Balancing a Zone System Bypass Duct


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Many traditional zone damper systems have bypass ducts. When bypass ducts are sized too large they generally allow too much supply air to flow back into the return. Obviously, this can cause operational temperature-related problems for the HVAC system. Additionally, the amount of supply air going to the zones is reduced causing temperature control and comfort problems.

Bypass ducts are designed to return supply air directly back into the return trunk when a zone closes down. This reduces overblow and the resultant noise issues in the open zones. However, many bypass duct linkages do not include a manual (hand) balancing damper as called for in ACCA Manual Zr. Thus, too much air returns through the bypass damper when the zones close down. The solution is to measure the airflow with zones closed and then to install a hand balancing damper and balance the bypass airflow.

The basic procedure for setting the airflow through a bypass duct uses static pressure (SP) measurements and equipment manufacturers (OEM) tables or charts. The OEM info includes the following items:

  • The air handler or furnace unit’s maximum and minimum airflows for heating and for cooling.
  • The air handler or furnace unit’s tables or charts that show the external static pressure (ESP) relationship to the units airflow in cubic feet per minute (CFM).

Once the minimum and maximum airflows for heating and for cooling are known, the bypass duct can be easily balanced. The set of steps below are designed to prepare the equipment for testing:

  1. Make sure the system is operating in as new as possible condition; coils & blower clean with a new air filter.
  2. Make sure all of the system supply registers and return grilles are wide open.
  3. Make sure the damper(s) in the bypass duct are closed.
  4. Make sure any makeup or outside air duct that is attached to the system is sealed or closed off so no outside air can enter the return ducting (leave closed until balancing completed).
    After preparing the equipment for testing, turn the system on to full-load cooling (for cooling mode, condenser may be off it is winter; for heating, the gas valve etc. may be shut off). Be sure to set the thermostat so the equipment will keep running, wait 10 minutes for the system to stabilize and then do the following:
  5. The Heating and Cooling Mode ESP measurements are then compared to the OEM tables or chart values and converted to CFM values for cooling and heating modes. Once the CFM values are known, determine if the highest CFM range is for heating or for cooling.
  6. Measure the ESP (see Figure 1 above)
  7. Then measure the static pressure of the supply trunk before any take offs that may remove airflow from the duct system (this is often done at the supply duct ESP test hole by removing the return SP hose from the test meter).
  8. Move the thermostat to the maximum heating position and repeat steps 6 & 7 above.
    Note: be sure to write down the ESP and SP values for each step.
  9. The system is then placed in the operational mode that requires the most CFM, and the thermostat is again set to keep the system operating during the balancing. After the HVAC system has stabilized (operated 10 minutes), do the following:
  10. Shut down all of the zones except for the one with the least designed airflow. Note: Manual ZR provides guidance on how much bypass airflow is allowable. The smallest zone should be designed accordingly.
  11. Open the bypass damper(s).
  12. Re-measure the SP on the supply trunk.
  13. Adjust the manual/hand damper on the bypass duct until the SP on the main trunk is back to the original value it had in the 1st test.
  14. Lock down the manual damper on the bypass duct and make sure the SP is still equal to the original value (if not readjust the manual damper and lock down again until it is, and then mark it too).

Before leaving, the zone thermostats/controls should be turned back to their normal operational setting, outside air dampers should be opened back up, and the ESP should be checked in both heating and cooling modes with only the smallest zone open to make sure the balanced airflow CFM falls within the OEM’s minimum airflow requirements for the unit, and meets Manual Zr requirements.

Don Prather

Posted In: ACCA Now, Technical Tips

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