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High Ventilation Loads in a Zone

Thursday, November 10th, 2011

Q: Why is the ventilation load calculation in one of my zones so high?

A: Very high ventilation values can occur when the required ventilation for a room exceeds the minimum required supply air, which will cause some load calculation methods to fail.

Solution

First, follow these steps to confirm that the problem is being caused by ventilation exceeding supply in a room and not something else:

  1. Run the Print Load Calculations command.
    • Ribbon: DM HVAC->Load Calculations-> Print Load Calculations
    • Pulldown Menu: DM HVAC->Load Calculations->Print Load Calculations
  2. Check the box next to Print Ventilation Schedule and select the zone from the list.
  3. Press the Calculate and Print Loads button.
  4. In the ventilation schedule for the zone, find the critical room, highlighted in bold. Load calculations for the zone are most heavily influenced by the critical room.

If the Discharge Outdoor Air Fraction for the critical room is less than 1, the high ventilation load is caused by something else.

If the Discharge Outdoor Air Fraction for the critical room is 1, you need to set a maximum ventilation-to-supply ratio for the project using these steps:

  1. Run the Project Info command.
    • Ribbon: DM HVAC->Building Definition-> dm_hvac-project Project Info
    • Pulldown Menu: DM HVAC->Building Definition->Project Info
  2. Check the box next to Maximum Room Ventilation Percentage, then enter a value less than 1 in the field provided (e.g., 0.25 for 25% outside air).
  3. Press the OK button to save your changes and close the dialog box.

When you recalculate the loads, the ventilation load should be significantly lower.

Cause

For rooms that require little conditioned air, the minimum required ventilation from outside air can exceed the minimum required supply air, resulting in a ventilation-to-supply percentage of 100% (or higher). The ventilation calculations used by  ASHRAE 62.1 and the International Mechanical Code are not designed to handle that percentage, resulting in a zone that “needs” unrealistically high ventilation.

Setting a Maximum Room Ventilation Percentage will cause the software to increase the supply air to maintain the specified ventilation-to-supply percentage.

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