Application

Application Notes
Dual Duct Sequence Configurations
15
Siemens Industry, Inc.
Application Note
A6V11692042
Smart Infrastructure – Building Products
2019-04-01
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 = Mixing control;
On an increase in heating load, the heating flow demand on the hot duct modulates from heating flow min
to heating flow max. Cold duct air flow will be the value of the difference between hot duct air flow and the
current value of ventilation flow demand, provided that the ventilation flow demand is greater than hot duct
air flow, otherwise cold duct air flow will modulate to zero. Once the cold duct flow modulates to zero, the
hot duct becomes the active duct.
If the last active duct = hot duct, then when the system exits the temperature deadband the behavior is as
follows:
• Exiting deadband on a call for heat when last duct is the hot duct and CtlStrgy = Mixing control;
On an increase in heating load, the heating flow demand on the hot duct modulates from heating flow min
to heating flow max. Hot duct air flow will be the greater of vent demand and heating demand. Cold duct air
flow will be zero. (Hot duct remains the active duct.)
• Exiting deadband on a call for cool when last duct is the hot duct and CtlStrgy = Mixing control;
On an increase in cooling load, the cooling flow demand on the cold duct modulates from cooling flow min
to cooling flow max. Hot duct air flow will be the value of the difference between cold duct air flow and the
current value of ventilation flow demand, provided that the ventilation flow demand is greater than cold
duct air flow, otherwise hot duct air flow will modulate to zero. Once the hot duct flow modulates to zero,
the cold duct becomes the active duct.
Use case 3: VAV DD - Ventilation in both ducts, snap acting control
In use case 3 configurations, the mechanical equipment consists typically of a single fan AHU with separate
elements for heating and cooling air distribution. The hot and cold air streams are drawn from the same
source(s) and driven by the same fan. Outside air ventilation is provided using both ducts.
Ventilation in deadband – When ventilation is provided in both ducts, different possibilities exist for ventila-
tion in deadband: cold duct (Duct 1), hot duct (Duct 2), and "Last duct". The dual duct can be configured ac-
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.