Application

Application Notes
Dual Duct Sequence Configurations
18
Siemens Industry, Inc.
Application Note
A6V11692042
Smart Infrastructure – Building Products
2019-04-01
Use Case 3c – VAV DD with ventilation in both ducts, last duct ventilation in deadband, snap
acting control:
Last active duct will stay open in deadband
to meet ventilation requirement. For behav-
ior outside of deadband see additional in-
formation below.
Ducts may close if ventilation requirement
goes to zero.
Parameter configuration for use case 3c:
• VavSuChovrCnd1 = Cooling
• VavSuChovrCnd2 = Heating
• VntDuctSprt = Both ducts
• CtlStrgy = Snap acting control
• VntSprtDdband = Last duct
In use case 3c, with the thermal load in deadband the air flow from the last active duct will equal the current
value of the ventilation flow demand (may vary per room operating mode or DCV); the inactive duct air flow will
be zero.
If the last active duct = cold duct, then when the system exits the temperature deadband the behavior is as
follows:
• Exiting deadband on a call for cool when last duct is the cold duct and CtlStrgy = Snap acting;
On an increase in cooling load, the cooling flow demand on the cold duct modulates from cooling flow min
to cooling flow max. Cold duct air flow will be the greater of vent demand and cooling demand. Hot duct air
flow will be zero. (Cold duct remains the active duct.)
• Exiting deadband on a call for heat when last duct is the cold duct and CtlStrgy = Snap acting;
On an increase in heating load, the cold duct closes (snap action) and the heating loop becomes the active
loop / duct. As the heating flow demand on the hot duct modulates from heating flow min to heating flow
max, the hot duct air flow will be the greater of vent demand and heating demand.