Page 9 - Zone Control Product Guide

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9
Determine Bypass Method
Controlled Pressure Relief method:
the unique Aprilaire
®
method of zoning that doesn’t require a
bypass damper and, therefore, minimizes system costs. The Controlled Pressure Relief method
assures a minimumof 85% of design airflow across the evaporative coil at all times, while not affecting
the temperature of non-calling zones by more than 1°F, assuring system performance and comfort.
By following our engineered design methods, which include slightly over-sizing the duct system and
utilizing Aprilaire factory-preset dampers, we relieve pressure into non-calling zones without affecting
the temperature (see Form #5001 for design details). Controlled Pressure Relief is perfect for
residential new construction applications.
Positives:
assures proper airflow; simplifies installation; lowers system costs.
Negatives:
may not be applicable to existing systems; each zone must represent at least 25% of
total system airflow.
Bypass Damper method:
allows the system designer to size the ductwork in the same manner as
a non-zoned application. The pressure relief is handled by use of a bypass damper directing
excess airflow back to the return or to a dump zone. This is the most common method when
adding zoning to an existing home.
Determine bypass damper sizing:
the purpose of the bypass damper is to maintain airflow
through the HVAC equipment when zone dampers close. Sizing the bypass damper is calculated
considering worst case scenario. Subtract the air flow of the smallest zone from the total system
airflow and the difference is how much air you need to bypass.
(
Total system CFM - Smallest
zone CFM = Amount of air to bypass).
Then, refer to the bypass damper sizing chart for the right
size bypass damper.
Positives:
utilizes existing duct design methods; good retrofit option.
Negatives:
potential of bypassed air conditioning the return or dump zone; requires plenum
temperature control (included with Aprilaire zone panels).
Oversize method:
entails sizing the ductwork (main and branches) in each zone to accommodate
approximately 70% of the total system airflow.
Positives:
no bypass damper is needed; low likelihood of air noise.
Negatives:
cost (duct materials); requires additional space; can result in lack of velocity or “throw”
from the registers.
Selecting Zone System Components
The following pages will assist you with selecting the right components for the job.
Select control panel based upon:
Type of equipment to be controlled (heat/cool or heat pump, single- or multi-stage)
Number of zones to be controlled
Additional feature needs (i.e., customer would like the ability to change thermostat
settings from any thermostat, in which case you would choose the Model 6504)
Select the correct type and size dampers for the job
Select the correct thermostats