Direct bypassing only slows the inevitable, the bonnet/plenum
temperature will get too hot or cold and eventually shut down the
equipment. Barometric Bypassing to an open ceiling or open foyer
is another option. With this type of zoning system, there are some
other options to Barometric Bypassing.
“Controlled Leakage” is a way to divert air to otherwise closed
zones if the smallest zone is the only one open. Each damper motor
has a setscrew to allow a MIN setting. By not allowing the damper
to close all the way, we have created a controlled leakage. This
works great for retrofit application, when ductwork may not be
able to be oversized as much as needed.
“Out Zones” are another alternative to eliminating excess air. This
principle works from the duct temperature. If the bonnet/plenum
becomes too hot or cold, the system will open an “Out Zone”.
Bottom Line: If the smallest zone plus any controlled leakage can
not handle 60 percent of the nominal CFM, then some type of
“Bypassing Option” must be considered. It may not be used very
often, based on patented damper movement (the system tries to
achieve setpoints in all the zones at the same time). When
designing with comfort in mind, rarely will only one zone be open,
while the others are closed. Bypass Determination will be
completed in Step 5 of the Duct Sizing Worksheet.
Installation of the Bypass Damper can either be a “Direct-Return”
or “Dump-Zone”. (See Fig. 2 and Fig. 3.)
Location of the Bypass Damper is very important. Listed below are
a few guidelines for proper location.
1. Location must be accessible for inspection and mainte-
nance.
2. Location must be in an area that has allowed the airflow on
the supply side to become smooth and allows bypassed air
to mix with return-air, before entering the equipment.
3. The leaving air temperature (LAT) must be installed up-
stream (ahead of) from the bypass inlet.
4. Do not locate too closely to an open return. The bypassed
air could cause the return to become positively pressured.
5. Consult Bypass Damper Installation Instruction for more
information.
B.
Task 6–Explanation of the Duct Sizing Worksheet
The Duct Sizing Worksheet will help size the supply ductwork for
a zoning system. Traditional methods, whether zoning was being
applied or not, have been to design duct work at .1 in. wc supply
and .08 in. wc return. But not everyone knew that this was based
on 100 ft of equivalent ductwork. Factoring in the equivalent
lengths of fittings could cause the Total Equivalent Length (TEL)
to go past 100 ft This could leave the ductwork undersized.
When zoning was to be applied to the system, we recommend 25
percent overissuing of the ductwork to handle the varying condi-
tions of airflow in the system. Some distributors/dealers have a
built-in “safety-factor” by designing the system with 30 percent
oversizing. Other manufacturers of zoning products have recom-
mended as much as 50–75 percent oversizing. In most cases, the
oversizing took care of any TEL’s over 100 ft.
The reason for this new Design Guide Worksheet is to help ease
the fear of designing a zoning system. The way the worksheet is
put together was to look at as many scenarios as possible, then
apply three design techniques to each example. Each scenario was
designed at:
1. 25 percent oversizing at .1 in. wc supply
2. 25 percent oversizing at .08 in. wc supply (to compensate
for TEL over 100’)
3. 30 percent oversizing at .1 in. wc supply (to compensate for
TEL over 100’)
In 99 percent of the applications, the ductwork sizes “crunched
out” to the same size.
So, if you have designed a zoning system in the past, use this guide
to see if the sizes match. If this if your first zoning design, have
faith that the sizes are not too large. Our patented damper
movement will adjust the airflow to where it’s needed. If you are
applying zoning to an existing duct system, compare what you
have to what you need. Then make the necessary adjustments to
the ductwork.
You should have completed Tasks 1–5 of the Duct Sizing
Worksheet by following Tasks 1–5.
NOTE:
The use of good take-offs and fittings are critical to the
TEL of any ductwork system. Take-off and fitting Total Equiva-
lent Length (TEL) examples are further explained in the Residen-
tial Air System Design (Catalog #791–443).
IV.
REFERENCE FOR DUCT SIZING WORKSHEET:
(SEE STEP 6 OF DUCT SIZING WORKSHEET, PAGE 9)
Using Table 1 determine the minimum Main Duct square inches
and the minimum Total Branch square inches by locating the
desired Zone CFM (from Task 3) along the left-verticle column. If
your desired Zone CFM falls between the listed CFM’s, use the
one closest to your calculated CFM. Follow the desired Zone CFM
across until you reach the Equipment Capacity, along the top,
required for your application. Write these values for each zone in
the area provided in Task 6.
NOTE:
If two or more zones share a main duct (see Fig. 6) then
add the zone CFM’s together and use that CFM to size the Main
Duct. Then as each zone is “branched-off”, it becomes the zone
“Main Branch” and would be sized based on the individual zone
square inches.
The listed areas (square inches) will provide a maximum zone duct
pressure drop of approximately 0.1 in. wc / 100 ft and a maximum
of 900 fpm for main ducts and 700 fpm for branch ducts for sheet
metal ductwork. For “Duct board”, multiply areas by 1.1, or for
“flex-duct” multiply areas by 1.25 to maintain same duct pressure
drop and fpm. If CFM in Step 3 is based on cooling and the design
is NOT based on 400 CFM/ton, divide that CFM by 400. Multiply
the areas by this number (Example: a system design of 350
CFM/ton of airflow, multiply areas by: 350/400 = .88).
NOTE:
The “grey-shaded” boxes represent zone CFM’s of less
than 20 percent of the total CFM. The square inches were
increased approximately 10 percent to help deal with situations
where limited zones may be open.
Fig. 2–Direct Return Installation
A00190
BYPASS
LEAVING AIR
TEMPERATURE
SENSOR
INDOOR
UNIT
SUPPLY AIR DUCTWORK
RETURN AIR DUCTWORK
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