Bypass Questions:

9.1
Q.
 What is a Bypass?
A.
 A Bypass System is a connecting duct installed between the supply and return ducts in order to relieve excess static pressure in the ductwork that is caused by the shutting down of all but one or two small capacity zones.
9.2
Q.
 Why do I need a Bypass?
A.
 A Bypass is necesary on many jobs because zoning will close various sections of ductwork.  This will cause high air velocity, and may result in increased air noise.
9.3
Q.
 What choices of Bypass do I have?
A.
 Your choices include:
  1. ModuPASS™ Modulating Bypass for variable speed blowers
  2. The Automatic Bypass Damper Control System (BPDC)
  3. Barometric Bypass Dampers (SPD series)
  4. The Wild Zone
  5. Blow-by Dampers
9.4 Q. What is the ModuPASS™?
A. The ModuPASS™ is a truly modulating bypass system.  It allows for very fine control. It was designed to compliment variable speed blowers.
9.5
Q.
 What is an Automatic Bypass Damper Control System?
A.
 The Automatic Bypass Damper Control System is installed in the supply duct to automatically monitor the static pressure in the ductwork and cause the Bypass dampers to open/close in order to control the pressure in the supply duct.  Set the enclosed pressure switch to the air pressure you wish to maintain.
9.6 Q. When should I use the ModuPASS™?
A. Use the ModuPASS™ when a bypass is required with a variable speed blower.
9.7
Q.
 When should I use an Automatic Bypass?
A.
 You should use an Automatic Bypass Damper Control if you encounter any of the following:
  1. Three or more zones,
  2. Zones of vastly different size,
  3. Unducted returns,
  4. The smallest zone is less than 15% of the total CFM, or
  5. Anytime you think you may need precise Bypass control.
9.8
Q.
 What are Barometric Dampers?
A.
 Barometric dampers are the familiar weighted arm type, relief dampers.
9.9
Q.
 When do I use a Barometric Bypass Damper?
A.
 Use a Barometric Damper with the 200 series control panels.  It will also be effective on simple systems with relatively even zones and good ducted returns.
9.10
Q.
 What is a Wild Zone?
A.
 A Wild Zone is a run or two that are left without a damper so that air circulates in them the entire time the fan is running. This adds capacity to your smallest zones and allows air to escape from the ductwork, effectively increasing the minimum capacity of the smallest zone's duct system.
9.11
Q.
 When do I use a Wild Zone or Blow-by dampers?
A.
 Wild Zones and Blow-by dampers may be used separately, or in conjunction with any other Bypass techniques.  They are used to dump some of the air into the conditioned space, or on jobs where space limitations preclude the use of a bypass damper. Be careful that you don't cause overconditioning in the zone where you dump air.
9.12
Q.
 What if I don't install a Bypass?
A.
 Without a Bypass System, you may try to push too much air into the available ductwork in a given zone(s). This may cause air noise at the registers.
9.13
Q.
When can I get away without a Bypass?
A.
 You may be able to get away without a Bypass if the zones' capacities are approximately equal, and the smallest zone can handle approximately 40% of the total CFM available.
9.14
Q.
 How do I size the Bypass?
A.
 Double the CFM of your smallest zone. Subtract that from the total CFM of the blowers. The difference is your worst case Bypass scenario. Calculate the size of the Bypass damper to be able to handle the difference at 2,000 ft/min. You may also view a PDF of the bypass sizing chart here.
9.15
Q.
 How do I install a Bypass?
A.
 Install a section of the appropriately sized duct between the supply and return trunks. Insert the Bypass damper into this piece of ductwork. The opening to the supply side must be before any trunk dampers and the opening to the return duct should be as far as practical from the equipment but before any reductionin the return duct size. This will allow for the supply air to mix with a maximum amount of the return air.
9.16
Q.
 Do I have to field adjust the Bypass pressure switch?
A.
 Yes. You must adjust the set-screw for the maximum static pressure you choose. See the instruction manual for procedure.
9.17
Q.
 Is there a minimum distance the return should be from the equipment?
A.
 We recommend at least eight feet from where the bypass feeds into the return duct to the equipment.  Shorter runs will increase the need for Leaving Air Temperature Controls.

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