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Check Your AC’s Cooling Performance To Ensure It’s Working At Its Best

This is Part 2 of 5-Part Series

This Series is dedicated to lowering cooling bills.  It provides in-depth details regarding problems that cause an air conditioner not to cool at its maximum ability.  Many times an air conditioner is running longer and harder than it needs to, due to operational problems that are reducing its ability to cool.  These problems: drive up cooling costs, cause more frequent break downs, and wear out the HVAC System sooner.

Often, the A/C is assumed to be doing “all it can” and must run longer to “keep up” with cooling-demand.  This situation presents itself only when the A/C is too small for the space it’s cooling.  If it’s sized properly to the space, and not able to keeping up with cooling-demand, the a/c has operational problems. 

Part 2 of our 4-Part Series Discusses:

  • Air-flow restrictions or blockages
  • What must be done to correct air-flow restrictions or blockages
  • #1 Reason for low air-flow
  • High MERV-Rated Air Filters their affect on HVAC System air-flow
  • How to decrease air-flow to a room that is getting too much air
  • How to increase air-flow to a room that is getting too little air
  • Homes with Central Air-Returns, and how closing a door may restrict air-flow to a room

Common Air-Flow Problems

A. Air Filter Is Clogged

Air-Flow problems are one of the most common reasons an HVAC System cannot keep up with cooling (or heating) demand.  Central Air Conditioning is designed to produce 400 Cubic Feet per Minute (CFM) for each Ton of cooling capacity (1 Ton = 12,000 btu’s).  So, a 3 Ton system moves 1,200 CFM.  With that much air-flow, restrictions or blockages have a huge affect on the System’s ability to cool as designed.

Air-Flow Problems Are Often Due To:

  • A clogged Air Filter (#1 reason).
  • High Efficiency, High MERV Rated air-filters that are not being changed frequently enough. High MERV Rated filters clog very fast (because the pores that air passes through are quite small).  High MERV Rated filters likely won’t last a full month like low rated filters do.
  • An Electronic Air Cleaner that’s too dirty.  At some point the Air Cleaner is so dirty that it can no longer clean the air.  This slows air-flow through the cleaner, and allows contaminants to clog the A/C Cooling-Coil and furnace components.
  • A clogged A/C Cooling-Coil (inside the furnace or ductwork and cannot be seen without removing a panel).  When contaminates pass through the filter (or by-pass it because it’s clogged and buckled, or no filter is in place) the contaminates stick to the A/C Cooling-Coil and furnace components.  Because the Cooling-Coil is wet (it’s what removes humidity from the air) contaminates stick to it very easily.
  • Ductwork leaking air into the attic.
  • Ductwork that is too small.
  • Ductwork with reduced or blocked air-flow (common with flexible ductwork, due to incorrect installation).
  • Ductwork and / or furnace components that are very dirty.
  • Specific ductwork lines needing air-flow boosted.
  • Specific ductwork lines needing air-flow reduced.
  • Homes with Central Air Returns (versus one in each room).  Closing a room’s door can reduce air-flow.

All of these are discussed in this portion (Part 2) of the 4 Part Series.

A. Clogged Air Filter

The #1 reason too little air is coming out of your ducts is due to a clogged furnace air filter.

  • Air filters must to be checked and changed / cleaned each month.
  • High MERV-Rated filters typically need to be replaced more frequently than once per month.
  • If the filter looks dirty, it is, and it’s time to change / clean it.
  • NOTE: If you have: several people living in the home, or shedding pets, filters may need to be changed more frequently than monthly.

TIP: Check the filter on the first day of each month (to make it easy to remember).

very dirty and buckled furnace air filter

dirty and clean furnace air filter

dirty electrostatic furnace air filter

Reusable filter shown on right


Reusable filters clog very fast and need to be cleaned monthly or sooner.


The disposable filter on the left was so clogged that it buckled.  This allows dust into the HVAC System, clogging the A/C Cooling-Coil, which creates an ideal environment for mold to grow.


B. Air Conditioner Cooling-Coil Is Clogged or Frozen

molded home central a/c evaporator coil

A/C Cooling-Coil that is clogged and shows signs of black mold growth.

home central a/c evaporator coil iced up

A frozen A/C Cooling-Coil can be due to insufficient air-flow through the coil (often due to a clogged air filter)

The A/C Cooling-Coil is inside the furnace, or ductwork attached to the furnace.  It cannot be seen without removing a panel.  Over time, this coil will clog, regardless of the quality of Air Filters used.   This is why HVAC System maintenance includes occasional A/C Cooling-Coil cleanings.

