Size the Ductwork
In New Construction: ductwork is sized as usual for the size of the home; trunks are sized for the
amount of air going to the zone.
Ductwork should be designed normally, typically .10 or .08 in. w.c./100 ft.
Proper duct design of a zoned system will:
• Maintain proper airflow through the HVAC unit when only one zone is open.
• And maintain proper airflow to all zones when all are open.
In Retrofit Applications: zones are somewhat limited by how the ducts were originally constructed,
but any home can be zoned provided there is access to the ductwork. See page 10 for zoning informa-
tion specific to retrofit applications.
Use ZD dampers for rectangular ducts. Use ARD or
RRD dampers for round ducts in residential systems up
to 2000 CFM.
For systems over 2000 CFM or commercial systems,
see page 11.
Install the Dampers
A supply air damper should not be placed closer than 6
ft. to a diffuser. The damper should also be at least 3 ft.
away from the plenum. (Less than 6 ft. between damper
and diffuser could create objectionable air noise in the
conditioned space and 3 ft. of duct off the plenum will
reduce turbulence.) These minimums are recommenda-
tions for optimal performance; however, each installa-
tion presents its own challenges, so each installation will vary. For instance, in some installations the
damper may need to be mounted directly on the plenum.
Allow for Access
Dampers should be installed in a location that can be accessed for service. If a damper needs to be
"buried," place a decorative register over it to provide access.
Balancing dampers should be included in the duct work, because a zoned home needs to be balanced
just as a non-zoned home does.
Zoning System Design Manual
ducts and damPers