open the heating coil on a call for heat.

Paul and Ira first worked together on a series of ASHRAE projects including the BACnet committee and Guideline 13 – Specifying DDC Controls. The formation of Building Intelligence Group provided them the ability to work together professionally providing assistance to owners with the planning, design and development of Intelligent Building Systems. Building Intelligence Group provides services for clients worldwide including leading Universities, Corporations, and Developers. More information can be found at www.buildingintelligencegroup.com We also invite you to contact us directly at Paul@buildingintelligencegroup.com or ira@buildingintelligencegroup.com

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