On new construction or when inspecting an existing HVAC system that
will include an EWC Controls Zoning system, you must determine if
the duct is correctly sized to handle the volume of air delivered
from the HVAC system. You should also determine if the HVAC system
was sized correctly for the home or building. In order to do all
of that, a load calculation should be performed. Once it has been
determined that the HVAC equipment and the duct work are correctly
sized, then you may install an EWC Controls Forced Air Zone System.
Otherwise, HVAC equipment and duct-work issues should be addressed
prior to installing an Air Zone system. EWC provides Zone System
Design Guidance that predates and supplements the new ACCA Manual
Zr or if you prefer, use the ACCA Manual Zr for design guidance.
Since the zoned heating and cooling requirements will be controlled
through zone dampers, the concern is how to effectively manage the
airflow and volume resulting from closed and semi-closed zone dampers.
This means that as zones are satisfied, the airflow to those zones
will be completely or partially shut off. This creates a volume
build-up in the supply duct which can be dealt with using a combination
of tools that help manage airflow and volume:
Tools that help manage airflow and volume:
- If possible, specify Modulating or Multistage HVAC systems
when zoning. This allows the zone system to match System
capacity to the Zone demands.
- Select the correct Zone Control system for
that HVAC system and the number of zones you want to create. Consider
Timer staging or Thermostatic staging.
- If necessary, modify the existing duct and/or install
new duct that has been sized at a friction rate that will
accommodate higher Cfm volumes at normal static pressures and normal
velocities. (Use ACCA Manual D to size your duct-work or use a duct
calculator and select .07 friction rate instead of the typical .10).
- If possible, install Dampers in the Branch Runs, rather
than Duct Trunks. Now you can select which branch runs
to damper and which runs to leave alone (Open Runs). This method
provides airflow to certain areas every time the HVAC system operates.
(Bathrooms, Foyers and Washer&Dryer areas should not be dampened).
Dampening on the branch runs is more costly than on duct trunks.
- Allow the Zone dampers to leak 10 to 20% air volume
when closed. This small leakage can offset the heat gain
or heat loss in a particular zone and reduces bypass volume. Large
zones can leak more air than small zones.
- Install a Modulating Bypass Damper, Route the
bypass air back to the return duct or to a rarely used area of the
home. The Bypass Damper is the most common tool available to manage
airflow and static pressure in a zoned HVAC system.
- Install a Balancing Hand Damper in the Bypass Duct.
The Balancing Hand Damper allows you set sufficient Pressure Differential
across the bypass duct, preventing the bypass duct from being the
path of least restriction.
- Balance the System. All HVAC systems needs
to be balanced and an air zoned system is no exception. Use the
Zone damper itself to restrict or allow more flow to a particular
zone and/or install balancing hand dampers in the branch runs.
Combine several of these methods together to effectively manage
excess air volume. The HVAC system must be able to deliver the proper
volume of air to any and all of the new zones:
- 100% of the equipment rated airflow (Cfm) must flow through
the HVAC system at all times. Do not restrict return airflow back
to the HVAC system.
- Do not create numerous small zones. Two to four large zones
works the best. Too many small zones makes it more difficult to
- Try to create zones using areas & rooms with similar heating
and cooling loads. Do not combine rooms with drastically different
- When sizing the bypass damper, assume the worst case scenario
which is the smallest zone may be the only one to call at any given
- EWC Controls always recommends using the Electronic Bypass Damper
(Part# EBD) on all zoning installations, although Barometric Bypass
Dampers (Part# PRD and PRD-RD) work also, but are not as precise.
- Install a balancing hand damper in the the bypass
duct to slow the bypass air down and achieve a slow mixing of bypass
and return air.
- Use the Zone Damper's travel limit adjustment and set up to
20% leakage on large zones. Smaller zones can be set lower or have
no leakage at all.
