Commissioning Instructions

Dual Maximum VAV Control

A6V12035398

Unrestricted

Dual Maximum VAV Control

Title 24 requires the DDC application to support Dual Maximum VAV control for

Vav/Fpb terminal units with reheat (modulating hot water or electric).

See 2019 nonresidential Compliance Manual section 4.5.2.1 for more information. In

summary, for systems with DDC at the zone level, the controls must be able to support

two different CFM maximums, one each for heating and cooling.

In cooling, this control scheme is like a traditional VAV reheat box control. The difference is

what occurs in the deadband between heating and cooling and in the heating mode. With tradi-

tional VAV control logic, the minimum airflow rate is typically set to the largest rate allowed by

code. This airflow rate is supplied to the space in the deadband and heating modes. With the

"dual maximum" logic, the minimum rate is the lowest allowed by code (e.g. the minimum venti-

lation rate) or the minimum rate the controls system can be set to (which is a function of the

VAV box velocity pressure sensor amplification factor and the accuracy of the controller to con-

vert the velocity pressure into a digital signal). As the heating demand increases, the dual max-

imum control first resets the discharge air temperature (typically from the design cold deck tem-

perature up to 85 or 90°F) as a first stage of heating then, if more heat is required, it increases

airflow rate up to a “heating” maximum airflow setpoint, which is the same value as what tradi-

tional control logic uses as the minimum airflow setpoint. Using this control can save significant

fan, reheat and cooling energy while maintaining better ventilation effectiveness as the dis-

charge heating air is controlled to a temperature that will minimize stratification. This control re-

quires a discharge air sensor and may require a programmable VAV box controller. The dis-

charge air sensor is very useful for diagnosing control and heating system problems even if they

are not actively used for control. (see figure)