Bypassing Using Out Zones; Glossary - Bryant ZONE PERFECT PLUS ZONEBB2KIT Zoning Design Manual

bypass provides no protection for the equipment even though it
increases the airflow through it. Particularly for furnaces, a
direct bypass can harm rather than help by overheating internal
components. Therefore, the amount of air bypassed during
heating with a direct bypass must be limited.
• If bypassing results in excessive temperature in the supply duct
and a limit is tripped, the equipment shuts down. To protect the
equipment, the zoning system does not allow the equipment to
be turned on more than 4 times per hour. If the limit is tripped
repeatedly, the amount of heating or cooling delivered to the
zones might decrease dramatically. Thus, a direct bypass might
initially help the performance of a system during a period of
limited demand, but it might eventually result in an abrupt loss
of conditioning.
NOTE: A leaving air temperature (LAT) sensor is required in
every Zone Perfect Plus system. If the system is programmed
properly, this sensor ensures that the equipment turns off when the
leaving air temperature exceeds a safe range. Although it might be
tempting to omit the LAT sensor to increase the system's perfor-
mance, you risk eventual equipment failure. The equipment's
internal limits are not intended to be tripped repeatedly.
Similarly, if the LAT range is not programmed properly, you can
cause the equipment's internal limits to trip repeatedly. Again, you
risk eventual equipment failure.

BYPASSING USING OUT ZONES

Although the OUT zone feature can be used to provide energy
savings to the homeowner, its primary use is to give the system a
place to direct excess airflow to avoid tripping limits that protect
the heating or cooling equipment when the airflow falls below the
minimum required to operate safely. By closing an OUT zone's
damper(s) if the zone becomes cooler than the coolest zone (in
cooling) or warmer than the warmest zone (in heating), the system
avoids over conditioning the OUT zone. Thus, using OUT zones
for bypassing provides the advantages of bypassing to a dump
zone, but avoids the disadvantages.
The following scenario illustrates bypassing in a system that has
both a direct barometric bypass damper and several OUT zones.
This example illustrates the operation of the system in the cooling
mode. Keep in mind that the system would operate similarly in the
heating mode.
Imagine a zoning system is running in cooling mode and less than
1/3 of the zones have a cooling demand. The air pressure in the
ducts starts to build and the barometric bypass damper starts to
open. As the air recirculates to the return of the air handler and into
the evaporator coil, the leaving air temperature drops.
When the leaving air temperature, as measured by the leaving air
temperature (LAT) sensor, falls to 46°F, the system opens all OUT
zone dampers 3 positions. This action relieves the duct pressure
and may allow the bypass to close. If the temperature drops
another degree, the OUT zone dampers open another 3 positions.
The system continues to progressively open the OUT zone
dampers as the temperature falls until 1 of the following events
occurs:
• The leaving air temperature stabilizes and the OUT zone
dampers maintain position. (This result is the most common.)
• The demand is met for all zones and the system shuts down.
• The OUT zone dampers are fully open (position 15).
• The OUT zones become colder than the lowest cooling system
set point and their dampers close.
• In extreme cases, the leaving air temperature can fall to 40°F.
Usually this situation is caused by factors outside the heating
and cooling system, such as doors and windows left open when
the rest of the home is satisfied. In this extreme case, the limit
for the system is reached and the system shuts down the cooling
equipment.
Barometric Bypass Damper
Device used to control noise and maintain airflow in a heating or
cooling system during minimum airflow. When the supply airflow
reaches a preset value, the barometric bypass damper opens.
Block Load
Heating or cooling load of the home as 1 space.
Btu
British thermal unit. A standard unit of heat—the amount of heat
required to raise the temperature of 1 lb of water 1°F.
Bypass
Configuration that allows additional air through the system that
bypasses the zone dampers, the zones, or both.
Cooling Demand
The difference between the current temperature in a zone and the
zone's cooling set point (when the temperature is higher than the
cooling set point).
Condition
Process of ventilating, heating, or cooling (and sometimes humidi-
fying or dehumidifying) an area.
Dump Zone
Conditioned zone to which air is bypassed.
Equipment Controller
Device that interfaces with the home's heating and cooling
equipment so Zone Perfect Plus can control that equipment.
External Static Pressure
Measured static in inches wc from the inlet to the outlet of the air
handling unit.
Heat Pump Temperature (HPT) Sensor
Device that measures the temperature of air leaving the indoor coil.
Zone Perfect Plus uses this temperature under certain circum-
stances to adjust the operation of the heating or cooling system.
Heating Demand
The difference between the current temperature in a zone and the
heating set point (when the temperature is lower than the heating
set point.)
ICM (Integral Control Motor) Blower
Variable-speed blower. Air handlers using ICM blowers are
designed to supply a fixed quantity of air into a duct system
regardless of the external static pressure.
Leaving Air Temperature (LAT) Sensor
Device that measures the temperature of air leaving the air handler
in heating or cooling. Zone Perfect Plus uses this temperature
under certain circumstances to adjust the operation of the heating
or cooling system.
Multi-Stage Equipment
Heating or cooling equipment that can adjust the capacity in
response to the demand of the zoning system.
OUT Zones
Zones (usually unoccupied) that have been selected to receive no
conditioning for a period of time unless the temperature exceeds
85°F or falls below 60°F. Zone Perfect Plus also uses OUT zones
to relieve the heating or cooling equipment under overload
conditions.
Outdoor Temperature (ODT) Sensor
Device that measures the temperature of the air outside. Zone
Perfect Plus uses this temperature under certain circumstances to
adjust the operation of the heating or cooling system.
PSC (Permanent Split Capacitor) Blower
Single-speed blower. A PSC blower runs at a fixed speed and
cannot change speed as the dampers move.
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GLOSSARY