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W180 ZONE CONTROL SYSTEM
AIRFLOW CHARACTERISTICS
Air, when enclosed, pushes equally against all sides of a
container. When an opening is provided in the container, air
flows out of the container until the pressure inside and
outside are equal. Air always flows from regions of higher
pressure to regions of lower pressure. The greater the
pressure differential, the faster the air flows.
In HVAC system design, the ratio of airflow pressure to
total pressure is controlled to ensure that each room re-
ceives the right amount of air flow to supply the heating or
cooling requirements. Total pressure has two components:
static pressure and velocity pressure.
Static pressure is the pressure exerted by air against the
sides of a container (in this case the duct). Static pressure is
measured as the difference between duct pressure and
atmospheric pressure, and can be positive or negative. In a
forced-air heating/cooling system, static pressure is sup-
plied by the blower. It is positive on the supply side and
negative on the return side.
When the container has an outlet so that air flows, part
of the static pressure is transformed into velocity pres-
sure, which is the pressure exerted by the air in the direction
of the airflow. Like static pressure, velocity pressure is
measured as the difference between duct pressure and
atmospheric pressure, but can only be positive. Velocity
pressure correlates directly with air speed.
At any given point in the duct, static pressure and
velocity pressure always equal total pressure. So if static
pressure increases and total pressure remains the same,
velocity pressure drops.
Total pressure is greatest at the face of the blower. As air
moves through the duct system, total pressure drops. It is
neutral once the air leaves the outlet and mixes with the
room air, but negative in the return air ducts. Pressure is lost
because of friction, leakage and turbulence. Friction is
caused by moving air rubbing against the sides of the duct.
Friction losses must be known to determine the blower
capacity required to provide adequate air flow to each zone.
Leakage should be minimal in a well-constructed duct
system. Turbulence is increased whenever the duct
changes size or direction; thus such changes should be
kept to a minimum and should be gradual.
Some factors that affect air flow and pressure in an
HVAC system are:
• Duct size. As the duct size increases, static pressure
increases and velocity pressure decreases.
• Transitions, angles, and rough surfaced ducts. These
all cause greater friction losses than straight, smooth
ducts and thus cause pressure loss. To simplify
calculations, these friction losses are measured as
equivalent lengths of straight, smooth ducts.
• Fan characteristics. Blade efficiency, fan speed, and
fan horsepower all affect the amount of pressure that
can be supported.
• Dampers. When a damper with seals closes, the total
pressure downstream matches the atmospheric pres-
sure and the total pressure in the rest of the system
68-0139
Appendix C: HVAC System Basics
ZONING
Purpose of Zoning
building receives the right amount of heating or cooling.
Zoning allows the occupant to independently control the
temperature in each area of the home or building. If desired,
all areas can be kept at the same temperature or each area
can be adjusted for occupancy patterns and uses.
bution patterns result in uneven temperature control. For
example, a building that is partly below grade can use
zoning to eliminate uneven temperature control between
the basement and the rest of the building. Large or sprawl-
ing buildings that might have long, unequal length duct
runs can use zoning to equalize the delivery of conditioned
air. Buildings with many large windows can use zoning to
compensate for solar heat gain and radiation losses at night.
by keeping various zones at different temperatures. Tem-
perature settings can be scheduled to fine-tune a zoned
system to match usage patterns.
Zone Selection
mary considerations in defining zones. Rooms in a zone
should be in the same area of the building. Also, they
should have similar uses and occupancy patterns. Rooms
that are subjected to heavy heat loads or heat loss because
of large windows, exposure to prevailing winds, or other
reasons should be zoned separately.
DUCT SYSTEMS
more suited to locations where heating is a primary con-
cern, and systems with high outlets on either outside or
inside walls are better choices where cooling is a primary
concern.
Loop Perimeter System
52
increases. Manual dampers in each takeoff duct are
adjusted after the system is installed to equalize the
air distribution.
• Register or grille design. To the extent that the grille
or register offers resistance to air flow, it adds to
pressure drop. The primary considerations when
choosing registers are throw—how far will the air-
stream move into the room before it dissipates; and
spread—how much the airstream will fan out imme-
diately after leaving the register.
Zoning is a way of ensuring that each area of a home or
Zoning is particularly useful where normal heat distri-
Zoning can add to comfort and possibly energy savings
Location, heat gain (loss), usage, and size are the pri-
The basic duct system types are:
• Loop perimeter.
• Radial perimeter.
• Extended plenum.
Generally, systems with low outlets on outside walls are
Zoning is not recommended for loop perimeter systems.
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