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.

A typical ductwork system

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.

Figure 1 –Duct balancing principle based on the 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:

Figure 2 –Start balancing diffusers by specifying the reference (R) and index (I) dampers. The furthest damper (downstream) becomes a reference damper (called R). Starting point for balancing is the damper that has lowest quotient, i.e. the relationship between measured and designed airflow rate, known as index diffuser (I).
  • 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.
Figure 3 – Pitot tube formula
Figure 3 –As air flow is turbulent inside the duct, there is variation in the velocity over the face area of a duct. This is why the average of several velocity measurement readings gives more accurate result. Air flow measurement can be done either using hot wire anemometer (velocity measurement) or pitot tube (pressure measurement). Typical duct velocities are 1-15 m/s (instrument resolution is typically 0.1 m/s). Pitot tube readings with the same velocities are 2-50 Pa (instrument resolution is typically 1 Pa). Therefore, the accuracy of the pitot tube measurement is poorer with low velocities.
Figure 4

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.

Figure 5

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.

Figure 6

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 90‒110 percent;
  • Attach the air flow measurement and ratio tables both before balancing and after balancing into the commissioning documents.