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The Basics of Ventilation

Almost everyone has had the feeling of walking into a building and the air just feels stale, but have you ever wondered why? The root cause of this feeling is not having enough fresh air or ventilation air circulating throughout the space. Building codes strive to prevent this by putting requirements on the minimum amount of ventilation air that needs to circulate throughout a space. Unless the building system is 100% outside air, the air circulating a space is a mixture of recirculated air and fresh air. While there are many types of Heating, Ventilation, and Air-Conditioning (HVAC) systems that a building could have, the diagram below depicts a common example of an Air Handling Unit distributing air throughout the space.

Figure 1: The diagram depicts fresh, outdoor air being introduced into an air handling unit system that is then distributed to the individual spaces. This also shows the return air splitting between being re-circulated in the system or released outside.

Healthcare facilities are designed based on guidelines produced by the Facility Guidelines Institute (FGI) referencing ASHRAE 170. Spaces that fit into any of the categories listed in the FGI guidelines are based on air changes in each space. An air change is defined by the amount of times in an hour that the entirety of the air in a space is replaced with new air. There are two types of air changes that need to be taken into consideration, total air changes and ventilation air changes. Each space is required to have a certain number of each to meet the minimum code requirements. For example, a general examination room requires 4 total air changes and 2 ventilation air changes. A key factor in ensuring the correct amount of ventilation air is being distributed into a space is knowing the outside air (OA) percentage of a system. For example, you have a system with 33% outside air and your space requires 4 total air changes and 2 ventilation air changes (just like the general examination room mentioned above). In order for you to meet those 2 ventilation air changes, you’ll actually need that space to have 6 total air changes, based on the system’s outdoor air percentage. If you only design to those 4 total air changes in this system, only 33% of those 4 total air changes would be ventilation air (about 1.33 ventilation air changes).

For non-healthcare spaces, ventilation requirements are dictated by the International Mechanical Code (IMC). There are several factors that go into calculating the ventilation requirements for a space, such as area of the space and ventilation rate factors for each space found in Table 403.3.1.1 in IMC Chapter 4. These ventilation rate factors include occupant density, people outdoor airflow rate in breathing zone, area outdoor airflow rate in breathing zone, and exhaust airflow rate. Another important consideration in calculating the ventilation air requirements for a non-healthcare space is the Zone Air Distribution Effectiveness. This factor determines how effectively the air is distributed throughout the space based on the configuration of supply, return, exhaust, and/or makeup air within the space. Examples of these configurations can be found in the chart below, taken from IMC Chapter 4.

Figure 2: The Zone Air Distribution Effectiveness chart, per IMC Chapter 4 Table 403.

Most people only recognize the need for ventilation air when it is apparent that there isn’t enough of it in a building, however, it is a critical consideration for building engineers during design. In addition to ensuring the building occupants are comfortable, it is important to note the health impacts as well. Having adequate ventilation air removes moisture, odors, and pollutants from the air to create a healthier and safer environment for building occupants. Ventilation is a key factor in keeping building occupants safe and healthy and shouldn’t be overlooked, especially in the current state of the world.

Written by:

Erica Norquist

Mechanical Design Engineer



FGI Guidelines for Design and Construction of Hospitals, 2018 - (2018). MADCAD.

International Code Council (ICC). (2015). 2015 INTERNATIONAL MECHANICAL CODE (IMC) | ICC DIGITAL CODES. International Code Council.


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