This article discusses the best way 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 Richardson, TX; Garland, TX; and Wylie, TX.  We service all homes in southern Collin County, TX and Denton County, TX with no additional 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 (owned by American Standard) and Coleman HVAC (owned by York HVAC).

Al’s also provides Full-Service Plumbing 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.

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

Photo Source: Dreamstime

SHOWN: A New Gas Furnace’s Heat-Exchanger 

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

How The Heat-Exchanger Works  

The heat exchanger is the metal component (inside the furnace and can’t be easily seen) that heats up while the burners are running.    In a furnace that’s operating properly & safely — the exhaust gases pass through the inside of the heat exchanger and into the flue — where they exit the home.

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.

Note: High-E furnaces have a 2nd, smaller heat-exchanger that extracts the heat from the exhaust gasses before they are pushed outdoors.

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 & contraction, often causes a creak in the heat exchanger.  Once cracked, the crack will continue to get worse.

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 won’t know.  **

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

An Important Side Note: CO2 Detectors Can Save Your Life

These symptoms often occur when exposed to small levels of carbon monoxideCarbon Moxoside (CO).  They mimic flu symptoms — but will clear up after you are out of the house.

• Headache
• Dizziness
• Weakness
• Upset stomach
• Vomiting
• Chest pain
• Confusion

How To Test A Gas Furnace For A Cracked Heat Exchanger 

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

• Exhaust Gasses Analysis.
• If the heat exchanger is cracked — oxygen level in the combustion gasses will rise while the furnace is heating up — because the heat exchanger’s crack gets wider as the metal gets hot.
• The video below shows this test as it’s occurring.  Click on white arrow in the center to view.
• In this video — oxygen levels in the exhaust gasses lower while the furnace is heating up.

This Video Shows Testing A Furnace WITHOUT A Cracked Heat Exchanger

Image Source: YouTube Embedded Video

Click On The White Arrow In Center Of Image To View The Video

In this video — the heat-exchanger is not cracked.  This is evidenced when the Oxygen Level (in exhaust gasses) Drops While The Furnace Is Heating Up (starts at Time: 1:55 / 2:14).

This Video Shows Testing A Furnace WITH A Cracked Heat Exchanger

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

Image Source: YouTube Embedded Video

Click On The White Arrow In Center Of Image To View The Video

Sometimes Owners Discover Their Heat-Exchanger Is Cracked Because Their Furnace Shut Down

A cracked heat-exchanger can cause the furnace to shut down.

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

• See “flame-rollout” of the burners’ flames for yourself in the video below (at time 1:40 / 3:38).
• See the flame touch the rollout-fuse — and the furnace immediately turns the gas off to the burners (at time 2:00 / 3:38).
• This is a safety-feature to protect you from carbon-monoxide poisoning.

• If a flame touches the rollout-fuse — the furnace will not operate until the fuse is replaced.
• If the flame-rollout cause is correctable — then a new fuse can be installed and the furnace will run again.
• Otherwise, it won’t do any good to replace the rollout-fuse.  It burned out for a reason, and the new one will do the same thing.
• Often, flame-rollout is caused by the furnace’s blower-fan forcing air into the combustion-chamber through a crack in the heat-exchanger.

• When a heat-exchanger cracks, you can replace it — or replace the furnace.
• Unless the furnace is so new that the heat-exchanger is still under warranty (part only – you pay the installation charge) — you’re better off replacing the furnace.
• Replacing a heat-exchanger will cost 2/3 — 3/4 the price of a new gas furnace (including installation).
• And then, you have an expensive new part in a furnace that’s nearing the end of its service-life (usually around 15 years).

Image Source: YouTube Embedded Video

Click On The White Arrow In Center Of Image To View The Video

Two Issues Can Accelerate Heat-Exchanger Failure

Caused By Excessive Heat-Exchanger Temperature During Heating Cycles

• Closing Off Too Many Air Ducts With A Standard (single fan speed) Furnace:  Your furnace was designed and built to operate with all ducts open.  When you close off more than 10% of the ducts — the furnace will draw in air at its rated air-volume level — but can’t push the air through the furnace (at that air volume) due to closed ducts.   Meanwhile, the furnace burners are heating at the furnaces’ rated air-volume — and the heat-exchanger is getting too hot.

Note: If the furnace has a variable-speed fan, it will adjust the blower-fan’s speed and burner’s output.  With this type of furnace, it’s ok to close off ducts.

• 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 will repeatedly become too hot, and fail sooner.

