Ductwork Air Flow Balancing

One of the most important requirements for the design of a duct system is the possibility and simplicity of air flow balancing. The system pressure gets balanced to the point where the fan pressure generation is equal to the sum of the pressure losses through each section of a branch. This is true for each system branch.

Another interpretation of the air flow balancing is that pressure losses need to be balanced at each junction. If the sum of the pressure losses in a branch does not equal the fan pressure, the duct system will automatically redistribute air, which will result in air flows that are different from those designed.

Designing a duct system means sizing the ducts and selecting the fittings and fans. Duct sizing is not the same as making pressure loss calculations, although the two are commonly confused. Balancing air systems may be accomplished in various ways.

The most common method to accomplish ductwork balancing is Proportional Method (Fig. 1). Each diffuser is adjusted to supply the right percentage of total air volume in the ductwork. Balancing is performed step by step based on the fact that the operation of each diffuser is dependent on the previous one. Balancing is made based on the ratio between measured and designed airflow rate. Most of the supply and exhaust ducts can be adjusted based on proportional method.

Proportional method process follows several steps. Balance the duct work in the following fixed order:

• Ensure that all VCD, VAV and fire dampers are in fully open position;
• Adjust each diffuser damper proportionally to the same ratio in one branch duct;
• Adjust each branch duct VCD damper in every main duct so that each branch duct has the same proportion of air flow rate;
• Adjust each main duct VCD damper in the system;
• After that each diffuser has the same ratio;
• Adjust the fan speed (or airflow by closing the main VCD damper after AHU unit) in the air handling unit:
• Each diffuser has the right airflow rate.

Start the balancing in the main/branch duct that has the highest ratio between measured and designed airflow. If the ratio is higher than 1:3 in the main duct, adjust damper to get ratio to 1:3, but open it before adjusting the main duct.

Duct balancing of each duct branch (Fig. 2) is done based on the following steps:

• Measure the airflow in each diffuser and calculate ratios; (Fig. 3 and 4);
• Select reference (last VCD damper of branch) and index (lowest ratio) dampers;
• Adjust reference damper so that the ratio is the same in both reference and index damper (index damper stays fully open);
• Repeat the same with second, third, etc. lowest ratio damper;
• Mark the positions to the dampers and lock them;
• Each branch balancing is completed once all the ratios are the same.

Velocity hood is a reliable way to measure the air flow rates in the square diffusers.

Adjust each branch damper using the similar Finally, the airflow rate in AHU needs to be adjusted: principle of reference and index dampers. (Fig. 5 and 6) Start from the duct with the lowest ratio and adjust the reference damper so that the ratio becomes the same. Hereafter, do not adjust diffuser dampers. Continue balancing in the branch duct damper that has the second lowest ratio.

Branch duct dampers are balanced based on the same principle starting from the lowest ratio damper and balancing the reference damper against it. And then repeating the same with the damper having the second lowest ratio.

The main duct dampers are balanced again based on the same principle. Ratios are specified and reference and index dampers are selected. In this case it is the same damper. This damper A stays in fully open position. Then select the damper with second lowest ratio (damper B) and close that until ratios are the same with Reference damper. Adjust the damper C accordingly.

Finally, the airflow rate in AHU needs to be adjusted:

• Adjust the total air flow rate in air handling unit to the designed value;
• After that the ratio between measured and designed air flow rate should be 1.0 in all diffusers, branch and main ducts;
• Take the following aspects into account:
• Before adjusting the total air flow rate, the building must be in normal operation mode (in design conditions);
• Record the ambient climate conditions;
• Measure the airflow rate after AHU. If that cannot be done in a reliable way, then measure the airflow rate in each main duct;
• Adjust the airflow rate with main damper in AHU or by adjusting the fan speed (recommended);
• Record the amperage and voltage of fan as well as rotating fan speed;
• Measure and record air flow rate of each diffuser and compare it to designed air flow rate. All readings shall be between 90â€’110 percent;
• Attach the air flow measurement and ratio tables both before balancing and after balancing into the commissioning documents.
HVAC COMMISSIONING GUIDEBOOK - Maija Virta (M.Sc.Eng)

What is the primary objective of air flow balancing in ductwork systems?
The primary objective of air flow balancing in ductwork systems is to ensure that the air flow rates in each branch of the duct system are proportionate to the design requirements, thereby maintaining the desired indoor air quality, temperature, and humidity. This is achieved by balancing the system pressure to the point where the fan pressure generation is equal to the sum of the pressure losses through each section of a branch.
How does the Proportional Method work in ductwork balancing?

The Proportional Method is a common approach to ductwork balancing, where each diffuser is adjusted to supply the right percentage of total air volume in the ductwork. This method involves measuring the air flow rates at each diffuser and adjusting the damper or valve to achieve the desired proportion of air flow. The goal is to ensure that each diffuser receives the designed air flow rate, thereby maintaining uniform air distribution throughout the duct system.

What happens when pressure losses are not balanced at each junction in a ductwork system?

If the sum of the pressure losses in a branch does not equal the fan pressure, the duct system will automatically redistribute air, resulting in air flows that are different from those designed. This can lead to uneven air distribution, reduced system efficiency, and increased energy consumption. Therefore, it is essential to balance pressure losses at each junction to ensure that the air flow rates in each branch are as designed.

How does duct sizing impact air flow balancing in ductwork systems?

Duct sizing plays a critical role in air flow balancing, as undersized or oversized ducts can disrupt air flow rates and pressure losses. Proper duct sizing ensures that the air flow rates and pressure losses are within the design specifications, making it easier to achieve balanced air flow. Conversely, improperly sized ducts can lead to air flow imbalances, increased energy consumption, and reduced system efficiency.

What tools are commonly used to measure air flow rates in ductwork balancing?

Several tools are commonly used to measure air flow rates in ductwork balancing, including pitot tubes, hot wire anemometers, and flow hoods. These tools enable technicians to measure air flow rates at each diffuser and adjust the damper or valve to achieve the desired proportion of air flow. Accurate measurement of air flow rates is critical to achieving balanced air flow in ductwork systems.

How often should ductwork systems be rebalanced to maintain optimal performance?

Ductwork systems should be rebalanced periodically to maintain optimal performance, as changes in occupancy, usage patterns, or system components can affect air flow rates and pressure losses. The frequency of rebalancing depends on the specific system and usage patterns, but it is generally recommended to rebalance ductwork systems every 5-10 years or when significant changes occur. Regular rebalancing ensures that the system operates efficiently, maintains indoor air quality, and reduces energy consumption.