This article discusses gas furnace failure, and identifies the components that may fail.   Al’s Plumbing, Heating & A/C in Plano, Texas 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.  Al’s is near your home in Plano, Allen, and Frisco.  We service all homes in southern Collin and Denton Counties with no travel charges.

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 and offer 24/7 Emergency Service.

Gas Furnace Failure — Which Components Fail And Why

Any of a number of components in a gas furnace can fail.  While most components can be replaced — the cost to replace a heat-exchanger (discussed below) may not be justified due to the furnace’s age.

Furnace Efficiency

• Standard-Efficiency gas furnaces are 80% efficient.  This means 20% of the heat generated is vented outdoors with the combustion gasses.
• These furnaces are what’s required (by law) in the southern U.S.
• These furnaces have one metal exhaust pipe for combustion gasses.

• High-Efficiency gas furnaces are 90% — 98% efficient.  2% to 10% of the heat generated is vented outdoors with the combustion gasses.
• 90% furnaces are required (by law) in the northern U.S.
• These furnaces have two plastic pipes.  One brings outdoor air in for the burners.  The other pipe carries exhaust gasses outdoors.

Image Source: ShutterStock

SHOWN: High-Efficiency Gas Furnace With Two White Plastic Pipes. 

One Pipe Brings Air In From Outdoors (for combustion).  The Other Removes Exhaust Gasses To Outdoors.

The Difference Between A Standard & A High-Efficiency Gas Furnace

The difference between a Standard Efficiency (80%) gas furnace and a High-Efficiency (90-98%) gas furnace is that a Hi-E furnace has an additional component  — called: “secondary heat-exchanger” that removes heat from the exhaust gasses before they’re vented outdoors.  In the 2nd link below — the location of the 2nd heat-exchanger is identified along the left side of the diagram.

Copy The Link Below Into Your Browser To See A Stanard-Efficiency Furnace Diagram:

https://www.google.com/search?q=furnace+diagram&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiNlvz-qP7cAhUDPq0KHVyMB34Q_AUICigB&biw=1097&bih=539#imgrc=w0g-UrmZbMwpTM

Copy The Link Below Into Your Browser To See A High-Efficiency Furnace Diagram:

https://www.google.com/search?q=high+efficiency+furnace+diagram&tbm=isch&ved=2ahUKEwiG35zFoZ7tAhVGTa0KHQkBCXUQ2-cCegQIABAA&oq=high+efficiency+furnace+diagram&gs_lcp=CgNpbWcQAzICCAAyBggAEAcQHjIGCAAQBxAeMggIABAHEAUQHjIICAAQBxAFEB4yBggAEAgQHjIGCAAQCBAeMgYIABAIEB5QmR5YlS1g8C9oAXAAeACAAfIBiAHNBJIBBTYuMC4xmAEAoAEBqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=9Jm-X4aVD8aatQWJgqSoBw&bih=539&biw=1097#imgrc=quxZP8FD4UrS2M

For More Details, Click On This Link To See American Standard Furnaces:  American Standard Furnace Brochure

• American Standard’s 80 is their Standard Efficiency Gas Furnace with a Single-Speed Blower Fan.  It vents 20% of the heat created outdoors with the exhaust gasses.

• American Standard’s Platinum 95 is their Highest Efficiency Gas Furnace.  It has a Variable-Speed Blower that adjusts its speed to match heating demand.   It vents as little as 2.7% of heat created outdoors with the exhaust.

Replacing Your Furnace — Can A Standard, 80% Furnace & Standard, Direct-Vent Water Heater Share The Same Vent?

This is a common question — due to concern about a standard-efficiency furnace’s exhaust fan.

• YES — Because a standard, 80% furnace’s exhaust-fan does not exert positive pressure into the flue pipe.
• The furnace’s exhaust, while inside the flue, is gravity-vented.
• This is why an 80% Standard-Efficiency furnace can share a 4″ common vent pipe with a standard, direct-vent gas water heater. **

Shown: An Older Furnace That’s Less Than 80% Efficient.  Note how large the exhaust vent is (5-6″)

• When installing an 80% furnace in an older home, with an existing gas furnace that’s less than 80% efficient — its exhaust vent pipe is too large for the new 8o% furnace.
• It’s common practice to install a new, 4″ vent pipe inside the old, larger vent pipe (often 5-6″).

Shown: A Standard, Gas Water Heater And A Hi-E Furnace Don’t Share A Common Vent

• When installing a Hi-E furnace in an older home, with an existing gas furnace that’s less than 80% efficient — its exhaust vent pipe (5-6″) is too large for the water heater alone.
• With a Hi-E furnace — a standard, direct-vent water heater requires its own 4″ vent pipe to outdoors.  ***
• It’s common practice to install a new, 4″ vent pipe inside the old, larger vent pipe (often 5-6″).

