BY-PASS SIZING CALCULATIONS
EWC
Controls
Inc.
385
Highway
33
Englishtown,
NJ
07726
800-446-31
10
F
AX
732-446-5362
E-Mail-
[email protected]2
The PRD must be installed horizontal & level between the supply duct and the return duct.
Position PRD door to open in the direction of your airflow by forcing it over the stop.
Make the PRD connection on the return so that the bypass air has a minimum 6 feet of return duct before it
enters the air handler, if space permits. If not do your best.
Position ARM in the “IDEAL ARM LOCATION” for the direction of your air flow and tighten the THUMB
SCREW. (Refer to Page 4)
Position the WEIGHT high up on the ARM and tighten the THUMB SCREW
Energize ALL Zones to operate the unit with the Fan running on the Highest speed (Usually a Cooling
demand, 2nd stage if applicable).
Return to the PRD & reposition the WEIGHT lower on the ARM until the PRD just closes completely. (It
should be closed to the point where any additional force will open the PRD).
Turn off all Zones but the Smallest Zone & Wait about 45 seconds.
Monitor the airflow for the Smallest Zone. Is there to much Velocity? Proceed to Step 9.
Is there insufficient Velocity? Proceed to Step 10.
Is the Velocity acceptable? Proceed to Step 11.
To Much Velocity: Reposition the WEIGHT Higher up on the ARM until the velocity has subsided to an
adequate level. If the velocity remains to high, reposition the ARM towards ‘6’ on the scale. Proceed to Step
11.
If the velocity is still an issue, double check your By-Pass size, calculations are on Page 1.
Insufficient Velocity: Reposition the WEIGHT Lower on the ARM until the velocity is at an acceptable
level. If the velocity remains to low, reposition the ARM towards the ‘9’ or ‘3’ on the scale (Depending on
the direction of airflow). Proceed to Step 11.
If velocity still remains low for the Smallest zone, to much air is being forced through the PRD, an Electronic
By-Pass Damper (Model EBD) may need to be considered. Contact Technical Support.
Congratulations, your PRD By-Pass damper is now properly set-up.
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BAROMETRIC BY-PASS SET-UP PROCEDURE
Start with the system’s Total C.F.M.
Calculate the C.F.M. for the smallest zone that can run by itself.
Subtract the Total C.F.M. from the Smallest Zone C.F.M.
The left over amount is the By-Pass C.F.M. Refer the By-Pass C.F.M. to the chart on Page 1, match to the
closest size. DO NOT Undersize the By-Pass Damper. The By-Pass can also be sized on a Duct Calculator
at 0.3”w.c. Friction per 100ft of Duct. (For quieter By-Pass operation, size the By-Pass Damper at a lower
Friction Value).
EXAMPLE: 5 ton system @ 400 C.F.M. Per ton = 2000 C.F.M 2000 C.F.M.
Subtract Smallest Zone = 500 C.F.M. -500 C.F.M.
1500 C.F.M.
1500 C.F.M. @ 0.3” Friction per 100ft. of duct = PRD 20 x 8
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