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

This is Part 1 of 5-Part Series

The U.S. Department of Energy States:  Air Conditioners Use 1/6 of Total U.S. Electricity.  On Hot Summer Days, Air Conditioning Creates Up To 43% Of Peak Power-Loads.  

The power grid must maintain a balance of electricity supply and demand (or it will shut down to protect itself).  When a power plant can’t meet electrical load (and cannot buy electricity from the power grid) one of two situations must occur: Brown-Outs and/or Rolling Black-Outs.  

Generating all the electricity needed to power air conditioners creates a lot of pollutants that contribute to Global Warming.  The Greenhouse-Effect is caused by the adding carbon-dioxide (a combustion by-product) to the earth’s atmosphere.   The worst offenders are coal-burning power plants, with natural gas plants producing only a a fraction of the air pollutants of coal.  Nuclear, hydro / water power, wind, and solar energy do not produce air pollution, but these sources of electrical power are small in relation to coal and natural gas.

Additionally, up to 15% of the electricity generated is lost during transmission through power lines and transformers.  This is an expensive problem that cannot be avoided, and adds to the price we pay for electricity.  

In Parts 1-3 of this 5-Part Series discusses many reasons why an air conditioner is not cooling at its maximum ability, and how to remedy the problem(s).  In Parts 4 & 5 are ways to improve your home’s energy-efficiency, which creates less demand on your air conditioner.  The end result: lower cooling costs, and HVAC Systems which break down less and last longer.

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 HVAC Systems 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 the A/C is sized properly to the space, and is not able to keep up with cooling-demand,  the a/c has operational problems and/or the home needs energy-saving improvements. 

Part 1 of this 4-Part Series Covers:

  • How to test your Air Conditioner’s cooling performance.
  • Air conditioner low on refrigerant.
  • A/C outside unit is very dirty, reducing air-flow through the unit, and reducing it’s ability to get rid of the amount of heat it’s designed to.
  • Cooling losses due to under-insulated or uninsulated ductwork.
  • Adding insulation to existing ductwork.
  • Properly installed flexible ductwork — what it looks like.
  • Improperly installed flexible ductwork — what it looks like.
  • Air-leakage from ductwork.

Any or all of these problems can cause an air conditioner to be under-performing, and running longer & harder than it needs to.

During Operation, the air coming out of your A/C (at the duct nearest the furnace) should be 14–20 Degrees Cooler than air going in (at the air-return grill nearest the furnace).  An air conditioner that is not producing this temperature difference could be low on refrigerant, the outside unit could be dirty, and other operational issues may exist.

An air conditioner cooling more than a 20 degrees likely has air-flow restrictions.  If the HVAC System is operating properly, but there is unsatisfactory cooling throughout the home, it is likely due to: Clogged Air Filter, HVAC Cleaning & Maintenance Needed, or Ductwork Problems.

Use A Thermometer To Make Checking Air Conditioner Performance Easy

handheld digital thermometer
There are many brands of handheld digital thermometers.  Their cost is modest and they provide a quick and easy way to check your A/C’s cooling.   

Steps To Perform an A/C Cooling-Performance Test

  • Install a new furnace filter.  If the furnace filter is visibly dirty, it’s restricting air-flow, and the output-temperature reading will be colder than with a clean filter.
  • Turn a/c on for 15 minutes and keep it running during the test.
  • Read the input-temperature of the air going into the HVAC System at the Return-Air Grill nearest the furnace.
  • Read the output-temperature of the air coming out of the a/c at the Supply-Air Duct nearest the furnace.   The air coming out should be 14–20 degrees lower.

return air duct in ceiling

return air duct at floor

air supply ceiling vent







  • Return-Air Grills are larger and can’t be shut off with a lever.  They can be in the ceiling, wall, or near the floor.
  • Supply-Air ducts are smaller and have a shut-off lever.  In most cases they are in the ceilings in DFW homes.


If your air conditioner is not producing the cooling described above (14-20 degrees) there is an operational issue.  If it’s producing output-air that’s more than 20 degrees cooler, there is also an operational issue (likely too little air-flow through the system).

The two most common reasons the a/c is not cooling properly are:

  • Air conditioner is low on refrigerant
  • Outside unit is very dirty and cannot perform as designed


The first evidence that refrigerant is low is the cooling temperature-spread is not present.  Also, an a/c low on refrigerant may form visible frost or ice.  If you see frost anywhere on your a/c (inside or outside) it’s likely low on refrigerant.

a/c low on refrigerant causing frost to form

a/c frozen due to being low on refrigeranta/c low on refrigerant causing frost to form

home central a/c evaporator coil iced up







  • 3 Outside (condenser) units showing frost.
  • An Indoor cooling-coil (evaporator coil) showing frost.  Note: The cooling coil is behind a panel (that must be removed) in order to see if it’s frosted.


very dirty and clogged a/c condenser coil

The outside unit is far too dirty.  This restricts air-flow through it, and reduces its ability to get rid of as much heat as it’s designed to.  The condenser unit needs to be cleaned at the beginning of each cooling season to ensure it’s able to perform at its maximum ability.