• A Hi-E furnaces pull outdoor air in for combustion — then vent the exhaust through a separate vent pipe.  These pipes are never shared.

• A Power-Vented Water Heater operates just like a Hi-E furnace. ***  They pull outdoor air in for combustion — then vent the exhaust through a separate vent pipe.  These pipes are never shared

** Source: https://www.creia.org/common-venting-of-gas-appliances

*** Source:  https://www.hotwater.com/resources/gas-water-heater-vent-configurations/#:~:text=A%20direct%20vent%20water%20heater,are%20then%20vented%20back%20outdoors.

There Are Up To 12 Components In A Gas Furnace:

(Hi-E Have 1 More — The Secondary Heat-Exchanger for the exhaust):

Repair Or Replace?  A Rule Of Thumb: If the repair cost, multiplied by furnace’s age, exceeds $5,000.  It’s likely more cost-effective to replace the furnace.

Example 1:  The repair cost is $500.00 and the furnace is 5 years old:  $500 X 5 (years) = $2,500.00.  It’s likely more cost-effective to repair the furnace.

                                                  Example 2:  The repair cost is $500.00 and the furnace is 12 years old:  $500 X 12 (years) = $6,000.00.   It’s likely more cost-effective to replace the furnace.

• Cold Air Return Connection —  Attached where the furnace intakes indoor house air to be heated.   This connection may leak, but won’t cause gas furnace failure.

The cold air return may be at the; top, side or bottom of the furnace.  It depends on how the furnace ductwork is installed.

This Older Furnace Has A Larger (5–6″) Double-Wall Metal Exhaust Pipe Than Today’s Std-E (80%) Furnaces.

An 80% Furnaces Require A 4″ Double-Wall Metal Vent Pipe.  This is the same vent pipe required for a gas water heater **

** Source: http://www.whirlpoolcomfort.com/Upload/46924D003.pdf

Image Source: Amazon.com Embedded Link

SHOWN: Double-Wall Vent Pipe.

Note: You Can See The Double-Wall Best At The “T” Vent Pipe, In the Upper Right Of The Photo.

Double-Wall Vent Pipe Has A Space Between The Inner & Outer Walls.

Only The Inner Wall Has Contact With The Exhaust.  The Gap Between The Inner & Outter Walls Keeps The Pipe Cooler — Because The Pipe Is Typically Near Combustible Materials (like wood).

• Warm Air Supply Connection — Attached To The Furnace where it Discharges Heated Air To The House.  This connection may leak, but won’t cause gas furnace failure.

The warm air supply can be at the; top, side or bottom of the furnace.  It depends on how the furnace ductwork is installed.  The furnace above discharges heated are from the top of the furnace.

• Heat Exchanger — This Sealed Component Allows Heat + Exhaust Gasses To Flow Inside It.  House Air Flows Along The Outside.  This is a common reason for gas furnace failure.

Image Source: DreamsTime

Shown: Heat Exchanger Inside A New Gas Furnace

The Heat-Exchanger Often Fails (cracks) in old furnaces.  This is because the metal expands each time the furnace makes heat — then contracts after the heating-cyle is over.  This eventually causes stress-fractures in the metal.  Once the heat-exchanger is cracked, the furnace is no longer safe to use, as it poses a carbon monoxide leakage risk.

Once cracked, the heat-exchanger or the furnace must be replaced.  Unless the furnace is so new that the heat-exchanger is still under warranty (part only — you pay the installation labor charge)  it makes little sense to put an expensive new part inside a furnace that’s at or beyond its typical service lifespan (about 15 years).

Typical Warranties — Parts only, you must pay the installation labor charges.

• Heat Exchanger – twenty (20) years.  Some brands have a lifetime warranty on heat exchangers. 
• All Other Parts – five (5) years 
• Parts Installation — Labor Not Included. 

Note: Electric furnaces don’t have a heat-exchanger.

• (Hi-E only) Exhaust-Gasses Secondary Heat Exchanger — Removes Heat From The Exhaust Gases.  This component is typically made of stainless-steel — and is unlikely to fail before the primary heat-exchanger would.  If this component fails, it causes gas furnace failure.  Much like primary heat exchanger failure — it typically not wise to replace this component because the replacement cost will be too much for a furnace that’s nearing or beyond its typical service-life (12-15 years)..