• The furnace filter needs to be checked each month, and replaced when needed.  If it looks dirty, it is and needs to be replaced.

Image Source: CanStockPhoto

• When a furnace filter gets dirty enough — it will buckle and the air will bypass it.  Then the dust gets caught in the central a/c cooling coil and ductwork.  In a short time, the cooling coil gets clogged.  This causes the furnace to run longer and hotter than it should.  And because the cooling coil is wet when the a/c runs (it’s what removes humidity) — it’s a perfect environment for mold to grow.

  Copy the link below into your browser to see both a clean and a clogged central a/c cooling coil:

  https://www.google.com/search?q=dirty+central+a+c+evaporator+coils&sxsrf=ALeKk00FvTeIXcsClmW8viT7WIQwlcnW7w:1600276473747&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj584m2lu7rAhVJSK0KHVaICaUQ_AUoAXoECA0QAw&biw=1280&bih=646#imgrc=N7pDpCOoUF4G6M

• Dust collects inside the ductwork — and  dust-mite’s feces is in the dust (it’s typically why we sneezed when near dust).  Additionally, with DFW’s very humid summers — dusty ductwork is an ideal place for mold to grow.  Each time the blower runs, it picks up dust mites’ feces, and mold spores (if molded) and makes them airborne inside your home.

Copy the link below into your browser to see (magnified) dust mite feces:

https://www.google.com/search?q=dust+mite+excrement&sxsrf=ALeKk03W01U5mq6Zlbx8MIHwLKsLSA4K1A:1606325284829&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj2s_v_m57tAhUKKqwKHdiWDD4Q_AUoAXoECAgQAw&biw=1097&bih=554#imgrc=ruWbNr7uizwBAM

Copy the link below into your browser to see badly molded HVAC ductork:

https://www.google.com/search?q=mold+hvac+ducts&sxsrf=ALeKk03bYIKQWpRY8UfYlemaFLhVMUCDOw:1606325352263&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjomY-gnJ7tAhVPMqwKHesvAy8Q_AUoAnoECAwQBA&biw=1097&bih=554#imgrc=C7qGBc7cX-EdgM

NOTE: Your Central A/C Cooling Coil (inside the furnace and can’t be seen without removing a panel) and Ductwork need to be cleaned at least every 5 years. 

The length of time between cleanings will be reduced if you have shedding pets, and with more people living in the home.  Older homes need these cleanings more than newer homes — because older homes leak more air.

These Installation Errors Will Cause A Furnace To Fail Before Its Time

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

• Ductwork that’s too small for the volume of air the furnace is rated to move.

• The furnace is too big for the house.  If it’s rated at 100,000 BTU, and your home needs a 75,000 BTU, the furnace is 25% too big — and the burners’ heating capacity is too high.    The air coming from ducts will feel too hot, and the home will heat up very quickly.  The house will typically seem too hot or too cold — this is caused by “short-cycling”.   Short-cycling leads to premature heat-exchanger failure because its expanding & contraction more, and more frequently, that it’s designed to.

• Note: A higher efficiency new furnace doesn’t need to be as large as the lower-efficiency furnace it’s replacing.
• A standard-efficiency furnace moves 20% of the heat it makes outdoors with the exhaust.
• A 90% furnace keeps 10% more of the total heat it makes (10% goes out with the exhaust) — so it doesn’t need to be as big as the one it replaced (if the old one is less efficient).

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

• 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).

• Note: 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.  Confirm that HVAC contractor’s furnace tune-ups 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.

Red-Tagging A Furnace

Image Source: YouTube Embedded Video

Click On White Arrow In Center Of Image To View The Video

If test readings provide proof that a furnace is not safe to operate — the furnace may get “Red Tagged”.  This means the furnace is immediately shut down and taken out of service by the HVAC Tech.  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.

• The mechanical system constitutes a hazard to health, safety or welfare — due to; inadequate maintenance, age beyond it’s customary service-life (dilapidation), fire hazard, damage or abandonment is hereby declared unsafe.

• Such unsafe equipment and appliances are hereby declared to be a public nuisance and shall be corrected by; repair, replacement, demolition or removal.

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

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.

Al’s Plumbing, Heating & A/C in Plano, Texas is near your home in Plano, TX; Allen, TX; and Frisco, TX.  We service all homes in southern Collin County; TX and Denton County; TX with no additional 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 HVAC (owned by American Standard) and Coleman HVAC (owned by 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.