The Importance of Air Balancing and Pressure Relationships

Air-based heating and cooling systems are very common in the US; there is a good chance you are sitting in a space right now that is heated or cooled using air. Understanding these systems at a conceptual level is easy. During the winter, hot air is delivered to space. The delivered hot air is warmer than room temperature, and when it mixes with the rest of the air in the space, the overall air temperature increases. Similarly, during the summer, cool air is mixed to decrease air temperatures (this is an overly simplified explanation of cooling; I am trying to avoid the rabbit hole of psychometrics). Air enters the space through a diffuser, which could be located in the floor, ceiling, or wall, and leaves through a grille. In this article, we will keep our focus only on the topic of air distribution.

Air distribution systems typically include a fan, ductwork, duct accessories (such as dampers), and air terminals (typically diffusers or grilles). When these systems are designed, a path is created that will deliver the desired amount of air to each space. Let’s look at the following scenario. A heating calculation indicates that an office needs 100 cubic feet per minute (cfm) of 120°F air to maintain 70°F indoor temperature during the coldest days of the year. A path is then designed to move the treated air from an air handler to the office; ducts are sized accordingly, and a fan is selected that can overcome the pressure and make sure the heat production raises the supply air temperature to 120°F. Additionally, the position of the air diffuser is chosen to create a natural mixing of air in the room and avoid furniture.

When we do our job well, these systems look great on paper. However, real-world conditions can vary from our perfect engineering world. For example, the HVAC contractor may need to install a few extra elbows to avoid a drop beam. Now, the static pressure in the system is greater than our initial expectation, and as a result, the office only receives 80 cfm of air. This out-of-balance system can have several downsides.

When a space receives less air than designed, it may end up being too hot in the summer or too cold in the winter because it doesn’t receive enough air to properly condition the space. The space may also need more fresh air to meet the code minimum. If the system was designed to have 20% outdoor air, then our initial 100 cfm total air would include 20 cfm of fresh air. If we only deliver 80 cfm to the room, we only provide 16 cfm of fresh air. The inverse problems occur when a space receives more air than designed. The extra air will overheat and over cool the space. If the space is too hot or cold, the system may try and compensate by running more frequently, causing added stress on the equipment with lots of cycling and ultimately reducing the lifespan of equipment.

Another important consideration during balancing is the pressure relationship between adjacent spaces. A space can be under positive, negative, or neutral pressure. Positive pressure spaces are like full balloons with air pushing out against floors, walls, windows, and doors. Unlike balloons, spaces are not airtight. Air from positive pressure spaces will typically leak into adjacent spaces (or outside) by sneaking under doors, through walls, or other gaps. Neutral spaces are considered in balance and do not force air movement in or out. Negative pressure spaces are like vacuums, trying to pull air into them. The air typically sneaks into these spaces using the same pathways described above. Often, we intentionally design our air-based systems to create positive or negative pressure in a space. Bathrooms should almost always be under negative pressure because we want to exhaust humidity and odors from bathroom air to the outside; if a bathroom is under positive pressure, that air will leak into adjacent spaces (hallways, bedrooms, etc.). We like to design hallways in multifamily buildings to have a slight positive pressure because the positive pressure will prevent contaminants from occupants (excess perfume, dog smell, smoke from burnt toast) from leaking into the shared hallway.

In manufacturing and healthcare buildings, pressure relationships are often even more critical. In a healthcare setting, pressure relationships are used to control airborne diseases, dust, and other contaminants. ASHRAE’s Standard 170 offers a detailed design guide and assigns a required pressure relationship for each type of space. For example, a “clean workroom” must be under positive pressure, while a “laboratory workroom” must be under negative pressure. Negative pressure is used to prevent airborne contaminants from escaping a space, and positive pressure helps to ensure air delivered to a space is clean. In a manufacturing facility, many machines and processes require specialty exhaust to capture contaminants and chemicals. Rooms, where hazardous materials are used or stored, should be under negative pressure to prevent chemicals from leaking into adjacent spaces. It’s also important to assess the overall pressurization in a building so it can be understood if the air will be pulled into the envelope or leak out of it. If there are a lot of rooms where negative pressure is needed, providing other areas with positive pressure is typical. This allows the “extra air” in the positive pressure spaces to be pulled into the negative pressure areas. The correct pressure relationships are only achieved when the system provides the designed air flows.

The best practice to ensure air-based systems perform as designed is to include testing, adjusting, and balancing work (also known as TAB) during the commissioning process. When the system has been designed well (i.e., ductwork and fans sized correctly), it should naturally come close to delivering the correct amount of air to each space, but it is unlikely to be perfect without tweaking. During the TAB process, the air-based system is turned on, and a technician is able to measure how much air is delivered at each supply register and returned at each return grille and will indicate if a space is under positive, negative, or neutral pressure. Then there are several ways a technician can adjust the airflow:

• Setting the position of balancing dampers
• Adjusting fan speed or fan pressure settings
• Verifying duct sizes

In the photo below, note the pink tags marking balancing damper handlers on each branch of the supply air ductwork. In some cases where airflows are far off the desired amount, it may be necessary to add dampers or even change the duct size. The TAB processes will also make sure outdoor air intake and exhaust flows are correct and ensure the operation of economizers or outdoor air dampers. Often, the TAB work is performed by an independent party that produces a report for the building owner. While many folks may consider TAB an extra expense to be cut out, it is the only way to ensure that an air-based system can perform optimally. Hopefully, that means the occupants are comfortable.

Nate Rogers P.E.

Staff Engineer