Figure 1-6: Round Bypass Damper - York Meridian Technical Manual

Zoning Design Guide
Zoning Design Procedures
mended that slot diffusers be located near the perim-
eter or outside wall with the airflow directed into the
room. Consult your diffuser supplier or catalog for
proper diffuser sizing and location.
Series fan boxes may be used instead of zone dampers
where higher induction rates are desirable. If the heat
loss on perimeter walls is high, such as large areas of
glass, the use of series fan boxes may be indicated to
maintain higher induction rates to offset "downdrafts."
If the heat loss is greater than 275 BTUH/LINEAR
FOOT, you should use high quality slot diffusers next
to the outer wall with the airflow directed inward to
counteract downdrafts during heating. Serious
downdraft problems occur when heat losses exceed 400
BTUH/linear foot and both high induction diffusers and
series fan boxes are recommended.
Step #5 - Bypass Damper Sizing
The function of the bypass damper is to allow a con-
stant volume air handling unit to be used with variable
volume zone dampers. The bypass damper modulates
on a signal from a duct static pressure sensor to "by-
pass" air from the supply duct back into the return air
duct. If the duct static pressure exceeds the adjustable

Figure 1-6: Round Bypass Damper

16
setpoint, then the damper opens to bypass more air, and
if the static pressure drops below the setpoint, it closes
to bypass less air.
Using a load calculation program, the bypass damper
should be sized to give you the maximum CFM of air
to be bypassed, typically 60 to 70 percent of the HVAC
units rated capacity.
To size the damper, select a damper from the table based
on calculated bypass CFM and a maximum velocity
between 1750-2250 FPM. When determining the by-
pass duct size, be sure to take into account any transi-
tion fittings and associated pressure drops. (See Tables
1-1 & 1-2: Damper Sizing Charts)
Whenever possible, use a single bypass damper and
round duct for the bypass. If space limitations or total
airflow requires it, multiple bypass dampers can be con-
trolled in parallel or a rectangular damper may be used.
For proper control of the Bypass Damper, the static pres-
sure sensor location is very important. Refer to Fig-
ures 1-8 Thru 1-10 for proper sensor installation loca-
tion information and guidelines.
Figure 1-7: Rectangular Bypass Damper & Kit
036-21478-001 Rev. A (0802)
Meridian Systems
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