If you are handy around the house, you may be able to do this.  If not, call Al’s for a Spring A/C Tune-UP.  The Technician will advise you if the condenser coil needs to be cleaned, and if other operational problems exist and need to be corrected.  With any needed maintenance or repairs completed, your A/C is able to cool at its maximum ability, resulting in lower cooling costs and increased comfort inside the home.  Your A/C will also last longer and require fewer repairs if it’s able to perform as it was designed to.


If A/C Is Working Right, But Not Cooling The House Well, The Problem May Be The Ductwork

Do The Temperature Degree-Difference Test For Farthest Supply-Air Duct In The House

  • This will tell you how much cooling / heating is lost while air travels farther through the ductwork (to the farthest vent from the furnace).


  • Air coming out of the nearest duct (closest to the furnace) is -15 degrees cooler
  • Air from the farthest duct is only -10 degrees cooler — there is a 5 degree cooling loss due to air moving through the ductwork (to the farthest duct).
  • A Rule Of Thumb: The air coming from the duct (farthest from the furnace) should be no more than 10% warmer (than the nearest duct).

Ductwork Inefficiency Results Primarily From 3 Things:

1. Thermal Losses From Under Insulated or Uninsulated Ductwork 

According to the U.S. Department of Energy (DOE) — Cooling and Heating Temperature losses (while the air travels through the ductwork in attics) range from 10-45%. If the temperature at the closest duct is -15 degrees, the cooling loss should be no more than 10% (no more than 1.5 degrees warmer at the farthest duct).   If the cooling loss is higher than 10%, consider increasing ductwork insulation.

Insulated Flexible Ductwork

photo of flexible ductwork with all components identified

Newer Flexible ductwork used in attics looks similar to this

Flexible ductwork insulation can range from R-4 to R-8

proper flexible ductwork installation with mastic at connections

The Flex-Duct on right shows R4.2 Insulation

Today’s Building Code Requires Flexible Ductwork Have A Minimum Of R-6 to R-8 Insulation.   In The Past, R-4 met Building Code.

In order to increase insulation for flexible ductwork, you can replace it with new & better insulated ductwork.  Adding insulation to metal ductwork can be done by applying Closed-Cell Spray Foam (CCSF) Insulation.  This is a big job and likely should be left to a Spray-Foam Insulation Contractor.   They have: the proper installation equipment, the skill & knowledge to install the foam correctly, and purchase the foam in large quantities (making the foam material less expensive).

Benefits Of Adding Closed Cell Spray Foam (CCSF) Insulation On Ductwork:

  • 2 Inches of CCSF creates R-12 insulation.  This is in addition to the R-Value of Existing Insulation.
  • Properly applied CCSF seals the ductwork from air leaks.  
  • CCSF also provides a moisture-barrier which will keep the ductwork from sweating during DFW’s Sub-Tropical Summer Climate humidity levels.

spray foam insulation added to attic ductwork

Attic ductwork after spray foam is applied


There can be big problems with flexible ductwork is if it’s installed improperly.  Flex-duct has specific requirements for routing and supporting it (with hangers).  If it’s installed correctly, flex-duct performs well.  If the installation is poor, flex-duct’s performance can be dramatically reduced.   Flex-duct is lightweight and more easily installed than (rigid) metal ductwork.   If you have flex-duct, take a look in your attic to see what your installation looks like, and compare to the two sets of photos below.

Properly Installed Flexible Ductwork

In the left photo, you see:

  • Flex-duct installed (as much as possible) near the attic floor (versus the roof where temps are much higher).
  • The ductwork is properly supported (and not sagging).
  • Curves are not too short or kinked (which reduces air-flow).

correctly installed flexible ductwork

proper flexible ductwork installation requirement for bends in ductwork

proper flexible ductwork hanging requirements

Flex-Duct has Installation Specifications that must be followed for the ductwork to performed as it’s designed to.  If these specifications are not followed, flex-duct will not perform properly — and it’s not the material’s fault.

Improperly Installed Flexible Ductwork

If your attic looks like these photos, your flex-duct installation needs to be corrected.  The repair cost will be repaid in lower energy costs.

poorly installed flexible ductworkpoorly installed flexible ductworkpoorly installed flexible ductwork

improperly installed flexible hvac ductwork. ductwork crimped because bend is too small

These photos show flexible ductwork installation that is not supported correctly.  Flex-Duct is to be installed using hangers, not components of the home’s construction to support it.  The photo on right shows a bend in the ductwork that is way too tight.

improperly installed flexible hvac ductwork. no metal collar between 2 flex duct
proper flexible ductwork hanging requirements, join 2 flexible duct with metal collar

The photo on the left shows two pieces of Flex-Duct that have come apart.  The diagram on the right shows how a metal collar must be used to connect 2 pieces of Flex-Duct (so the pieces can be properly attached to the collar).

flexible ductwork that has failed and is leaking a lot of air

  The photo below is flex-duct that has torn because it’s worn out.  Flexible ductwork has a lifespan of around 15 years.