- Whenever possible, group numerous zone dampers together on the
individual branch ducts, rather than installing single dampers on
the primary ducts. (Don't confine all the airflow too close to the
- Do not install dampers on branch runs serving bathrooms, washer/dryer
areas, foyers or any room that should have constant airflow. (Open
- Always use the SAS (Supply Air Sensor) to prevent the supply
air temperature from rising too high or falling too low.
- Make good use of and utilize the capacity control features available
on all Ultra-Zone Control systems such as:
- Timed or Thermostatic staging depending on preference &
- 50% Rule will inhibit Y2 until a sufficient number of
zones are active.
- Outdoor Air Sensing to inhibit Auxiliary operation in
- Return Air Monitoring will allow or prevent stage up
- Communicating Zone Control systems can minimize bypass
The most common tool for managing excess air volume in a zoning
system is the Bypass damper. This is done by tapping into the supply
air plenum, installing a Bypass damper and Balancing Hand Damper,
which allows you to route the excess air back into the return duct,
or divert the excess air into a rarely used area of the home that
has an open return grille. The Bypass graphic below reflects bypassing
into the return duct. Use and reference this graphic diagram when
planning and designing a system.
When tapping back into the return duct, the tap is recommended to
be at least 6 feet away from the equipment if you have the room
and clearance. This is done to ensure that the hot or cold air coming
off the plenum has ample time to mix with the return air before
going across the coil again. In addition, a Balancing or Restricting
Hand Damper should be installed in the bypass duct. It's the perfect
way to ensure sufficient restriction of bypass air-flow and proper
mixing of Bypass air with Return air.
Supply Air Temperature Sensors are mandatory when you install an
Air Zone system. The sensor will protect the HVAC equipment heat
exchanger from overheating and the DX coil from freeze-up conditions.
EWC Controls includes the Supply Air Sensor (Part# SAS) with every
micro-processor based Ultra-Zone control panel.
If the bypass method is used either exclusively or along with other
tools, the bypass duct should be sized to manage the airflow and
volume under the worst case scenario, which means the smallest Cfm
zone may be the only zone calling at any given time. That scenario
will cause the most volume build-up. The calculation is done by
taking the total Cfm capacity of the smallest zone and subtracting
that number from the total Cfm delivered by the HVAC system. If
you have airflow from Damper Leakage and Open Runs, subtract that
Let's try one together!
Example: 3 Zone system:
Zone 1 = 700Cfm, Zone 2 = 600Cfm, Zone 3 = 700Cfm
Total system CFM
Less smallest zone’s CFM
Less 20% Damper leakage Cfm
Less any Open run Cfm
Equals Bypass flow Cfm
Smallest Zone Cfm
700Cfm x 20% = 140Cfm x 2 = 280Cfm
(3) 6” branch runs @ 100Cfm = 300Cfm
The bypass duct would be sized to handle 800Cfm @ 900Fpm (feet per
minute) velocity (12”x12”). If you don't have room for that size
bypass duct, select a Round model that is close to 800Cfm (12”).
If you prefer a smaller size bypass damper, the air velocity will
be higher. Just make sure to install a Hand Damper also, so you
can adjust the pressure differential and slow the air down as it
enters the return duct. For typical By-pass Damper Cfm capacities,
see the chart below. Contact EWC Controls for specs. on other bypass
models and sizes.
12” x 8” EBD
12” x 10” EBD
12” x 12” EBD
20” x 10” EBD
20” x 12” EBD
CFM @ 900fpm
CFM @ 900fpm
As stated previously, there are several tools you may use to manage
excess air volume. EWC Controls recommends using at least two or
more of these tools in order to effectively manage airflow and volume
on any Air Zoned system. Use the Bypass Graphic as a guide when
designing your systems. Doing so will save you valuable time and
labor. Contact EWC Controls for Technical Support if you have any
questions or concerns. EWC Controls will help you make all of your
zoning installations Excellent Without Compromise.