This article is about ways to determine if a gas furnace has a cracked heat-exchanger.   Al’s Heating & A/C provides repairs of all brands of Central A/C, Gas or Electric Furnaces, & Heat Pumps.  Al’s sells and installs new HVAC systems from American Standard (made by the same company as Trane), Ameristar (made by same company as American Standard), and Coleman HVAC (made by the same company as York HVAC).    Al’s also provides repairs and replacements of every plumbing component throughout your home.

Calls Al’s today to discuss any concerns or problems you are having.  We can arrange an appointment at your convenience.

image says red tag furnace? we can help

How To Determine If A Furnace Has A Cracked Heat Exchanger 

photo with words cracked heat exchanger


Most problems with a gas furnace can be repaired, though the repair cost is not always justified.  Most repairs require replacing a component that will return the furnace to normal and safe operation.  A furnace with a cracked Heat-Exchanger is usually an exception.      Note: Electric furnaces do not have a heat-exchanger.

Why A Heat-Exchanger Cracks

Heated metal expands.  As metal cools, it contracts. This recurring expansion & contraction is part of the normal furnace heating cycle. All heat-exchangers eventually fail due to metal fatigue.  Constant expansion & contraction has a similar effect as bending a paperclip back and forth.  Over time, the metal fatigues and the paper clip breaks.

Some factors that can accelerate heat-exchanger failure.  These factors are usually:

  • Poor furnace maintenance
  • Improper installation
  • Furnace manufacturer design

In the past, the only way to check for heat-exchanger crack(s) was to perform a visual inspection.  Today there are scopes & camera equipment to help search for cracks.  But even with these devices, it’s still not possible to see the entire heat-exchanger.   Unless the furnace is disassembled to the point so you see the entire heat-exchanger, a visual inspection can miss a crack(s).   In the photos just above, you can see how enclosed the heat-exchanger is inside the furnace.

If You Are Told Your Furnace Has A Cracked Heat-Exchanger

  • Ask the technician to show you the crack.
  • If the tech cannot show you the crack.  Stop there and find a different HVAC contractor.
  • Ask the tech to show you the results from a Combustion-Gases Analysis.
  • If this test was not performed, and the tech is not equipped to perform one.  Stop there and get a different HVAC contractor.

Immediately Get A Second Opinion

  • Find a different HVAC Contractor who performs Combustion-Gases Testing / Analysis (it’s called different names).  This step is often part of a furnace tune-up.  Get tune ups only from HVAC contractors who include exhaust-gases testing.
  • Don’t tell them why you want the test. Simply order a furnace tune-up.  Ask to confirm their furnace tune-up includes combustion-gases testing.
  • Short of seeing a visible crack, a combustion-gases analysis is the surest way to determine if the heat-exchanger is cracked.


Here Is What The Tech Looks For — While The Burners Are Lighted And The Blower Fan Is Running:

  • If a crack is present: When the blower fan is running, enough air should be blown through the crack to raise the combustion analyzer’s oxygen reading (out of normal range)  Too much oxygen in the exhaust-gases is a sign of a cracked heat-exchanger.
  • A second (less reliable) way is to observe the burners just before & just after the blower fan turns on.  When the blower fan starts, if the burners’ flames begin to distort and / or turn more yellow, these are signs of a cracked heat-exchanger.

photo of a blue and a yellow fire flame

When operating correctly a gas-fired device will have nearly completely blue flames.  If the flame is yellow, it is an indication something is wrong.

photo of gas furnace flames distortion

Distorted Flames Coming From The Burners Indicate A Problem

photo of gas furnace burners operating properly

Burners’ Flames In A Furnace That Is Running Properly

This YouTube Video Shows Flames Distortion & Color Changes When Blower Fan Turns On Distorted & Yellow Burner Flames Due To Cracked Heat-Exchanger

(You will see both flames distortion and the flames color changing from nearly all blue to mostly yellow).

