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20 Causes of Low Indoor Airflow on Ducted HVAC Units!

Updated: Nov 20, 2023

In this HVAC training article, I discuss 20 AIRFLOW Problems that you may find on ducted gas furnaces, air conditioners, heat pumps, air handlers, and packaged units. Some problems happen over time due to lack of maintenance while others may have occurred during the initial installation.

Low air flow is a big deal, so it is important to figure out what is causing this issue. For a furnace it could overheat. For an air conditioning system, the evaporator coil could freeze. If the problem isn’t that bad, you can still have longer run times, lower capacity and lower electrical efficiency. The system's lifespan can also be reduced. Let's be aware of what these problems are in order to fix them. Below, I discuss each of the 20 problems.

Problem #1

Dirty Indoor Air Filter

It is important to change out the air filters, so that they don’t get clogged with dust. Keep in mind, while most systems have one air filter, some systems may have multiple filters. You need to look at the interior walls and the ceiling in each room of the building to see if there are multiple FRAG's (filter return air grilles) and change the filters in each one. Remember, you could find more than one filter in a row in the main return duct. This usually happens on accident when the occupant doesn't know the location of the original filter and adds another because they think that the system is not currently equipped with one. Take your time, investigate, look for filter racks cut into the duct, in the return near the furnace/air handler, inside the unit, and at the grilles. After you initially investigate, you will know for future service.

Problem #2

Air Handling Cabinet – Loose Insulation

This area may be overlooked. Inspect the inside of the air handler, furnace, or packaged unit cabinet. The glue that holds the insulation in place may come undone. This happens more often then you may think. Be aware because when the blower motor shuts off, the insulation may fall back into it's normal spot but while the system runs, the insulation sticks to the side of the blower motor squirrel cage and restricts the airflow.

Problem #3

Dirty Blower Wheel

When doing a preventative maintenance, clean off the blower wheel fins inside the blower squirrel cage. Make sure that they are as smooth as possible. You can use a brush and shop vac to clean them off. The blower wheel becomes less effective at moving air as dust accumulates on the blower wheel.

Problem #4

Dip Switches Never Set Properly

If the airflow dip switches or pin connector on the main control board were never set properly, this will cause an incorrect airflow speed. The system's control board determines what speed the ECM blower motor should run at. Set the airflow speed based on the unit's BTU/HR heat removal or heat gain capacity. For air conditioning, refer to the outdoor unit rating plate for the BTU/HR size.

Problem #5

Find The Supply Registers

You will need to find all of the supply registers. This may require you to move around furniture etc. to ensure that they are all exposed and fully open.

Problem #6

Return Grill and Supply Registers: Too Small and/or Not Enough

Be sure to consider the size and number of the return grilles and supply registers for the system size. This is an initial design mistake that we find when servicing existing units.

Problem #7

Filter Is Too Restrictive

If the air filter is too restrictive, you can go with a less restrictive type. The filter manufacturer should have the pressure drop noted on either the packaging or in the service literature. The higher the pressure drop, the more restrictive to the airflow. There are different types of filters available with varying pressure drops. A pleated filter that is 3” or 4” wide may be a good choice if there is room for a wide filter rack to be installed. Wide filters are often used to lower the pressure drop while still filtering the air because more surface area equals less restriction.

Problem #8

Secondary Heat Exchanger Clogged With Dust

When noticing low airflow, the issue could be the secondary heat exchanger on a 90% efficient furnace. This is the first coil downstream of the blower motor that could get clogged with dust and block the airflow.

Problem #9

Airflow Going Unchecked

It is important to measure the airflow going across the coil. You can do this by using the Temp Rise formula, a Hot Wire Anemometer, or by measuring the total external static pressure and comparing it to the manufacturer's guidelines.

Check out these videos related to this process using the Temp Rise Formula, External Static Pressure and Hot Wire Anemometer.

Problem #10

Supply Flex Not Installed Properly On The Collar

Look at the supply flexes to see how they are installed. Issues with incorrect installation will cause it to fall off over time, especially if it gets kicked in the attic or there are no hangers holding it in place in the crawl space. The flex could also fall off due to it's weight over time or as a result of the air pressure in the duct. The supply air could be flowing right into the crawl space. Make sure flexes are installed properly and tightly onto the collar with zip ties, screws and/or tape.

