Weak Airflow From AC Vents? Why and What to Do

If your air conditioner (which had a strong airflow in the past) has a weak airflow now, there is something definitely wrong with one or more of its components. Before calling an HVAC technician to fix it for you, I would recommend trying the tips I will suggest in this post first.

AC vent

Common reasons for weak airflow from AC vents:

  • Clogged Air Filters: Dirty or clogged air filters can restrict airflow, making the AC less efficient. Regularly replacing or cleaning the filters can improve airflow.
  • Blocked Vents: Obstructed or closed vents can limit the distribution of cool air. Ensure that all vents are open and unobstructed by furniture, curtains, or other objects.
  • Ductwork Issues: Leaks, gaps, or blockages in the ductwork can reduce airflow. Professional inspection and ductwork maintenance may be necessary to resolve these issues.
  • Frozen Evaporator Coils: If the evaporator coils freeze due to refrigerant issues or lack of proper airflow, it can result in weak airflow. Thawing the coils and addressing the root cause is essential.
  • Incorrect Thermostat Settings: Incorrect thermostat settings, such as fan settings set to “auto” instead of “on,” can limit airflow. Adjust the thermostat settings to improve airflow consistency.
  • System Malfunctions: AC systems can experience malfunctions in components like the blower motor, capacitors, or wiring. Professional HVAC technicians can diagnose and repair these issues.
  • Undersized HVAC System: An AC system that is too small for the space it needs to cool may struggle to provide adequate airflow. In such cases, upgrading to a properly sized system may be necessary.

What to do to address weak airflow:

  • Check and Replace Air Filters: Regularly inspect and replace air filters to ensure they are clean and not clogged.
  • Open Vents: Ensure that all vents in your home are open and unblocked to allow proper air circulation.
  • Inspect Ductwork: Have a professional HVAC technician inspect and clean your ductwork to identify and address any issues affecting airflow.
  • Thaw Frozen Coils: If your evaporator coils freeze, turn off the AC and allow them to thaw. Address any refrigerant or airflow problems that may have caused the issue.
  • Adjust Thermostat Settings: Make sure your thermostat settings are optimized for airflow, such as setting the fan to “on” for continuous circulation if needed.
  • Professional Inspection and Repairs: If the issue persists, or if you suspect a malfunction, contact an HVAC professional for a comprehensive inspection and necessary repairs.
  • Consider Upgrading: If your AC system is undersized for your home, consider upgrading to a more appropriate system for improved airflow and cooling efficiency.

Causes of Weak Airflow from AC Vents and How to Improve It

Let us now look at the causes and solutions in more details

1. Dirty Air Filter

Apart from just cooling the air in your house, the AC also cleans it by removing the dirt particles suspended in the air. That is done by the filter which is located in the return air duct, to trap dirt particles in the air coming from the house on its way to the cooling/evaporator coil.

An AC filter should be changed after every 3 months. Failure to change the filter causes it to be clogged by debris, and as a result less air will flow through it. That could be the reason for the reduction in airflow in your AC vents/registers.

If you haven’t changed your air filter for some time, I suggest you change it as soon as possible. Luckily, air conditioner filters are inexpensive and easy to replace. You will however need to first locate the filter and know what size you need.

Also, avoid using air filters with a very high MERV rating since they clog up easily and restrict airflow to the house. A filter with a MERV rating of between 8 and 12 should be fine.

While changing the filter, start by turning off the AC and then make sure that the arrow on the filter points towards the furnace/air handler. That is the direction of airflow in your house.

2. Dirty Coils

dirty ac coil

Another reason why your AC could be having a weak airflow is a dirty coil, especially the condenser coil (located outside the house). The condenser is equipped with a fan that pulls in air to the coil through the sides of the unit to cool the refrigerant.

As the fan is pulling in air, loose debris from the surrounding will be pulled in as well and end up clogging the coil fins. When that happens, it becomes difficult to cool the refrigerant and as result less air will be cooled which could by why you have a weak airflow.

By cleaning the condenser coil, you will be able to fix the problem. The following are the steps to follow when cleaning a condenser:

  • Turn off power to the AC through the breaker next to the unit or inside the house. You should turn the thermostat off as well.
  • Remove the side panels all over the condenser to access the fins.
  • Use a brush with soft bristles (very important not to damage the fins) to remove the debris starting from the top and all the way down. Clean the inside and outside of the coil.
  • Spray a coil cleaner all over the coil. If you don’t have a commercial coil cleaner, a mixture of dish soap and water in a spray bottle will do just fine.
  • Rinse the coil using water, again from the top to the bottom, inside and outside of the coil.

For a detailed guide on how to clean the condenser coil check out this post. Also, it is important to make sure that the condenser unit is not obstructed by anything like bushes or physical objects. There should be about a feet of free space all around the coil.

