Air balancing is a fundamental design skill for HVAC engineers and consultants . Depending on the intended purpose of each construction area, it may require negative, positive or neutral pressurization. This is achieved by adjusting the supply and exhaust airflows: higher air supply causes positive pressure, while higher air exhaust causes negative pressure.
Although the ideal scenario would be to naturally ventilate all areas of the building, this is not possible in practice. For example, there is no way to use natural ventilation in areas completely surrounded by other rooms, as well as in underground levels. The purpose of ventilation can range from human comfort to facility safety: ventilation in residential and commercial environments focuses on providing air of breathable quality, while industrial ventilation is often used to keep dangerous gases away from certain areas or below a certain concentration.
Indoor spaces are subject to a lot of airflow and are typically measured in cubic feet per minute (CFM). We tend to think only about external air supply and exhaust air, but we consider that there is also unwanted air escape (exfiltration) and air gain (infiltration). Unwanted airflows typically occur around the edges of windows or doors.
Poorly balanced ventilation systems often lead to air quality problems. For example, negative pressurization can draw pollutants from above the ceiling or outside, and air can rush in when a window or door is opened.
Design a well-balanced ventilation system.
Calculation of intake and exhaust air
Before air balance calculations, it is important to know the required supply and exhaust air. There are many valid procedures, as indicated by the following codes:
- New York Mechanical Code , CH. 4 – Ventilation Using Table 403.3 Minimum Ventilation Rates
- ASHRAE 62.1 – Ventilation for acceptable indoor air quality
- In the case of hospitals, ASHRAE 170 – Ventilation of Healthcare Facilities
This article focuses on the New York Mechanical Code calculation procedure. To illustrate the concept, suppose a cellar is divided into the following areas:
| LIVING ROOM | AREA | AIRFLOW DIRECTION | MINIMUM AIR FLOW | DESIGN AIR FLOW |
|
Electric room Garbage Room Hall Store |
100 ft2 150 ft2 500 ft2 200 ft2 |
To supply Exhaust To supply To supply |
0.06 cfm/ft2 1 cfm/ft2 0.06 cfm/ft2 0.12 cfm/ft2 |
6 cfm 150 cfm 30 cfm 24 cfm |
Total air supply is 60 cfm while exhaust air is 150 cfm. Since the exhaust is 90 cfm higher, the result is negative pressurization. Increasing the supply to balance airflow is acceptable as the values given in the code are only minimum values.
Assume that all intake airflows are increased to the following values in order to avoid negative pressurization:
- Electrical room: 25 cfm
- Aisle: 125 cfm
- Storage: 25 cfm
This results in a total intake air of 175 cfm, which is greater than the 150 cfm of exhaust air. This means that the cellar areas are pressurized in relation to the waste room, preventing the spread of unpleasant odors. Because the airflow must be balanced at the end, the extra 25 cfm is released by exfiltration, but the trash odor is confined to the intended location.
Air Balance Troubleshooting
If a ventilation system has air balance problems, don't immediately assume the cause lies with the fans themselves. Consider that system components such as shock absorbers can be damaged and also that air ducts can become disconnected. When in doubt, the best recommendation is to obtain the professional opinion of an HVAC design engineer.
When ventilation systems are equipped with variable frequency drives for fan speed control, air balancing is simplified. VFDs can adjust the rpm of the supply and exhaust fans to match the ventilation load while keeping airflows balanced.