While you can clean this coil yourself, it may be preferable to have an HVAC Technician do it.   In most cases, the cleaning can be done with the coil in place.   Or, it’s dirty enough, the coil will need to be removed for cleaning.  This means the refrigerant must be recovered from the A/C and the system recharged with refrigerant after the Cooling-Coil has been cleaned and reinstalled.

C. Furnace Blower Is Dirty

very dirty furnace blower wheel

new furnace blower wheel

very dirty furnace blower motor


This is what can happen very fast if the furnace is operated without a filter, a filter was so clogged that it buckled.

The first step in furnace maintenance is to keep it clean.  The photo at left shows the furnace blower wheel so dirty it can hardly move air.  The center photo is a new blower wheel (for comparison).  The photo on the right is a furnace blower motor so dirty it cannot cool itself, which will cause it to overheat and fail.  These problems are particularly likely to happen if someone operates the furnace with no air filter, or the air filter was so clogged that it buckled.

D. High Efficiency Air Filters Reduce Air Flow

The Higher The Filter’s MERV Rating:

  • The Better It Cleans The Air
  • The Faster The Filter Clogs & Reduces Air Flow.  The air-flow reduction will be dramatic with high MERV Rated filters.

Air filters are rated as follows:

MERV Rating Particle-Size Filtering Efficiency (in Microns)
1-4 3.0-10.0 (microns)*: less then 20% of contaminants will this size will be filtered out
6 3.0-10.0:  50% will be filtered out
8 3.0-10.0:  85% will be filtered out
10 1.0-3.0: 50%-65%  +   3.0-10.0: 85% or more will be filtered out
12 1.0-3.0: 80%-90%  +   3.0-10.0: 90% or more will be filtered out
14 0.3-1.0: 75%-85%  +   1.0-3.0:  90% or more will be filtered out
16 0.3-1.0: 75%+ will be filtered out

* NOTE: The Width Of A Human Hair Is 60-100 Microns.

HVAC System Air Flow is reduced by High Efficiency Air Filters.   The higher the MERV Rating, the better the filter cleans the air (because the pores the air passes through are smaller). Because the pores are so small, the filter clogs very fast.  For this reason, these filters typically don’t last a full month.  While the reduction in air-flow is nominal when the filter is new, the higher the MERV Rating, the faster the filter will clog (due to the smaller pores).

As high MERV Rated filters clog, they have an enormous impact on HVAC Sytem air-flow.  For this reason, they must be monitored and replaced more frequently than lower rated filters.   A filter with a rating higher than MERV-8 will likely need to be changed more frequently than once each month.  If a home has shedding pets and several family members, a MERV-8 filter may also need to be replaced more frequently than monthly.

This video shows how an air filter can reduce air flow: Air Filters & HVAC System Air Flow.  Note:  This is not an endorsement for the brand of filter featured in the video.  Al’s has no opinion regarding specific brands of air filters only because we don’t have first-hand knowledge of all brands.

An appropriate balance between HVAC System Air-Flow and your Air-Cleaning Needs can be achieved.   Your specific air-cleaning needs will largely dictate how high the MERV Rating needs to be.  And keep in mind, more is not always better.  Align your filter choices to your needs, knowing that the higher the MERV Rating, the more frequently the filter must be replaced to ensure proper air-flow though out your HVAC System.

If your home has members who: are highly allergic or smoke, it’s likely a higher MERV Rated filter’s air-cleaning ability is worth the decrease in air flow (as long as it’s changed frequently enough). The composition of the residents in the home is the best guide to the MERV Rating that’s best for you.   For Example: A home without highly allergic residents, smokers, and shedding pets is likely well served by a MERV-8 rated filter (perhaps lower).

static air pressure test MERV ratings on air filters

  This is a guide as to what size particulates are filtered out by different MERV Ratings.

very low MERV rated furnace filter

merv 6 rated furnace filter

merv 11 rated furnace filter

merv 16 rated furnace filter

The blue filter on the left is a MERV-2 or lower.  It’s designed to catch only the particulates which can clog the HVAC System. Most filters have their MERV Rating clearly displayed.

MERV Ratings go as high as about 16. Beyond that, HEPA filters are used.   The MERV-16 filter on the right is 5 inches thick

Thicker filters filters impact air flow much less because they have more surface-area for air to pass through.  Note:  In order to use thicker filters, a modification may need to be made to the furnace.  This is not always possible.