Copy The Link Below Into Your Browser To See A High-Efficiency Furnace Diagram:

https://www.google.com/search?q=high+efficiency+furnace+diagram&tbm=isch&ved=2ahUKEwiG35zFoZ7tAhVGTa0KHQkBCXUQ2-cCegQIABAA&oq=high+efficiency+furnace+diagram&gs_lcp=CgNpbWcQAzICCAAyBggAEAcQHjIGCAAQBxAeMggIABAHEAUQHjIICAAQBxAFEB4yBggAEAgQHjIGCAAQCBAeMgYIABAIEB5QmR5YlS1g8C9oAXAAeACAAfIBiAHNBJIBBTYuMC4xmAEAoAEBqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=9Jm-X4aVD8aatQWJgqSoBw&bih=539&biw=1097#imgrc=quxZP8FD4UrS2M

The Secondary / Condensing / Exhaust Gasses Heat-Exchanger is identified in the left-hand column of descriptions in the diagram.

Image Source: CanStockPhoto

Blower Motor & Assembly

• The blower fan moves air through the furnace and ductwork.  Though failure of this component causes gas furnace failure, the cost to replace this is often justified (but based on the furnace’s age.

Note

Note: A gas furnace has two fans that you can hear running during a heating cycle.

• The first motor that starts is the Induced-Draft Blower (see photo below).  It helps move the exhaust-gasses outdoors.

• The second motor to activate is the Blower Fan (see photo above).  It moves the household air through the furnace.  The blower is notable louder.

• Induced-Draft Fan — Pulls Exhaust Gases Out Of Furnace & Helps Vent Them Outdoors.  This fan quitting causes gas furnace failure, but is typically cost-effective to replace (depends on furance age).

 SHOWN: Draft-Inducer Fan Assembly.

Click On Photo To: View Product, See Details, Or Purchase From Amazon.com.

NOTE: Each furnace requires a specific Induced-Draft Fan.  The unit shown is not a universal replacement part.

• Gas Burners — Burn Gas To Make Heat.  Burners rarely fail — but will accumulate soot over time.  Soot reduces their efficiency and safety.   Cleaning the burners is typically part of a Fall Furnace Tune-Up (ask to be sure).   Should a burner fail, it will contribute to, or cause, gas furnace failure.

Excess soot on burners is the result of incomplete combustion.   This is caused by:

• Insufficient combustion air.
• Improper air flow adjustment.
• Change in gas pressure since the furnace was installed.

Copy The Link Below Into Your Browser To See A Photo Of Gas Burner’s Soot:

https://www.google.com/search?q=gas+furnace+burner+with+soot&tbm=isch&ved=2ahUKEwjt0oGl76rtAhVKUKwKHYFWAzYQ2-cCegQIABAA&oq=gas+furnace+burner+with+soot&gs_lcp=CgNpbWcQDFAAWABg7KAmaABwAHgAgAEAiAEAkgEAmAEAqgELZ3dzLXdpei1pbWc&sclient=img&ei=FzbFX63rH8qgsQWBrY2wAw&bih=484&biw=960#imgrc=RvuKUhhYoPd1BM

Gas Ignitor

A failed gas ignitor is likely the most common gas furnace failure.  The ignitor is an electrical component that lights the gas burners.   A failed ignitor causes gas furnace failure, but isn’t expensive to replace.

Each time the ignitor cycles, it gets very hot & the metal expands.  Once the burners light, the ignitor shuts off and the metal cools & contracts.  Over time, this recurring cycle typically creates a stress fracture in the ignitor’s metal.  On the metal cracks, it can no longer conduct electricity and stops working.

The ignitor glows orange (much like the coils in a toaster) as it gets ready to light the burners.  Once it’s hot enough, the furnace’s gas valve will release gas to the burners.  Once the burners light, the ignitor shuts off.

Image Source: YouTube Embedded Video

Click The White Arrow In Center Of Image To See A Gas Furnace Ignitor Glowing Orange (starts glowing at Time 0:35 / 2:17)

SHOWN: Gas Furnace Ignitor.

Click On The Photo To See The Image, View Details, Or Purchase This Item From Amazon.

Note: Each gas furnace requires a unique ignitor.  The ignitor shown is not a universal replacement part.

Gas Valve

The gas valve controls the flow of gas to the burners.  Its most important duty is to shut the gas off if something goes wrong with the furnace.  Replacing this component is typically cost-effective.

Click On The Photo To See The Image, View Details, Or Purchase This Item From Amazon.

NOTE: Each gas furnace required a unique gas valve.  The valve required is based on: Furnace Brand & Model And BTU Heating Capacity.

• The gas control valve’s most important function is to shut the gas off if something goes wrong with the furnace.   EXAMPLE: In a failing gas furnace, the burner’s flames sometimes extend beyond where they belong.  If this occurs — there’s a safety-device (the flame-rollout switch) that detects this and messages the furnace’s control board (its brain).  The control board immediately shuts the gas off to the burners and shuts the furnace down.

