HVAC Zoning Bypass Damper – More than you ever wanted to know

Bypass Damper [And How it Works] – TECHNICAL WARNING!
A bypass duct is a short duct placed between the supply plenum and the return air plenum.  The purpose of the bypass duct is to allow the excess pressure from the supply plenum to escape into the return air plenum. Excess pressure can occur in the supply plenum when less than all the zone dampers are open. This excess air must pass through a bypass damper that is mounted in the bypass duct. Excess pressure is restricted from freely flowing back to the return air plenum by adjusting the bypass damper. A bypass damper must be adjusted so that when all zones are open the bypass damper is closed, but lightly closed so that as supply dampers close and pressure mounts in the supply air plenum; this pressure pushes the bypass damper open. Excess pressure escaping through the bypass damper is taken back into the return air system and re-circulated through the equipment.
There are TWO TYPES OF BYPASS: Barometric and Modulating, see below.
Modulating Bypass (Static Pressure Controlled)

Zoning Bypass Duct and Damper Diagram

The static pressure operated bypass requires the use of a motorized power open/power close modulating damper. As pressure increases in the supply plenum, the static pressure sensor will register this increase and power the motorized damper open slowly so as to relieve the excess pressure. As the pressure is removed, the static pressure sensor may register less pressure in the supply plenum, consequently powering the bypass damper closed so as to make pressure. This constant hunting for the correct pressure requires a motorized damper that moves from open to close in approximately 35 seconds. In this application, no dead band is required.

Set-Up and Adjusting a Modulating Bypass Damper with Static Pressure Controller
1. Power-open/Power-close damper can be mounted in any position in the bypass duct.
2. The static pressure controller (SPC) must be mounted so that the diaphragm is in the vertical position (6 to 8 inches of the pitot tube.)
3. The pitot tube should be inserted into the supply plenum between 2 and 4 feet from the beginning of the plenum. Insert the pitot tube perpendicular to air flow. The purpose of the tube is to measure AVERAGE supply air plenum pressure and report this pressure to the static pressure controller.
4. Attach the provided 12 inch rubber tube to the static pressure controller on the inlet barb labeled High-Pressure Inlet. Attach the other end of the 12 inch rubber tube to the installed pitot tube.
5. Wire the SPC, PO/PC Damper, included relay and a field supplied transformer as shown the included wiring instructions below
6. Start the indoor blower in high speed and make certain all zones are open.
7. Using a flat head screw driver, turn the set screw on the SPC clockwise ¼ turn so that the motor continues to power the damper closed. If the damper reverses itself and starts to open, turn the set screw another ¼ turn clockwise until the bypass damper has closed and is remaining closed.
8. Slowly turn the set screw counter clockwise until the damper starts to open. Immediately reverse the motor by moving the set screw clockwise slightly so that the position is located where the damper is barely staying closed with all Zones OPEN. This becomes the setpoint for the static pressure controller.
9. As the supply dampers open and close the SPC will constantly hunt to maintain the static pressure thatwas established during the above start up procedure.
10. When all zones are satisfied and the indoor blower is de-energized, make certain that the bypass damper is powering open so that when it begins its next cycle, the bypass damper will start modulating from the OPEN position. If the relay is not wired correctly, the damper will close. On the next cycle with only one zone open, the bypass will start from the closed position and cause several seconds of excess air noise in the zone that is open.

Barometric Bypass
A barometric relief damper is not motorized and houses a blade in the bypassed air stream attached to a shaft.  The blade is opened as air pressure in the supply plenum builds and escapes through the bypass duct. As the damper blade opens the attached shaft rotates. The shaft extends outside the damper. There is a shaft collar with a ½ inch hole that slides over the end of the ½ inch shaft screws through the side of the shaft collar and tightens onto the shaft. When weights are applied to this bolt, it is used as a weighted arm. The shaft collar allows the weighted arm to be positioned at any location (360 degrees) on the shaft. By sliding the weight toward the end of the bolt, leverage is applied to the attached shaft, closing the blade in the bypassed air stream and adding restriction to the bypassed air flow.
Set-Up and Adjusting a Barometric Bypass
1. Make certain that the arrow label on the barometric bypass damper is pointed in the direction of air flow; from the supply plenum toward the return air plenum.
2. Make certain that the shaft is located horizontally in the air stream and the blade is actually hanging down from the shaft inside the damper. This insures that the damper will swing open from the bottom and pivot from the shaft.
3. The barometric bypass damper shaft extends 3 inches on both sides of the damper. This allows room to insulate the bypass damper completely and still allow the weighted arm to pivot freely from the end of the shaft.
4. An eight inch long bolt with a ½” hex head is used as the weighted arm. Tighten the ½” hex head bolt that is threaded into a shaft collar that slides over the end of the shaft. Loosen the bolt slightly to allow rotation of the shaft collar on the shaft. This allows positioning of the weighted arm a full 360°.
5. Locate the weighted arm at approximately the 8:00 o’clock position or the 4:00 o’clock position depending on which direction the air is moving through the bypass damper. This weight should add restriction to the air moving past the blade inside the bypass damper. Retighten bolt onto the shaft and tighten the locking nut positioning the shaft collar onto the shaft.
6. As the air pressure increases the blade inside the damper will be forced to pivot open which will turn the shaft. The weighted arm attached to the shaft should be lifted from it’s at rest position.
7. The weight that slides up and down the 8” arm allows more or less leverage or weight to the arm. The weight includes an Allen screw so that an Allen wrench will tighten the weight after it is in position.

More helpful links:
Barometric Bypass Damper
Modulating (Static Pressure) Bypass Damper

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