Why Air Conditioner Ductwork Sizing Matters


Ductwork sizing is crucial in ensuring a safe and comfortable indoor environment. It also impacts indoor air quality and the level of heating macomb township mi and cooling energy bills.

If ducts are too large, this can reduce air pressure, making it difficult for air to be circulated throughout the property. To achieve the correct air pressure, the ductwork may need to be altered or a more powerful HVAC system that complements its sizing. 

If too small, it can cause air pressure to build up in the HVAC system, causing ductwork to become worn and deteriorate more quickly. It can also force the equipment to work harder and become stressed and damaged. It is vital to ensure the proper sizing of ductwork is installed, ensuring the HVAC system can function optimally and without causing undue wear and tear. 

Standard Sizing

For the HVAC system to function optimally, the duct size must adequately handle airflow intake and its return. For this, there is a need to determine the distance between an outlet and the HVAC unit. Also, you will have to consider the unit’s output and the amount of area the unit needs to condition. 

Multiple factors are considered when calculating what ductwork size should be installed. These factors include:

  • Cubic feet per minute (CFM)
  • Friction loss rate
  • Airflow velocity
  • Pressure of the HVAC system
  • Length of the ductwork
  • Home square footage
  • Ductwork material

Besides the size of the entire property, you will also need to factor in the size of each room. This is where cubic feet per minute comes in. When it comes to materials, there are multiple options, including fiberglass, sheet metal, fiberboard, flexible plastic, and even fabric. In many cases, a combination of flexible and rigid ductwork is used. 

Due to the many factors involved in calculating ductwork size, variable results can be found. There is no universal standard ductwork size. The closest you can come to a standard size is by comparing rectangular with round duct sizes. 

How to Calculate Size Needed

Rectangular or square ducting can be sourced in standard sizes that range from 3-7 inches to 35-40 inches. Airflow needs per CFM and velocity will determine the sizing. You will need to check the HVAC systems guide that will indicate the maximum airflow per CFM with corresponding duct sizes. Know that the ductwork material, length of the ducts, and the number of corners or branches in its design will also impact the velocity of supply and return.

With round ducting, the standard sizing can range from 4-over 41 inches in diameter. The sizing to be installed will depend on the airflow needed per CFM and velocity. Like with the rectangular and square ductwork, the length and number of branches in the design will also affect duct design. The goal is to ensure that suitable air velocity and pressure can be achieved and comfortably sustained throughout the HVAC system. 

Incorrect Sizing 

Duct size will determine how well airflow intake and return are achieved. If the airflow and velocity are poor, the HVAC system will work inefficiently, likely leading to overworking and premature wear and tear which can lead to costly AC repairs. This can then cause breakdowns and a need for repairs or parts replacement. When an HVAC system is working inefficiently, this can also cause high energy consumption and higher energy bills, besides the costs of repairing and possibly replacing the system. 

When ductwork is too small, the HVAC system has to work harder to push heated or cooled air to the needed rooms. This overwork can cause the system to deteriorate quickly. If the ductwork is too large, the level of airflow velocity will be too low. This means occupants in the rooms that need to be heated or cooled will hardly feel the air blowing through the vents. 

Even with the benefit of online calculators, it is challenging for a layperson to properly consider and calculate all the factors determining what size of ductwork is most suitable for a home. The process involves complex calculations and the use of tables to work out the correct ductwork sizing. Engaging a certified HVAC technician to do ac services lakeside mt will ensure all relevant factors are considered, and all code standards are met.