In contrast, multi-stage equipment shifts to low-stage operation
(typically 60 percent of total capacity) when the air distribution
system becomes effectively smaller. Because the equipment re-
duces its minimum airflow limit, it is able to continue to operate.
The reduced capacity generally is adequate to supply the necessary
conditioning since only 1 or 2 zones need conditioning under these
circumstances.
The following multi-stage equipment can be used with the zoning
system:
• Two-speed air conditioners.
• Two-speed heat pumps.
• Two-stage furnaces.
• Two-stage electric heat used without a heat pump.
NOTE: When using multi-stage heating or cooling equipment
with Zone Perfect Plus, the zoning system must be allowed to
control the equipment staging. Therefore, there must be separate
inputs for low-stage and high-stage operation, and the equipment
must be configured to operate from these inputs only. Outdoor
temperature switches, staging algorithms, and so forth should not
be used.
C. Using Variable-Speed Blowers with Zoning
PSC blowers are single-speed blowers. A PSC blower runs at a
fixed speed and cannot change speed as the dampers move. A PSC
blower usually can produce about 0.5 in. of static pressure.
In contrast, ICM blowers are variable-speed blowers. Air handlers
using ICM blowers are designed to supply a fixed quantity of air
into a duct system regardless of the external static pressure. The air
handler can "sense" the external static pressure and change the
motor speed to deliver the desired airflow. An ICM blower
typically can produce about 0.9 in. of static pressure.
In a properly-designed system, the use of an ICM blower can help
assure adequate airflow through the system regardless of damper
positions. Air handlers using ICM blowers typically are pro-
grammed to produce 325 CFM per ton (of equipment capability).
The use of such an air handler does not reduce the damper or duct
size requirements of a system, but does help the system run
properly and quietly without a bypass.
D. Bypassing with Zoning
When only 1 zone requires conditioning, airflow through the
ductwork can fall below the system’s minimum requirements. This
situation can cause problems for the system’s equipment and can
lead to discomfort for the home’s occupants.
There are 3 standard methods that have been developed to
maintain minimum airflow, reduce duct velocity, and reduce duct
noise:
• Bypass air to an unconditioned space.
• Bypass air to a conditioned space.
• Bypass air directly to the return.
The term "bypass" means to allow additional air through the
system that bypasses the zone dampers, the zones, or both. The 3
methods described above use a bypass damper, which receives air
from the supply plenum. A barometric damper, which opens when
the supply air pressure reaches a preset value, generally is used for
the bypass. Thus, the barometric bypass damper acts as a pressure
regulator, releasing enough air to reduce the supply plenum
pressure to an appropriate level.
In addition to the 3 standard bypass methods, Zone Perfect Plus
provides an additional bypassing method—OUT zones. OUT
zones can serve as an excellent bypassing method. You can think
of an OUT zone as a temporary dump zone. Ideally, an OUT zone
should be available in the system at all times.
Each of the conventional bypass methods, as well as bypassing
using OUT zones, is discussed in the sections below. As you
evaluate the usefulness of these methods for a particular installa-
tion, keep in mind that bypassing using OUT zones can work well
in conjunction with a barometric bypass.
NOTE: A bypass damper reduces air noise by reducing airflow
velocity in the ducts. A direct bypass to the return does not add
load to the system.
BYPASSING TO AN UNCONDITIONED SPACE
Conditioned air can be bypassed to an unconditioned space. This
type of bypass is considered an indirect bypass.
Although there is no limit on the amount of air that can be
bypassed using this method, this type of bypass wastes energy. In
addition, this method creates negative indoor pressure, which pulls
unconditioned air into the home. This method is rarely used in
zoning installations and we do not recommend it.
BYPASSING TO A CONDITIONED SPACE
Conditioned air also can be bypassed to a conditioned space, called
a dump zone. This type of bypass also is considered an indirect
bypass.
Although there is no limit on the amount of air that can be
bypassed using this method, this type of bypass reduces the
effectiveness of the zoning system. The dump zone tends to be
over conditioned and uncomfortable.
This method is acceptable if you have a zone that can function as
a dump zone, such as a large basement. However, there rarely is a
zone suitable for this use.
BYPASSING DIRECTLY TO THE AIR RETURN
There are advantages and disadvantages to bypassing directly to
the air return using a barometric damper. The advantages of a
direct barometric bypass include:
• A direct bypass serves as a pressure regulator, bypassing
enough air to hold the static pressure in the plenum to a safe
level. Thus, a direct bypass prevents excessive air velocities,
which can cause noise and drafts.
• Because a direct bypass returns conditioned air back into the air
handler, it makes the leaving air colder in cooling and warmer
in heating. A direct bypass increases the Btu content of the air
entering the duct system while airflow (CFM) remains at the
same level. The result is that more capacity can be forced into
the system without increasing the airflow.
• During cooling, the process of making the leaving air colder
removes more water from the air. When a system is bypassing
a large amount of air (only a small portion of the zones require
conditioning), there is relatively little total demand on the
system. During such periods of light load, humidity often can
be a problem. The direct bypass automatically increases the
removal of water when it is most needed.
The disadvantages of a direct barometric bypass include:
• It is very difficult, if not impossible, to determine how much air
is being bypassed under a given condition. Traditional calcu-
lation methods can assure that the bypass is large enough, but
do not prevent the bypass from becoming too large. The
amount of air that can be bypassed often is greater than the
amount that the equipment can tolerate.
NOTE: The dimensions and attachment points of the bypass
ductwork have more influence on the amount of air that is
bypassed than the ratings of the bypass damper.
• Airflow into the ductwork might be reduced excessively,
resulting in insufficient conditioning.
• Because too much air can be bypassed, entering and leaving air
temperatures can range outside safe values for the heating and
cooling equipment, possibly resulting in equipment failures or
shortened equipment life. A direct bypass, unlike an indirect
bypass, does not remove Btu’s from the equipment, although it
does allow more airflow through the equipment.
Although it is commonly believed that the equipment is
protected as long as proper airflow is maintained, this belief is
true for an indirect bypass but not for a direct bypass. A direct
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