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High Ventilation Loads in a Zone

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

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.

Solution​

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

  1. Run the Print Load Calculations command.

  2. Check the box next to Print Ventilation Schedule and select the zone from the list.

  3. Press the 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 Zd for the critical room is less than 1, the high ventilation load is caused by something else.

If the Discharge Outdoor Air Fraction Zd 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.

  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  button to save your changes and close the dialog box.

Setting a ☐ Maximum Room Ventilation Percentage will increase the required supply air when necessary to maintain the specified ventilation-to-supply percentage.

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