How to Increase Airflow in Ductwork


Ductwork is the small metal or plastic pipes attached to your air handler, which distributes cooled (or heated) air throughout your home. Ducts allow for a cost-effective and energy-efficient way of spreading the conditioned air to different rooms in your home. It’s often necessary during an HVAC system installation to add more ductwork, depending on the size of your home, even if it was already installed by another company when you bought your home. Unfortunately, many people think that all they have to do is install new registers in their basement or attic space.

Though this will help increase airflow through the ductwork, adding records probably won’t be enough because not enough cool air will enter into the house from outside at night. So it’s essential to make sure you are adding registers in your basement or attic space that is proportionate to the size of your house. Today I am going to discuss a technique on how to increase airflow in ductwork.

How to Increase Airflow in Ductwork

Reasons for Low Airflow in Ductwork

Inadequate Fans: 

Smaller diameter fans cool airless effectively. Foes tend to install smaller diameter fans because they are less expensive than more giant fans. Also, the smaller diameter fan will run slower and is more likely to be rated for continuous duty so that it runs 24 hours per day instead of 8 hours per day like more giant diameter fans. The bigger the percentage of time a fan is running in its no-load condition (a low kW load), the shorter life it will have – even if it’s operating at speeds as high as possible for which it isn’t designed. It may seem counterintuitive, but when a fan runs at a lower speed/temperature with a higher air volume flow rate, then it lasts longer unless you’re using undersized fans for your ductwork.

Blockage in Duct:  

Even the most efficient fans can’t overcome a significant amount of blockage in the ductwork and will be severely hindered if there is more than 50% flow restriction. In addition, a dirty filter has a resistance that increases with air velocity. Therefore it’s equally as important to keep your filters clean as it is to have enough airflow. Both undersized ducts (2″ instead of 3″) AND dirty filters should never be used together because they combine to make even an oversized fan fail quickly.

Poor Installation:

Another major cause for low airflow in ductwork is poor installation techniques such as improper sealing around grilles, cleanouts, diffusers, or loose connections at fittings that allow air leaks through friction or turbulence. Tube fittings are costly compared to a few screws, and they should be used for all new ductwork installations. In addition, each fitting should be sealed with a metal gasket, airtight tape, or silicone caulk to prevent any air leaks around the outside of each connection.

Pipe Size:

Another major source of low airflow in ducts is turbulence created by pipes that are too large for the application, which often happens when using an overly large flex hose between the HVAC blower and the fan coil unit due to mistaken ideas about proper practice related to “dampers”. In most cases, it’s required that these oversized tubes have enough excess capacity inside them (via an accordion effect) so as not to restrict airflow – especially when operating at lower speeds/higher pressures than designed for.  

Low Airflow in Ducts Is Turbulence

A Guide on How to Increase Airflow in Ductwork

Ensure that the ductwork is designed with a suitable size and dimension. The sizes of ducts vary depending on the location they are in. If the room available for installation is small, then you can opt for smaller ducts, whereas if it is large, then larger ones should be chosen. Ducts that are too tight will reduce airflow, thus increasing pressure drop and causing overheating of terminals, motors, etc.

These problems are resolved by increasing air volume flow through the duct system; this can be achieved by replacing the original smaller systems with larger ones or adding more turns or elbows to get sufficient turns. If a smaller system is used, an additional length of an exhaust hose with a 50% bigger diameter than ideal must be attached to avoid too high backpressure in the system. On the other hand, if larger systems are used, that would require an additional length of ductwork before reaching the location with the same size as originally installed.

As per industry standards, for 1-inch diameter main and 2-inch branch lines, the standard lengths of the exhaust hose are 12 feet while 6 feet for flex couplings. The maximum length that can be allowed to ensure optimum operating conditions is 24 feet from the fan or blower discharge connection point to the smallest diameter elbow and then doubling it to get the total minimum required length between main and all branches and also double the above-mentioned dimensions accordingly for each additional 100 cfm.

For example 500cfm=100ft; 1000cfm=200ft etc.For ducting sizes smaller than ½ inch, the calculation of minimum required length is different, and as per the ASHRAE handbook, it is 5 ft for 1/8 inch main diameter ducts. While calculating the length, other things to be considered while calculating the length are bends and offsets, which must not exceed a 45-degree angle from the straight ducts. At this point, you need to divide each length by 6 feet or 2 meters for each bend.

So if you have two elbows in the same direction, you need to double these figures again so that your cutouts will be right. Ductwork being used should always be equal to or larger than what was originally installed because if we try using smaller ones, then a higher pressure loss occurs, which results in poor airflow.

For larger buildings with many locations, multiple branch returns may be required to minimize the total circulation resistance. The exhaust duct must have the same size as the installed exhaust for every location to ensure proper airflow. It can be done by adding more elbows or turnouts in the existing ductwork system and maintaining a ratio of 2-3 feet of unused length for every elbow,

which is added because once you open an elbow, then there are two separate systems so if you wish to double your current size of those branches then these figures will be doubled as well: one is from nearest elbow to farthest terminal and another one from next nearest elbow, etc.

If we choose this method then make sure that there are at least 50% times more size than originally installed for every elbow you will add because the sum of all branches should be equal to or larger than originally installed ductwork.

Precautions While Performing How to Increase Airflow in Ductwork

  • Use a duct-cleaning machine to clean the inside of the vents. This would help in removing dirt and debris that might be clogging up your vent.
  • There can also be unwanted material like dead animals or birds that could have become stuck on the grill at some point. You should make sure you check thoroughly before increasing airflow in ductwork.
  • Remove all big furniture from the room, which will allow better air circulation in case of fire situations. Ask for permission over any additions, changes, or modifications of ductwork. Make certain they are well made with proper permits and licenses as required by law.
  • All work must be done even if some parts of the system are not blocked. For example, if the filter is dirty, but there’s still good airflow, that’s fine. Once air goes through the hole and into the room, it will continue to circulate.
  • To check for increased airflow in ductwork, use a CO meter or a smoke machine. These can be used to test your codes and make sure you’re doing everything right. It’s instrumental when you want to increase airflow in ductwork because it allows you to know if your changes affected what flows out.


I hope you have obtained all the information about how to increase airflow in ductwork. Also, ensure proper safety while performing the process. Thank you and have a nice day!

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