A Cracked Heat-Exchanger:

Either The Heat-Exchanger Or Furnace Must Be Replaced

photo of new furnace's heat exchanger

New Furnace Heat-Exchanger

photo of heat exchanger in old furnace

Heat-Exchanger In Old Furnace

furnace heat exchanger crack visible in photo

Cracks In Heat-Exchanger Are Circled In Red

Your Options If The Heat-Exchanger Has Cracked

  • Replace The Heat Exchanger.  If the heat exchanger is under warranty, this is a good way to go.  If the part is under warranty but not available, ask the contractor to call the manufacturer and request the cost of the heat exchanger be removed from the new furnace’s price.  It’s rare that labor charges are part of the Heat-Exchanger Warranty.
  • If the heat exchanger is not under warranty, compare the cost to replace it (parts & labor) compared to a replacement furnace.   Also consider the factors just below.
  • Replace The Furnace.  Factor in the existing furnace’s efficiency & age.  There are several other expensive furnace components that can fail with time.  Two are the control-board (the furnace’s brain) and the blower fan.  If a furnace is out of warranty and more than 10 years old, it’s likely best to replace it.

How An Exhaust-Gases Test Can Reveal A Cracked Heat-Exchanger

photo man drilling hole in furnace vent pipe photo of flue gases testing hole in pvc furnace vent pipe 

Holes Are Drilled Into Furnace Exhaust-Vent To Insert The Exhaust Gases Tester’s Prob (Probe Is Just Above The Yellow Tester In Photo On The Right).


This Video Shows Using Combustion-Gases Analyzer To Test For A Cracked Heat-Exchanger. 

 YouTube Combustion-Gases Testing / Analysis


As described above (and shown in the YouTube videos) a cracked heat-exchanger forces air (from inside the house) into the furnace’s combustion chamber.  This raises the level of oxygen in the exhaust gases.  You will also likely see the effects of the additional air-flow, showing as the burners’ flames becoming distorted and / or flames chaning color from mostly blue to lots of  yellow.

When natural gas is burned, the amount of oxygen in the exhaust-gases is reduced (from the earth’s natural oxygen level -21.9%).  This is because oxygen was consumed during the combustion process.    If no crack(s) exist, when the blower fan turns on, there will little change in the exhaust gases oxygen level.  If a crack(s) exist, the oxygen level will increase notably.

Higher than normal Carbon Monoxide (CO) readings may indicate a crack.  A test reveals if a furnace is producing CO.  CO is a symptom of improper gas combustion.  A clean and efficiently operating gas furnace produces very small amounts of carbon monoxide.  A dirty and inefficiently running furnace may produce large amounts of CO.

CO production can also be due to a furnace needing maintenance or dirty burners.  As air enters & exits the burner, it brings dust particles with it. These dust particles continue to accumlate on the burners.   It’s just like the dust that accumulates in your home as a result of air infiltration.  And there is a tremendous amount of air moving through the burner, so it becomes dusty faster than your house.

In The YouTube Video Demonstrating Exhaust-Gas Testing:   

YouTube Combustion-Gas Analysis Testing For A Cracked Heat-Exchanger

The Steps (in the video) Show How The Tech Determines The Status Of The Heat-Exchanger:

  • The tech says he’s providing a second opinion on a furnace reported to have a cracked heat-exchanger (at time in video: 0:10 / 5:17).
  • He checks for a hole in the exhaust-vent from when the exhaust-gases were tested.  He confirms no hole exists ( 0:29 / 5:17).
  • The tech first examines the flames.   He confirms the flames have no distortion and their color is blue.  (1:15 / 5:17).
  • He inserts the tester’s probe into the exhaust-vent.  At the time the probe is inserted, it shows an oxygen level shows 20.9%. (1:18 / 5:17).
  • The tester shows the oxygen level in the exhaust-gases = 11.1% (at time 2:44 / 5:17).  This shows the exhaust-gases oxygen level is lower (like it’s supposed to be) while the blower is running.  At time of the test, the furnace was using 9.8% of the available oxygen during the combustion of the natural gas.
  • This test shows the exhaust gases are within normal range (you can hear the blower fan running).  This would not be the case if the heat-exchanger was cracked.
  • The tech states:If The oxygenation level is within range.  You do not have a cracked heat exchanger.”  (2:43 / 5:17).


