What causes low air flow at air conditioner vents?

Wednesday, September 19, 2018

The most likely cause of poor air flow is also the simplest to solve: a dirty air filter. Be sure to check your air filter first. 


    Also, we occasionally inspect air conditioning systems that have filters installed in two locations—both behind the return air registers and at the base of the air handler (inside unit of the air conditioner). Sometimes a homeowner forgets about the second filter layer and it clogs up almost solid, severely restricting air flow.

   Then again, even with an understanding that there are two filtration layers in a system, you might miss that there are two or more return air registers in the home. Most newer homes of 2,000 square feet or more have a large return air register in the living room or hallway to the bedroom cluster at one side of the house and a second, smaller air filter at a register in the ceiling of the master bedroom. 

   Also, some systems have a small return air register on an outside porch or soffit for introducing fresh air into the system. One or more of the locations gets forgotten and ends up with a clogged filter. One way to recognize that there is a filter behind the register is the small securing thumbscrew on the frame, as in the photo below.


   Two layers of filtration are not recommended, and we always suggest elimination of one set of filters; or, at a minimum, changing the filters at the return air registers to ones with a MERV rating of 3 or so. For more information on the MERV rating system, go to our blog What is the right MERV number for my air conditioning filter?

   A second issue that can cause restricted air flow is an evaporator coil with a thick blanket of dust, hair or lint on it. The evaporator coil is  a continuous pipe filled with refrigerant gas in the air handler that loops back and forth to form a flat panel or tent-shape, which is surrounded with a layer of thin fins. Air pushed through it by the blower in the air handler is cooled. 

    Having no air filter in place or allowing the air filter to get so dirty that it buckles and ruptures means that the evaporator coil becomes an unintended filtration device and starts collecting small bits of debris from the air between its fins. Eventually a thick layer builds up and interferes with both the air flow and the ability of the evaporator coils to absorb heat and thereby cool the air. In the photo below you can see a layer of lint/dust building up on the left side of the inverted V-shape of the evaporator coil, and a somewhat cleaner coil surface on the right with metal still visible.

   In some air handlers you can pull out the filter at the base and look up at the evaporator coil to find any dirt buildup. But in most units you can’t, and will have to depend on an a/c service tech to do some disassembly to check.

   Several other defects that can cause decreased air flow are listed below, but all will likely require a service technician to evaluate them:

  • A malfunctioning blower.
  • Damaged ducts or leaking duct connections. The taped duct connections can deteriorate over time. Also, if rats or squirrels get into the attic, they have been known to gnaw and tear the duct material.
  • A loose connection between a duct and register will allow air leakage just above the ceiling and reduced air flow into the room.
  • A poorly balanced duct system. To learn more about balancing an air conditioning system, check our blog Why are some rooms colder or warmer than others?

• • • • • • • • • • • • • • • • • • • • • • • • • • • • 

  To learn more about heating and air conditioning systems, see these other blog posts:

How can I find out the SEER of my air conditioner? 

My air conditioner won't turn on. What's wrong? 

How can I find out the size of my air conditioner? 

How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

Where is the air filter for my central air conditioner and furnace? I can’t find it? 

Does an old air conditioner use more electricity as it ages? 

How did homes stay cool in Florida before air conditioning? 

What is wrong with an air conditioner when the air flow out of the vents is low?

Why has the thermostat screen gone blank? 

Why does it take so long to cool a house when an air conditioner has been off for a while? 

Why is my air conditioner not cooling enough? 

What are the most common problems with wall/window air conditioners?  

Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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