This article details how high MERV Rated filters affect air flow and impact the HVAC System: MERV -Air Filtration and Air-flow Restriction

 Electronic Air Cleaners Require Monthly Cleaning

This YouTube Video describes one way to clean Electronic Air Cleaners Collector-Plates.  Cleaning Electronic Air Cleaner Collector-Plates

There are several cleaning methods, but this video was chosen because it demonstrates how dirty an Electronic Air Cleaner can get after just one month.  Note: As collected particles accumulate on the Collector-Plates, the effectiveness of an Electronic Air Cleaner drops dramatically.  The collector plates need to be relatively clean in order to attract polluting particles as the Air Cleaner is designed to.  At some point, an Electronic Air Filter can get so dirty that it no longer is able to clean the air.  If this occurs, contaminants pass through it into the HVAC System.











Electronic Air Cleaner’s Collector-Plates must be cleaned monthly or sooner.

E. Existing Ductwork (or parts of the ductwork) Is Too Small (or Too Big)

metal hvac ductwork shown in many sizes

Flexible and Metal Ductwork comes in many sizes.  This to ensure a correct ductwork size is available for all needs.

The larger the HVAC system (more BTU’s of Heating Capacity, or Tons of Cooling Capacity), the more air movement the ductwork must accommodate.  400 Cubic Feet per Minute (CFM) of air movement is standard for each Ton of Cooling Capacity (1 Ton = 12,000 BTU’s).  If a larger capacity HVAC System is installed, the existing ductwork must be upgraded, or it will be too small for the larger system to operate as designed.

This may be remedied by:

  • Replacing the existing ductwork with larger ductwork
  • Adding additional ductwork lines to supplement the existing ductwork.  This is particularly true of return-air ducts.

If Only 1 or 2 Rooms Are Warmer Or Cooler Than The Rest Of The House:

  • When some ducts have too much air-flow (more than is needed),  This can often be corrected by adding dampers to reduce air flow.

damper for hvac ductwork

Air Duct Damper

If Only 1 or 2 Rooms Are Not As Warm Or Cool As The Rest Of The House:

  • When some ducts have too little air-flow.   When air-flow to other ducts is good, air-flow to one duct can be increased by adding an air duct booster.

ductwork booster fanAir Duct Booster

If the air-flow to all other rooms is good, you won’t want to use a damper to lower air-flow from one duct hoping to increase air-flow to another.  Reducing air-flow to one duct increases air-flow to all remaining ducts.  What if you need to boost air-flow from one duct?   This can be accomplished by adding a ductwork booster fan. Adding this fan will take a little air-flow from all other ducts in order to increase air-flow to one duct.

For rooms father from the furnace, the ductwork leading to the rooms must be larger (near the furnace) than is required for rooms close to the furnace. The reason ductwork gets smaller at the far end is to increase air-movement speed.  If this practice is not followed, rooms father from the furnace may not receive sufficient air-flow.

hvac ductwork reducing in size


Central Air-Returns

Many homes have “Central Air Returns”.  Other homes have an air-return in every room (except baths and kitchen).  A Central Air-Return is quite large (and often noisy).  It’s common to see a central air-return grill just above or below a furnace located within the living space of the home (versus in the attic).

hvac central air return

Shown: A Central Air-Return Grill Located Above The Door Which Covers The Furnace

In homes with Central Air Returns, the amount of air flow into a room can be substantially reduced by simply closing the room’s door.  Without an air-return in the room, closing the door may reduce air-flow into the room because there is no way for air to exit the room (when the door is closed).

If this occurs, the door can be shortened 1-2 inches, to allow air to exit the room while the HVAC System is running.  Another solution is shown below.  Note: Because DFW is a “cooling climate” the solution below helps to remove warm air better than shortening the door (warm air rises and cold air sinks).

hvac air balancing grills over bedroom door

SHOWN: Air Balancing Grills over a bedroom door

“Air Balancing” Grills.  Many newer homes with Central Air Returns have return-air grills on both sides of the wall above bedroom doors.   These are not connected to the HVAC System.  Their purpose is to allow air to exit the room, when the furnace is running and the room’s door is closed (to ensure adequate air-flow into the room). This can be added to any home, though it may create a privacy issue with noise-transfer.

In homes with a return-air grill (similiar in size to the one in the photo above) located in a different location in the room (not above the door) is typically an air-return that is connected to the HVAC System.  To determine if it’s a Return-Air grill, or an Air-Balancing grill is to check to see if light is visible.   If you can see light through it, it’s an “air-balancing” grill.  If you can’t see light through it, it’s connected to the HVAC system.


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