The Benefits Of A Combustion-Gases Analysis

  • With a cracked heat-exchanger, the amount of oxygen inside the combustion-chamber rises while the blower fan is running.  This is because air is entering the combustion-chamber through the crack in the heat-exchanger.  This raises the amount of oxygen in the exhaust gases.
  • It tests under actual operating conditions.
  • It can be easily done during a normal service call that includes combustion gases testing.
  • It may also indicate how dangerous a crack is.

Example: With a severe crack, the (combustion-gases) oxygen level rises considerably while the blower fan is running.  This gives the tech more confidence the heat-exchanger is cracked.   Test readings provide documentation that a furnace with a cracked heat-exchanger is not safe.  It may be “Red Tagged” meaning it’s immediately shut down and taken out of service.

SOURCE:    Established in 1926, The Air Conditioning, Heating and Refrigeration weekly news magazine is the HVAC Industry’s direct communications link for HVAC Contractors and those buying their services.


furnace red tag indicating the furnace is not safe to operate


Red-Tagging  is when a Furnace Technician or Utility Representative shuts a furnace down because it is unsafe.    The best way to avoid a furnace being red tagged is to keep the heating system well-maintained.

If Your Furnace Is Red Tagged: 

  • Ask the technician to show you specifically what caused the furnace to be red tagged.
  • Get a second opinion right away.
  • Tell the contractor you have a red-tagged furnace.   Many contractors will make every effort to provide for Emergency or Next Day Service when a furnace is disabled.


If You Have Any Gas-Fired Appliance In Your Home, You Need Carbon Monoxide Detectors

Any gas appliance can be operating safely one day and become dangerous the next.  A cracked heat-exchanger is a good example.

photo of carbon monoxide alarm

 image says red tag furnace? we can help


Call Al’s Plumbing, Heating & A/C For 24/7 Emergency Furnace Service

Why To Get A Fall Furnace Tune Up

Get A Furnace Tune-Up Each Fall.  This can save you the worry, and higher Emergency Service rates because your furnace shut stopped working because of  something that would have been corrected during a Tune-Up.

Example 1:  A dirty Flame-Sensor will stop a furnace from working.  It’s most likely to happen when it’s very cold and the furnace is running a lot.  As part of a Furnace Tune-Up, the Flame-Sensor will be cleaned.

The Flame Sensor is a located at the burner assembly.  It sits in front of the burner’s flame.  The purpose of the flame sensor is to confirm a flame is present whenever the gas valve is open.  If the flame sensor is dirty,and cannot determine a flame is present, the furnace will close the gas valve and shut down to protect the home from a gas explosion.

Example 2: Air entering & exiting the burners has dust particles in it. These dust particles begin to form a coating of dust on and around the burners.  This needs to be cleaned away for proper and efficient burner operation

photo of a furnace flame sensor

Furnace Flame Sensor Circled In Red

You Can Also See The Dust Accumulation On the Metal Just Above The Flame Sensor



What A Heat Exchanger Is, And What It Does

photo of new furnace's heat exchanger

SHOW: Furnace Heat-Exchanger

HOW The Heat-Exchanger Keeps Combustion-Gases Separate

From The Home’s Indoor Air  

  • Burners create heat + combustion-gases.

  • Heat + combustion-gases move inside the heat-exchanger’s chambers / pipes.

  • The heat is extracted as it warms the heat-exchanger’s chambers / pipes.

  • Exhaust-gases then exit the furnace.

  • Blower fan turns on and blows the home’s indoor air along the outside of the heat exchanger’s chambers.

  • Heated air is then recirculated throughout the home.

This YouTube Video shows how a heat-exchanger works: YouTube (courtesy of Daikin) How A Furnace Heat Exchanger Works



Al’s Heating & A/C provides repairs of all brands of Central A/C, Gas or Electric Furnaces, & Heat Pumps.  Al’s sells and installs new HVAC systems from American Standard (made by the same company as Trane), Ameristar (made by same company as American Standard), and Coleman HVAC (made by the same company as York HVAC).    Al’s also provides repairs and replacements of every plumbing component throughout your home.

Calls Al’s today to discuss any concerns or problems you are having.  We can arrange an appointment at your convenience.