Duct Sizing With Ultra-Zone
When laying out a duct system for a job that will include an EWC Controls
zoning system, many people think that there needs to be drastic changes to
accommodate the zoning feature, this is not so. The ducts still need to deliver
an adequate amount of CFM to the zoned area as they would if it were not zoned.
The square footage of the home or building is not changing. What will change is
the amount of square footage that will need to be heated or cooled at the same
time. With zoning allowing for setback thermostats and different temperature
settings throughout the home or building during the day and night, the demand on
the heating and cooling equipment should be a lot less than it would be without
an EWC Controls zoning system. The demand is less because the amount of square
footage will be less due to zoning off areas that will not need to be heated or
cooled during certain times of the day.
Since the heating and cooling requirements are now going to be controlled
through zones, the concern is the excess pressure or build up of pressure from
closed zones. What this means is that when some zones are in setback mode, the
airflow to those zones will be shut off. That airflow will cause a pressure
build up in the duct system which will be dealt with by using a form of "Bypass"
to alleviate that pressure build-up. There are several ways of dealing with this
excess pressure and what works for one contractor may not work for another, but
EWC has some long standing rules that should be followed. These rules are
important to follow to insure proper airflow throughout the system and to insure
proper operation of the EWC Controls zoning system.
- 100% of the rated CFM must always move through the duct work when
zoning with Heat Pumps and High cooling demand areas. (i.e. Florida)
- Always assume the worst case scenario, which is the smallest zone will be
the only one to call at any given time.
- 65%-70% of the rated CFM should always move through the duct system with
standard heating systems.
As stated previously, there are several ways to accomplish these rules. EWC
Controls recommends two types of methods for two different types of
applications. The first application is on 2 and some 3 zone jobs. On these jobs
it is possible to oversize the supply ducts to handle the increased pressure of
closed zones. For example, oversize each zone supply duct to handle 65% -70% of
the CFM. This way when the other zones close, the one zone calling can handle
the pressure safely. The supply ducts should never be oversized more than 20% of
their intended capacity. (This method will reduce the velocity of the system so
it is important to know your customer’s expectations of the system.)
Another possible method is the Bypass damper, this method will take the
excess pressure from the supply duct and return it back to the return duct. This
is done by tapping into the supply air and running a duct back to the return air
and mounting a bypass damper in that run. (See below)
When tapping back into the return duct the tap is recommended to be at least
6 ft. away from the equipment the farther the better. This is done to insure
that the hot or cold air coming off of the plenum has time to mix with the
return air before going across the coil again. Temperature sensors are mandatory
when using the bypass method. The sensors will prevent any damage to the
equipment from overheating or coil freeze-up. EWC recommends the use of the SAS
(Supply Air Sensor) with the Ultra-Zone control panels.
If the bypass is used, it would be sized to handle the excess pressure build up under
the worst case scenario rule. The reason we use the worst case scenario is that
this is the worst possible condition for the airflow. This condition will cause
the most excess pressure build up that is possible. The calculation is done by
taking the total CFM capacity of the smallest zone, let’s say 600 CFM, and
subtract that from the total CFM of the system, let’s say 2000 CFM.
Total system CFM
Less smallest zone’s CFM
The bypass duct would be sized to handle the 1400 CFM which
would be the excess pressure when only that one zone calls. The bypass method
must be used on zoning systems that will have unequal size zones, applications
of 4 zones or more and might be needed on some jobs that will have a smaller
number of zones. This will be determined by the contractor at the time of
installation. For by-pass damper CFM capacities see the chart below.
A third method, which is used quite frequently is called the "Dump Zone".
This method will still use a bypass damper, but
it will not go through the
return duct. The dump zone will take the excess pressure build-up and dump it
into a non critical area of the home. Non-critical can be interpreted many ways,
what EWC Controls has seen most commonly used are basements, entrance foyers,
work shops or mechanical rooms. These areas are considered non-critical because
they are non-living areas were temperature control is not as important. This
method should be used responsibly by the installer. Determining what is a
non-critical area is not to be taken lightly, this should be a thought-out
process keeping the homeowners needs and expectations in mind.
Using some method of bypass is the most effective way of zoning a home or
building. Bypassing will accommodate the homeowners or occupants demand for a
controlled environment. Some Zoning systems call for bypassing the air into a
zoned area, this will NOT allow for maximum comfort. This method allows the
bypassed air to over shoot the thermostats temperature set point. What this type
of system does is it will leave the zone dampers partially open when they should
be fully closed, allowing for the bypass air to flow into an already satisfied
zone. This will negate the effect of zoning and drastically reduce the comfort
level, and savings potential of the home. Zoning is the control of the airflow
from the HVAC equipment that allows for Individual Room Temperature Control.
With proper installation there is no reason why an individual room can not
maintain a I degree differential from set point.
Duct design will be uniquely different from one zoning job to another and
because of that, unfortunately there is no single way of laying out a zoning
system. What EWC Controls has tried to do is to give you the critical
information that is needed before a zoning system can be installed. We have also
tried to show a few common techniques that are being used by others who are
installing the EWC Controls zoning systems. These are just a few suggestions,
other methods and techniques can be used if they are more successful for your
Bypass dampers and their CFM capacity
12"x8" PRD 1,000 CFM
12"x 10" PRD 1,200 CFM
12"x12" PRD 1,400 CFM
20"x 8" PRD 1,600 CFM
20"x 10" PRD 2,000 CFM
20"x 12" PRD 3,000 CFM
8" PRD-RD 400 CFM
10" PRD-RD 750 CFM
12" PRD-RD 1,200 CFM
14" PRD-RD 1,800 CFM
16" PRD-RD 2,400 CFM