Commissioning Instructions

5
Siemens Industry, Inc.
A6V11668992
Restricted
2019-03-12
Duct Pressure Reset and VAV Saturation
Overview
When the pressure in the duct system is too low, VAV boxes may open to maximum or
near maximum and still not get enough air. If pressure is too high, boxes could be nearly
closed and the AHU could be forcing air through them, wasting energy. An efficient way to
counteract these issues is to change the static pressure setpoint based on demand from
the zone. Duct static pressure should be just low enough that a single box is almost fully
open. At least one, and preferably only one, is ideal.
By using central functions it is possible to collect data from each VAV box and use it to
automatically reset duct pressure if necessary. ASHRAE recommends choosing from
available strategies such as flow setpoints, damper position data, and saturation signals.
This document focuses on the simple and efficient saturation signal method to achieve
duct pressure reset.
Each VAV (or FPB) application dynamically calculates the saturation level for the supply
air damper. StrtnLvl (Saturation level) is the value used to determine if the damper is
open more than a set amount. DlyOnStrtn (Switch-on delay saturation) is the time delay
before a damper that is starving for air (i.e., open more than a set amount) initiates a
signal change. Both parameters are configurable and are under Supply air VAV in the
Default values task area in ABT Site.
If any box is starved of air, a signal (VavSuAflStrtn / SU FLO SATUR) is sent to the central
function which then counts the total number of starved boxes. The central function shares
the total number of starved boxes with the AHU, which can then use the data to reset
static pressure to a level that is adequate for current demand.
SU FLO SATUR is a calculated binary object with two states: 0:Satisfied / 1:Starved. The
value is true (Starved) when:
Damper position is equal to or exceeds the value of StrtnLvl,
AND
Airflow error exceeds a configured error level,
AND
Both conditions stay true for longer than the value of DlyOnStrtn.
StrtnLvl (default 90%) can be adjusted to increase/decrease the terminal box application’s
sensitivity to a saturation condition. It can be set higher if there is concern only with boxes
being close to completely starved. Conversely, the level can be lowered if the goal is to
collect data on boxes that are nearing saturation. 90% represents a balance between the
two strategies. The configurable time delay (DlyOnStrtn) default is 60 seconds. This can
be adjusted along with the saturation level to achieve desired results.
Group Master object
The Group Master object in the Central Function application for supply air coordination
collects data from supply air Group Members to support duct pressure reset. When the
VavSuAflStrtn signal = Starved, the collected information being shared with the AHU fan
controller can be used to regulate an increase to the static setpoint.