Cleaning the evaporator coil is just as important, at least once a year. Since the air filter does not remove all the dirt, some of it end up clogging the evaporator coil, blocking the flow of air to the coil.

Again, start by turning off the power and removing the access panel. In some units you will also need to remove the furnace filter and disconnect the return air duct.

For a detailed guide on how to clean an AC evaporator coil, check out this post.

3. Closed or Blocked Air Vents/Registers

There are 2 types of air vents in your house. Return air vents pull air from your house and are connected to the return air ducts while supply air vents supply your house with cooled air and are connected to the supply air vents.

Sometimes people close some of these vents accidentally or when you want to direct air to a certain room in the house. Check if any of the vents in the house is closed and open them. That can fix the problem for you.

If you have wall or floor vents, make sure that they are not obstructed by objects like furniture. That can also reduce the airflow to the house. Ensure that there is at least 1 foot of free space between the vent and the closest object.

To check if the vents are blocked, you will first need to remove the grilles. In some cases you will need a screwdriver but in some you will only need to pull some tabs.

With the grille out of the way, use your hand to pull gunk from inside the ductwork. Most of the clogs are usually very close to the vent/register. Avoid using sharp objects like screwdrivers or drain snakes since they can damage the ductwork.

4. Evaporator Coil is Frozen

During the cooling cycle, the refrigerant enters the evaporator coil as a very cold liquid. If there are problems with the AC system, the refrigerant will keep on expanding, lowering its temperature to below freezing point.

That makes the evaporator coil so cold that instead of the condensate draining from the coil, it starts to ice over. After sometime, the entire evaporator coil will be a block of ice. A frozen evaporator can’t cool the air in the house which can explain the reduction in airflow.

So why do air conditioners freeze? It could be caused by one of the following:

  • Dirty air filter
  • Low refrigerant level
  • Dirty coils
  • Clogged or leaking air ducts

If you look and notice that your evaporator coil is clogged, turn off the AC and give the ice some time to thaw. Thawing will however not fix the problem. You will need to go to the bottom of the problem and establish why the AC froze in the first place.

5. Low Refrigerant Level

An AC refrigerant (commonly known as Freon or coolant) is the one responsible for cooling the indoor air. It changes forms (between liquid and gas) while travelling between the inside and outside AC units.

For an air conditioner to work properly, the amount of refrigerant in the AC system has to be right. Luckily, unlike a car whose gas needs to be topped up, an AC is a closed-loop system so you don’t need to regularly top it with the refrigerant.

However, a leak in the system can lower the amount of refrigerant flowing in the system, which also means that there will be a reduction in the airflow in the house.

Unfortunately, the average homeowner cannot measure the amount of refrigerant in the AC system. Even if they can, they will also need refrigerant to recharge it, which is only sold to licensed technician.

If you suspect that the refrigerant level in your AC is low (sometimes through a hissing sound) contact an HVAC technician to test it and recharge it.

6. Faulty Blower

HVAC blower

A blower is part of your inside HVAC system. It comprises of fan blades and a motor which pull warm air from the house and supply cold air back to the house. If there is a problem with the blower, there will be no or reduced airflow.

Some of the signs of a faulty blower is rattling noises from the inside unit or blower compartment. That is usually caused by broken blades or problems with fan belt or bearings.

Dirt in the blower blades can also impede its activities resulting in reduced airflow. In some cases you may need to replace the motor.

Inspecting the AC blower is not easy. Again, I would recommend hiring a technician to inspect and fix the blower.

7. Leaking or Clogged Air Ducts

Perhaps the reason why there is less air being blown in your house is because the cooled or warm air is leaking out through the ductwork somewhere in the attic or elsewhere in your house.

Leaky ductworks are very common. If not that, there could be a clog somewhere in the ductwork restricting the flow of air to the house. The only way to find out if that is the problem is by inspecting the ductwork.

You can trace your ductwork from the furnace to the rooms in your house looking for leaks but I would also recommend hiring a technician. The technician will inspect the entire ductwork and clean it and if there are leaks they will seal them.

8. Other Issues

Sometimes the problem of a reduction in airflow in an air conditioner is caused by other problems in the system. To start with, can you hear any unusual noises from your air conditioner (both indoor and outside units)?

If you can fix the noise cause, you can restore the strong airflow in your house. Also, check if the thermostat is set up correctly or if you need to replace the batteries.

While outside your AC unit, make sure that the condenser fan is spinning when the unit is running. If the fan is not spinning, you may need to replace the motor or capacitor. To find out which is the problem start by testing the capacitor.

Your air conditioner could also be having a weak airflow because it is too old or it’s undersized. Sometimes replacing an old air conditioner is a better idea than trying to fix it.