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This article discusses the best ways to determine if a gas furnace has a cracked heat-exchanger.  Al’s Plumbing, Heating & A/C in Plano, Texas is near your home in Plano, Allen, and Frisco.  We service all homes in southern Collin and Denton Counties with no travel charges.   Al’s provides maintenance & repairs for all brands of Central A/C, Gas & Electric Furnace, and Heat Pumps.  Additionally, we sell and install new HVAC Systems from American Standard (same company as Trane), Ameristar (same company as American Standard) and Coleman HVAC (same company as York HVAC).

Al’s also provides Full-Service Pplumbing Maintenance, Repairs & Replacements for every plumbing component in your home.  Al’s sells and installs Rheem Professional Series gas & electric water heaters, and tankless water heaters.  Call Al’s today to discuss any concerns or problems you have with your HVAC System or Plumbing.  We will arrange an appointment at your convenience.

photo of heat exchanger in gas furnace

Photo Source: Dreamstime

SHOWN: A New Gas Furnace Heat-Exchanger 

Click Here To Read: AlsPlumbing.com Time For A New Furnace?

How To Determine If A Gas Furnace Has A Cracked Heat-Exchanger

How The Heat-Exchanger Works  

The heat-exchanger keeps exhaust-gasses and the home’s indoor air separate: 

  • Burners create heat + combustion/exhaust gases.
  • The heat + combustion-gases flow inside the heat-exchanger.  Then the exhaust-gases exit the furnace and home
  • The blower fan turns on — and the home’s indoor air passes along the outside of the heat-exchanger.
  • Then heated air is recirculated throughout the home.

This YouTube Video Shows How a Heat-Exchanger Works: How A Furnace Heat Exchanger Works

Why A Heat-Exchanger Cracks

Each time a gas furnace goes through a heating-cycle — the metal heat-exchanger expands as it heats up.  At the end of the heating cycle — the heat-exchanger cools off and contracts.   Over the years, metal-fatigue caused by recurring expansion and contraction often causes a creak in the heat exchanger.  Once cracked, the crack will get worse as time passes.

A cracked heat-exchanger can leak Carbon Monoxide (CO) into the home’s indoor air.  This poses a safety and health hazard for the home’s occupants.  In lower amounts, Carbon Monoxide will make you sick.  In high concentrations — it can kill you.  Since CO has no odor, color or taste — it can’t be detected by our senses.  This means that dangerous concentrations of CO can build up indoors and we can’t detect it. **

NOTE: CO2 Detectors Can Save Your Life.

** Source: https://www.health.state.mn.us/communities/environment/air/toxins/index.html#:~:text=Since%20CO%20has%20no%20odor,problem%20until%20they%20become%20ill.

 

How To Know If A Gas Furnace Has A Cracked Heat Exchanger 

There are two primary ways to check for a cracked heat exchanger.  Both must performed by a qualified HVAC Technician.

  • Exhaust Gasses Analysis.
  • If the heat exchanger is cracked — oxygen level in the exhaust / combustion gasses will rise while the furnace is heating up (because the crack gets wider as the metal gets hot).
  • Visual Inspection Of Furnace During Operation.
  • If the heat exchanger is cracked, when the blower-fan turns on — often the burners’ flames distort due to air coming through the heat-exchanger’s crack.

 

Click Here To See The Burners’ Flames Distort When The Blower-Fan Turns On: Cracked Heat Exchanger Causes Distorted Flames When Blower-Fan Turns On

 

In some cases — owners discover they have a cracked heat-exchanger because the furnace shut down.

  • Furnaces are equipped with a safety-feature called a “Flame Rollout Limit Switch”.
  • If the flames come outside the proper area (like in the video above) — the Flame Rollout Limit Switch senses it and shuts the furnace down to protect itself and you.

 

Click Here To See A Flame Rollout Limit Switch: Photo Of Flame Limit Rollout Switch

 

 If A Heat-Exchanger Is Cracked — The Oxygen Reading In The Furnace’s Exhaust Rises To Abnormal Levels. 

This Will Be Detected With A Combustion Gasses Tester.

 

Click Here To See A Video Testing A Furnace WITH A Cracked Heat Exchanger:  TEST: Furnace WITH A Cracked Heat Exchanger

In This Video — Exhaust Gasses Oxygen Level Rises Substantially While The Furnace Is Heating Up (starts at Time: 1:10 / 2:06).

 

Click Here To See A Video Testing A Furnace WITHOUT A Cracked Heat Exchanger:  TEST: Furnace WITHOUT A Cracked Heat Exchanger

In This Video — Exhaust Gasses Oxygen Level Drops While The Furnace Is Heating Up (starts at Time: 1:55 / 2:14).

 

The Two Common Factors Can Accelerate Heat-Exchanger Failure. 