Problem #11

Clogged Evaporator Coil

The evaporator coil could be clogged with dust. This happens as a result of not having a filter installed on the system. This is often an overlooked airflow blockage because it is not easily seen. A significant static pressure drop across the indoor evaporator coil signals a clogged coil.

Problem #12

Frozen Evaporator Coil

This can happen due to 1 of the 3 following issues:

(1) Low refrigerant charge

(2) Liquid line restriction

(3) Low indoor air flow

With a frozen coil, the air cannot pass through to the supply ducts. We have an additional article which covers “Troubleshooting an Air Conditioning System with a Frozen Evaporator” . This article differentiates between a low refrigerant charge, low airflow and a liquid line restriction.

Problem #13

Acoustical Liner Blocking Air Flow

A duct that has internal acoustical liner may have some of the liner pulled off of the interior wall, partially blocking the duct. Liner is glued and buttoned in place. If the glue or buttons let go, the liner can get sucked back into return and block the airflow. The acoustical liner may have been used as duct insulation instead of installing insulation on the outside of the duct. The real use of an acoustical liner is to quiet the noise in a short return duct. Make sure that the interior of the duct is clear of obstructions.

Problem #14

Leaky Joints In Duct Work

Check to make sure that all joints are sealed up well. We don’t recommend using tape directly on the metal because the metal has oil on it so the tape will not seal properly. Seal the joints with Duct Sealant (Mastic). This can be purchased in a caulk tube or tub.

We have links below:

Duct Mastic in the Caulk Tube:

Duct Mastic Tube:

Problem #15

Improper Size for Supply / Return Trunk Ducts

The internal duct size and static pressure may not be sufficient to allow the correct amount of airflow needed across the indoor evaporator coil. In order to find the BTU/HR size of the outdoor unit, look at the rating plate of the outdoor unit. The BTU size is usually incorporated into the model number. Design or improve the existing duct work to accommodate the BTU size by using Manual D for residential systems.

Problem #16

Squished Duct

Look over the ductwork to locate improperly installed and squished ducts. Some ducts may not be squished but have too many turns or be too long. In this case, airflow will not make it to the supply register at full capacity.

Problem #17

Too Many 90 Degree Turns In the Main Trunk Duct

Keep turns to a minimum when designing ducts. Remember every time you add a 90 degree turn, it isn’t just the length of the turn that is added to the total length of the duct. The turn is adding a significant equivalent length of duct to the system.

Problem #18

Supply Trunk Duct Is Reduced Too Far too Soon

I often find that the main trunk duct is reduced too small in size before the last supply runs are added to the main trunk. This results in low airflow to each of the supply registers that these flexes or round ducts are connected to.

Problem #19

An Animal Has Made A Nest with the Return or Supply Air Flex

A collapsed flex may be found in an unsealed crawl space, such as under a mobile home. If the return flex has been ripped open by an animal, the flex will collapse in on itself when the system turns on. It is very important to make sure that the crawl space is sealed up.

Problem #20

Duct Touching the Concrete Floor or Ground

A duct making direct contact with a concrete floor or the ground in a crawlspace will eventually rot out and leak. I have seen this quite a bit where a downflow furnace has the supply duct under the home and the supply duct lowers down too far and is in direct contact with the wet ground. Other times, the concrete is continually wet and the duct rusts. The duct eventually rots out and the building owner calls the service tech out on a low airflow call.

I hope this review was helpful with solving your airflow issues. Check out our video on this topic.

If you want to learn about refrigerants and how they work in a system, check out our “Refrigerant Charging and Service Procedures for Air Conditioning” book . Test your knowledge with our 1,000 question workbook along with the answer key! We also have quick reference cards for use out in the field! Bundle Packs are a great way to save and get faster shipping! Check out

Check out our free Quizzes to test your knowledge here!

Check out our Free Calculators here!

Published: 04/14/2021 Author: Craig Migliaccio

About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 16 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at & &



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