In an older home, it’s not uncommon to find metal ductwork that is not insulated or sealed for air leakage.   If it is insulated, there may be very little.   If it was sealed, it’s likely duct tape was used.  Duct tape fails very fast in the high temperature settings (like DFW attics).

The photo below shows where someone added some insulation on top of uninsulated ductwork.  You will also see that only the main / trunk-line has insulated added, the branch lines (round ductwork) have no insulation.


This ductwork was a good candidate for adding Closed-Cell Spray Foam Insulation.  There will be the benefits of  both added insulation + air-leakage sealing.

existing attic metal ductwork covered with spray foam insulation

“After Photo” with Spray Foam Insulation Added.

Today’s Metal Ductwork

metal ductwork with inside insulation

properly insulated metal ductwork

Today’s new Metal ductwork can be purchased: Uninsulated, Insulated on the inside, or Insulated on the outside

2. Air Leakage From Ductwork      

The U.S. Department of Energy (DOE) states research found that 30–40% of the air traveling through ductwork leaks out.

Read the entire article here: Article About Leaking Ductwork — #1 Energy Waster

Flexible ductwork does not last as long as metal.  Even if the ductwork installation was perfect, flexible ductwork degrades with age.  According to Certified Home Inspections in Oregon,  Flexible Plastic and Fiberglass ducting has a lifespan of 14-16 years (which is longer than the air conditioner and furnace are expected to last).    You can see more details with this link: Flexible Ducting Lifespan.

photo of flexible ductwork with all components identified

In DFW attics during summer,  ductwork is subjected to temperatures of up to 160 degrees.  Replacing the HVAC Components, but reusing flexible ductwork (installed when the previous system was installed) is a bad idea.   Flexible ductwork needs to be replaced each time the HVAC Components wear out and are replaced.

As the plastic liner in flexible ductwork ages, it’s more likely to split, especially where there are bends in it.  Also, some flexible ductwork moves each time the furnace blower turns on and off, creating movement year round.  Additional benefits of new flexible ductwork:  it’s new and clean & clear of: dust, pet hair & dander, mold spores, and other built up contaminates.

flexible ductwork that has failed and is leaking a lot of air

Worn out flexible ductwork with tears and holes

1/3 Or More Of The Money Spent Cooling & Heating Your House May Be Ending Up In Your Attic Due To Leaking Ductwork!

The one energy-saving effort of having ductwork inspected and repaired or replaced could reduce your cooling and heating costs by up to 40%.  Additionally, if your older home has uninsulated ductwork, your savings are further increased by adding insulation.  It’s not difficult to realize you may reduce cooling and heating bills by 1/2 or more if you correct both uninsulated + leaking ductwork.

Actual Photos Of Existing Ductwork

ductwork sealed with duct tape

uninsulated and leaking hvac ductwork

leaking hvac ductwork

disassembled and leaking hvac ductwork









Older Homes’s HVAC Metal Ductwork:

  • Was installed back when little or no attention was paid to ensure it did not leak air
  • Even well installed ductwork can come loose, or be bumped by a worker in the attic.
  • Duct Tape was a common method of sealing (if any).  Duct tape fails within a year.


Duct Tape Fails Quickly In Very Hot Environments Like DFW Attics, And Has Been Has Been Replaced With Mastic

ductwork sealed with mastic

Newer Ductwork Installation With Mastic Sealing Every Connection

Benefits of Sealing Ductwork With Mastic Sealer


  • Seals areas of air leakage
  • Goes on soft and pliable
  • Does not harden or crack over time
  • Because mastic is applied on the outside of ductwork, it won’t be affected by future ductwork cleanings

spinning brush ductwork cleaning

Photo Showing Spinning-Brush Used For Ductwork Cleaning

Most Ductwork Cleaning includes a spinning-brush to break away any soil that is stuck.   Ductwork sealing products (installed inside the ductwork) may be subject to damage or degradation due to the spinning brush.


3. Insufficient Or Blocked Air-Flow Due To Ductwork

We have discussed how poorly installed flexible ductwork can restrict or block air flow.   While blocked air flow is less likely with metal ductwork, restricted air flow can easily be a problem due to ductwork being too small.  This problem can be further compounded by installing a new HVAC System that produces a larger amount of air-flow than the existing ductwork was designed for.  Existing ductwork can often be modified by increasing the size of some ductwork, or adding additional ductwork lines.

A 1996 Study For The Arizona Public Service Co revealed: *

  • 14% of newly constructed homes had an air-flow of 90% of normal air-flow requirements (400 CFM)
  • 39% tested at 80%
  • 14% tested at 70%
  • 7% tested at 60%

Many ductwork problems began when the home was built.  Years later, if a larger replacement HVAC System (with a larger air-flow capacity than the original) was installed — air-flow problems are increased.  It’s safe to say that while most homes built today have better ductwork systems, some may still not have the ductwork system needed.

* Source: Troubleshooting Ductwork Is Easier Than You Think — See link below to the complete article.   We have provided this article which goes into more detail than our blog post: Ductwork Troubleshooting

Continued In Part 2 Of 3

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