Note: Most gas furnaces’ gas control value has both the Gas Regulator and Selenoid Valve inside.

• Gas Regulator — Reduces the gas pressure (coming into the house) —  to the lower gas pressure that’s required by the furnace.

• Gas Selenoid — Electrically opens and closes to start & stop gas flow when the thermostat tells the furnace to make start or stop making heat.

Gas (Pressure) Regulator

The gas regulator’s purpose is to reduce the pressure of incoming gas to what the furnace needs.  A gas furnace typically requires about 1/4 pound per square inch of gas pressure.   The pressure of natural gas delivered to the home is typically the same.

The natural gas pressure of the gas main line (under the street or alley) can be up to 60 psi, depending on the number of homes or businesses served by that line.  The pressure of natural gas delivered to the home is typically 1/4 lb. per square inch.   The is a gas pressure regulator by the gas meter to ensure proper gas pressure entering the home.  Replacing this component is typically cost-effective.

Image Source: Can Stock Photo

SHOWN: The Gas Meter’s Pressure Regulator Is Round.  It’s at the far left of the gas meter in the photo above.  

If for any reason the gas pressure coming into the furnace is higher than 1/4 pound per square inch, the furnace’s gas pressure regulator will reduce the gas pressure before it enters the burners.

If the gas pressure coming into the burners were too high, burner flames would be too large and the furnace may overheat.  If it does, it will shut itself down to protect itself, your home and you.

If the gas pressure regulator fails, it typically shuts the gas off permanently, resulting in gas furnace failure.  The regulator must be replaced to get the furnace operating again.

 Gas Solenoid Valve

The gas solenoid valve opens to allow gas to flow into the burners.  Like the regulator, if the solenoid valve fails — it will shut the gas off permanently, resulting in gas furnace failure.  The selenoid valve must be replaced to get the furnace operating again.  Replacing this component is typically cost-effective.

NOTE: Some furnaces have a combination gas: valve, regulator and selonoid all combined into one unit.

Red Tagging A Furnace

A red tag means that an HVAC technician or utility company inspector has found a critical safety problem.  The tag indicates that it’s no longer safe to operate — and must be repaired or replaced.  Laws and ordinances for red tagging a furnace vary by location.   In some locations, the technician is required by law to shut the furnace down as a safety measure.    In another location, the furnace may be tagged but left in operating condition.

A red tag indicates that a furnace no longer meets the safety standards required by law.    A red tag provides written documentation to prove that the owner is aware the furnace is unsafe, and typically a photo of the tagged furnace is taken.   This releases the technician and their employer from responsibility should the owner continues to operate the furnace, or restarts it after it was shut down.

Why Is A Furnace Red Tagged?

Usually, it’s because the furnace’s heat exchanger is cracked.  A cracked heat exchanger can allow carbon monoxide into the home’s air as it passes through the furnace.  Carbon monoxide is harmful in low doses and can be fatal in large amounts.

What Should You Do?

Get a second opinion from a contractor equipped to do an exhaust-gasses test.  If the heat exchanger is cracked, the oxygen level in the exhaust typically rises as the furnace is heating up — or when the blower fan turns on.  This rise in oxygen level is because air is entering the exhaust through the crack in the heat exchanger.

• The 1st YouTube video below shows a furnace that HAS a cracked heat exchanger.

• The 2nd You Tube video shows a furnace that DOES NOT HAVE a cracked heat exchanger.

• Details about oxygen level rise are described below each image.

• Both tests are done by the same HVAC Company.  It appears (based on his voice) that the same technician did both tests.

Image Source: YouTube Embedded Video

SHOWN: YouTube Video Confirming Heat Exchanger IS Cracked (at time: 3:00 / 3:16 )

Click on the arrow in center of image to see the oxygen level rise substantially when the furnace’s blower fan turns on.

NOTE: Oxygen Level rises from 7% to 20% when the blower turns on.  This is due to the air being forced through the crack in the heat exchanger (into the exhaust) by the blower.

Image Source: YouTube Embedded Video

SHOWN: YouTube Video Confirming Heat Exchanger Is NOT Cracked (at time 2:30 / 5:17)

 Click on the arrow in center of image to see stable oxygen level during the furnace’s heating cycle.

NOTE: Oxygen Level Is Around 11% While The Blower Is Running — This Is Within Normal Operating Range.

To Learn More About Heat Exchangers And Cracks Click On The Link To Our Article: AlsPlumbing.com Gas Furnace Heat Exchangers

Al’s Plumbing, Heating & A/C in Plano, Texas 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.  Al’s 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.

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.