Application

Application Notes
Dual Duct Sequence Configurations
12
Siemens Industry, Inc.
Application Note
A6V11692042
Smart Infrastructure – Building Products
2019-04-01
cording to the relative costs of providing hot air or cold air. For example, if it takes more energy to provide hot
air, then it makes sense to use cold air for ventilation in the deadband. If energy costs are approximately equal,
then "last duct" configuration may be preferable.
Mixing control vs. Snap acting control – Some people associate mixing hot and cold air flows in the dual duct
box with wasting energy. Others appreciate the gradual temperature modulation that comes with mixing. Snap
action prevents the mixing of hot and cold air flows in the dual duct box. Select according to project specifica-
tions or end-user preference.
Use Case 2a – VAV DD with ventilation in both ducts, cold duct ventilation in deadband, with
mixing control:
Cold duct will stay open in deadband to
meet a ventilation requirement. As heating
load ramps up, cold duct damper modulates
to zero as hot duct damper modulates open
(i.e., mixing control during ramp up of heat-
ing mode).
Ducts may close if ventilation requirement
goes to zero.
Parameter configuration for use case 2a:
• VavSuChovrCnd1 = Cooling
• VavSuChovrCnd2 = Heating
• VntDuctSprt = Both ducts
• CtlStrgy = Mixing control
• VntSprtDdband = Duct 1
In use case 2a, with the thermal load in deadband the cold duct air flow will equal the current value of the ven-
tilation flow demand (may vary per room operating mode or DCV); the hot duct air flow will be zero.
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.