This Is Due To Excessive Heat-Exchanger Temperature During Heating Cycles

 

Click Here To See A Furnace & Central A/C Diagram (near upper right-hand corner of the diagram):  Furnace & Central A/C Diagram

 

  • Poor furnace maintenance — If the furnace is not maintained properly (such as dirty air filter or dirty A/C cooling/evaporator coil) — the heat-exchanger can repeatedly become too ho — and this can cause it to fail sooner.  The furnace filter needs to be checked each month, and replaced when needed.  If it looks dirty, it is, and needs replaced.

Click Here To See A Severely Neglected Furnace Filter: Very Dirty Furnace Filter

 

Click Here To See How Dirty Central A Central A/C Cooling (evaporator) Coil Can Get: Very Dirty Cooling / Evaporator Coil

 

  • Improper installation — If the furnace was installed improperly,  the air flow may be restricted — and cause the heat-exchanger to become too hot.  This may cause the heat-exchanger to fail sooner.

 

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

  • Ask the technician to show you the crack.
  • If the tech can’t show you the crack — that doesn’t mean there isn’t one.
  • Only a small portion of the heat-exchanger is visible without dismantling the furnace (and that’s not necessary — see just below).
  • Ask the tech to show you the results from a Combustion-Gases Analysis.
  • If this test was not performed, or the tech is not equipped to perform one.  Stop there and get a second opinion.

Immediately Get A Second Opinion

  • Find a different HVAC Contractor who performs Combustion-Gases Testing / Analysis (sometimes called exhaust-gasses analysis).
  • This step should be part of a fall gas furnace tune-up.  Get fall furnace tune-ups only from HVAC contractors who include combustion / exhaust-gasses 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 / exhaust-gasses testing.
  • Other than seeing a visible crack — a combustion-gases analysis is the most reliable way to determine if the heat-exchanger is cracked.

 

Benefits Of A Combustion-Gasses Analysis

  • It tests under actual operating conditions.
  • It can be done during a normal service call that includes combustion gasses testing.
  • It can detect if the heat-exchanger is cracked.
  • It may also indicate how dangerous a crack is.

Red-Tagging A Furnace

If test readings provide documentation that a furnace with a cracked heat-exchanger is not safe — the furnace may get “Red Tagged” meaning it’s immediately shut down and taken out of service.  Red Tagging can also be done by a gas-utility employee.

According to the 2006 International Mechanical Code (IMC) — part of Building Codes used by most states:

“An Unsafe Mechanical System Is One That:

  • Is unsafe, constitutes a fire or health hazard, or is otherwise dangerous to human life, as regulated by this code is hereby declared as an unsafe mechanical system.
  • Use of a mechanical system regulated by this code constituting a hazard to health, safety or welfare by reason of inadequate maintenance, dilapidation, fire hazard, disaster, damage or abandonment is hereby declared an unsafe use.
  • Such unsafe equipment and appliances are hereby declared to be a public nuisance and shall be abated by repair, rehabilitation, demolition or removal.”

SOURCE: https://www.achrnews.com/articles/109584-red-tagging-a-furnace-who-is-responsible

 

A Cracked Heat-Exchanger Is Cracked — Either The Heat-Exchanger Or The Furnace Must Be Replaced.  

Your Options Are:

Replace The Heat Exchanger. 

  • If the heat exchanger is still under warranty — this is a good way to go.  Keep in mind that furnace warranties are almost always for parts only — you have to pay installation labor charges.
  • 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.
  • 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 your furnace’s age & efficiency.   If a furnace is out of warranty and more than 10 years old, it’s likely best to replace it versus putting an expensive part into an old furnace.
  • There are several expensive furnace components that can fail with time.  Two of the most expensive are the control-board (the furnace’s brain) and the blower fan.
  • If you’ve replaced the heat-exhanger, and either of these components fails — you’ll have spent more money fixing the furnace than its replacement cost.

 

Why It’s Wise To Get A Fall Gas Furnace Tune-Up Each Year

This can save you the worry, and higher Emergency Service rates because your furnace stopped working during cold temperatures, often because of something that would have been discovered during a Tune-Up.

Example:  A dirty Flame-Sensor will cause a furnace to stop running.  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 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.

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Al’s Plumbing, Heating & A/C in Plano, Texas is near your home in Plano, Allen, and Frisco.  We service all homes in southern Collin and Denton Counties with no travel charges.   Al’s provides maintenance & repairs for all brands of Central A/C, Gas & Electric Furnace, and Heat Pumps.  Additionally, we sell and install new HVAC Systems from American Standard (same company as Trane), Ameristar (same company as American Standard) and Coleman HVAC (same company as York HVAC).

Al’s also provides full-service plumbing maintenance, repairs and replacements for every plumbing component in your home.  Al’s sells and installs Rheem Professional Series gas & electric water heaters, and tankless water heaters.  Call Al’s today to discuss any concerns or problems you have with your HVAC System or Plumbing.  We will arrange an appointment at your convenience.