100% Outside Air or Ventilation in a Zone

Thursday, November 10th, 2011

When calculating the outside air ventilation requirement for a zone using ASHRAE 62.1-2004 or the International Mechanical Code, the zone can require 100% outside air.

This high ventilation requirement is caused by one of the rooms in the zone requiring 100% outside air. The room typically has a small load in it that does not require very much air to condition. The supply air required for the space is lower than the ventilation air required, resulting in 100% outside air. If any space in a zone requires 100% outside air, the whole zone will require 100% outside air when using these calculation methods.

The solution is to increase the supply air required for the critical space. The percentage of outside air required in the space will go down, resulting in the percentage of outside air in the zone going down as well.

Here are the specific steps in Design Master HVAC to take to diagnose and solve this problem:

  1. Run the DM HVAC->Building Definition->Project Information command.
  2. Confirm that the Ventilation Calculation Method is not set to Minimum. If it is set to Minimum, then something else is causing the problem.
  3. Run the DM HVAC->Load Calculations->Print Preview command.
  4. Confirm that the Print Ventilation Schedule toggle is checked. Confirm the zone in question is selected to be printed.
  5. Press the Calculate and Print Loads button.
  6. Locate the Ventilation Schedule section of the print out. The critical room will be highlighted. This room is the one you will need to modify. The Discharge Outdoor Air Fraction should be 1 for this room. Check for other rooms that are set to 1. If there are any, they will need to be modified also.
  7. Run the DM HVAC->Building Definition->Rooms->Edit Rooms command.
  8. Use the Find button to select the room from step 6 that needs to be modified.
  9. Select the toggle next to Minimum Supply Air. Specify a value that is greater than the outside air required for the room. The outside air value can be found in the Zone Outdoor Airflow column of the print out from step 6.
  10. Run the DM HVAC->Load Calculations->Print Preview command again and confirm that the Outdoor Air Discharge Air Fraction for the critical zone is less than 1.

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