You are here: Home > Support > Knowledge Base > High Ventilation Loads in a Zone

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.

Related Articles

Filter by
Post Page
HVAC Load Calculations
Sort by

ASHRAE Glass Zone, Roof, and Wall Types

These charts illustrate the difference between the available glass zone, roof, and wall types defined by ASHRAE.

24

Roof, Wall, and Glass Zone Type Tables

The roof, glass, and wall types defined in DM HVAC are taken from these tables in the 1997 ASHRAE Fundamentals.

18

Ventilation Calculations

Design Master HVAC offers two methods for calculating the ventilation air required in a zone: minimum, and ASHRAE 62.1.

14

People Diversity in Load Calculations

The people diversity for a zone can be set using the People Diversity section of the Zones dialog box.

14

Correctly Using ASHRAE 62.1 or IMC Ventilation Calculations

When calculating ventilation rates, make sure you set the calculation method in Project Info as well as set the ventilation rates for rooms types according to the code being used.

8

Missing Roof Load

If your calculated roof load looks wrong, use the Edit Room command to make sure the roof load isn’t being categorized as a plenum return.

8

Products

Electrical for Revit

Electrical for AutoCAD

HVAC for AutoCAD

Photometrics for AutoCAD

System Requirements

Support

Contact Support

Install & Update Instructions

Online Training

Knowledge Base

User Manuals

Tutorials

Release Notes

Downloads

Wish List

Remote Support

Company

About

Customer Stories

Awards

Newsletter

Blog

Pricing

Electrical for Revit

Electrical for AutoCAD

HVAC for AutoCAD

Photometrics for AutoCAD

Free Trial

Electrical for Revit

Electrical for AutoCAD

HVAC for AutoCAD

Photometrics for AutoCAD

©2001-2020, Design Master Software, Inc