Gas Furnace Failure — Which Components Fail And Why

Gas furnace failure can result from any of a number of components in a gas furnace.  While most components can be replaced, the cost to replace a heat-exchanger (discussed below) may not be justified due to furnace’s age.

• Today’s standard-efficiency gas furnaces are 80% efficient.  This means 20% of the heat generated is exhausted outdoors with combustion gasses.
• These furnaces have one metal exhaust pipe for combustion gasses.
• Today’s high-efficiency gas furnaces are 92% — 98% efficient.  Only 2% to 8% of the heat generated is exhausted outdoors with the combustion gasses.
• These furnace have two plastic pipes.  One brings air in from outside for the burners.  The other pipe carries exhaust gasses outdoors.

Click Here To See A Diagram Of A High Efficiency (92-98%) Gas Furnace: Diagram Of High-Efficiency Gas Furnace

Image Source: Dreamstime 

SHOWN: High-Efficiency Gas Furnace With Two White Plastic Pipes.  One Brings Air In (For The Burners).  The Other Removes Exhaust Gasses

There Are Up To 12 Components In A Gas Furnace:

• Warm Air Supply Duct — Attached To The Furnace Where the furnace Discharges Heated Air To The House (in dashed lines at the top of furnace on the left — same location on right, but no dashed lines).
• Cold Air Return Duct —  Attached To The Furnace Where It Intakes Air To Be Heated (lower left dashed lines on furnace on the left — same location on right, but no dashed lines).
• Blower — What Moves The Air  (At bottom of furnace — light green on left furnace diagram above  — same location on right furnace)

• Heat Exchanger — This Sealed Component Converts Heat From The Burner (including exhaust gases) Into Heat For The House (without exhaust gases) — at the top of the furnace, just below the Warm Air Supply Duct (see red arrow)
• Condensing Heat Exchanger — Removes Heat From Exhaust Gases (none on left — on right, in the middle of the left side, under the red arrow)

• Exhaust Gases Vent Pipe — Attached to the Furnace.  Carries Exhaust Gases Out Of The House (the round pipe at top left of furnace on the left — on right diagram, a white pipe with “flue gas out” written above it).

• Intake-Air Pipe:  Attached to the Furnace.   High-Efficiency Furnaces use outside air for combustion (rather than removing heated air from inside the house). For High-Eff., 2 white pipes are present, one carries brings fresh air to the furnace and the other moves exhaust fumes from the furnace (to outside the home).

• Induced-Draft Fan — Pushes Exhaust Gases Out Of Furnace & House Through The Exhaust Vent.  (Fan shown near the top of the furnace – dark green on left — same location on right
• Gas Burner — Burns Gas To Make Heat (in the center at the front with light yellow pipe around it — in middle at the front of the side on furnace on right)

• Gas Valve — Controls the flow of gas to the burner.  It is designed to shut gas supply off if something goes wrong with the furnace. (Gas valve is located near the burner in left diagram (near a pale yellow pipe, on right side of furnace).   On the right, no location is shown (one is part of the furnace though).  All gas furnaces have a gas valve.  Most valves combine both the Gas Regulator and Selenoid Valuve into a single component.
• Gas Regulator — Reduces the gas pressure coming into the house to the lower amount needed by the furnace.
• Gas Selenoid — Electrically opens and closes to start & stop gas flow when the thermostat tells the furnace to make heat

Note: The gas valve, regulator and selenoid are often within one housing.

Gas Ignitor

A failed gas ignitor is one of the most common reasons a gas furnace failure.  Each time the ignitor cycles, it gets very hot & the metal expands.  Once the burners light, it shuts off and the metal contracts.  This recurring cycle often breaks the ignitor’s metal.

The gas ignitor is electric.  It’s purpose is to light the burners (older furnaces used a pilot light).

The ignitor glows orange (due to being very hot) when it’s ready to light the burners.  Once it’s hot enough, the furnace’s gas valve will release gas to the burners.  Once burners light, the ignitor shuts off.

SHOWN: GAS FURNACE IGNITOR

SHOWN: Gas Furnace Ignitor.

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

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.

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

Gas (Pressure) Regulator

The gas regulator’s purpose is to reduce the pressure of incoming gas to what they burners require.  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.

If for any reason the gas pressure coming into the furnace is higher than 1/4 pound per square inch, the gas pressure regulator will reduce the gas pressure as it enters the furnace.  If the gas pressure coming into the burners is too high, burner flames will be too larg and the furnace may overheat.  It 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 Selenoid Valve

The gas selonoid valve is electrically operated.  It opens to allow gas to flow into the furnace.  Like the regulator, if the selenoid valve fails, it will shut the gas off permanently, resulting in gas furnace failure.  The selenoid valce must be replaced to get the furnace operating again.

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

Gas Burners

GAS BURNERs don’t have any moving parts, so they seldom fail.  What typically causes the burner to not light is the gas-valve that controls gas flow to the burner.  If a gas-valve fails, the repair cost is typically modest (unless the furnace is quite old).

NOTE: The gas burners need to be cleaned periodically.  If they are dirty, they will not burn mostly blue, rather will have a lot of orange or red flame.  Cleaning the burners (if needed) is part of a gas furnace tune-up.

Image Source: Shutterstock 166297982

SHOWN: Gas Furnace Burners That Are Working Correctly Have A Nearly Completely Blue Flame

Heat Exchanger

Photo Source: Dreamstime

SHOWN: Gas Furnace Heat Exchanger. It Is At The Top Of The Furnace (it can’t be seen once the furnace is installed).

A cracked heat-exchanger is the most serious gas furnace failure.  A crack means the furnace is no longer safe to operate.  In most cases the furnace must be replaced.  There are exceptions where a new heat-exchanger can be installed, though the cost is typically prohibitive unless the furnace is still under warranty.

Identifying & Shutting Down an unsafe furnace is called “Red Tagging”.  A red tag is attached to the furnace stating that it has been shut down and is dangerous to operate.  It is the responsibility of the person who discovered an unsafe furnace to red tag it.  Without this step, there is no written verification the owner of the furnace knows it’s unsafe.  Typically a photo will be taken with the red tag visible.

Click Here To See A Furnace Red Tag: Gas Furnace Red Tag

The Red Tagged furnace will be shut down.  In most cases the HVAC Tech will shut the gas off to the furnace to shut it down. The furnace may or may not be disabled by an HVAC Tech.  Also the City, Fire Department or Gas Supplier Representative can Red Tag a furnace.

The purpose of the heat-exchanger is to keep combustion-gasses separate from the home’s indoor air.  The heat-exchanger is a combination metal chamber and passageway.  It starts at the burners and ends near where the exhaust-vent connects to the furnace.

As the burners heat the heat-exchanger, heat & combustion gasses travel inside the heat-exchanger’s pipes.  The combustion-gases are vented from the furnace to outdoors.  The heat-exchanger heats up while the burners are operating.

The furnace blower circulates air over the outside of the heat-exchanger’s pipes.  The heat exchanger must have an air-tight seal to separate combustion-gases from the house’s air.  Flue gases include carbon monoxide.  Combustion gasses pose a health risk and are fatal in large enough concentrations.

The metal pipes in the heat-exchanger heat up & expand during each heating cycle.  Once the burners shut off, the heat-exchanger cools & contracts.  Over the years, this recurring process often creates cracks the heat-exchanger.  Once cracked, it’s no longer safe to operate the furnace.

Click Here To See A Cracked Heat-Exchanger: Cracked Gas Furnace Heat Exchanger

The cost to replace a furnace heat-exchanger in an older furnace is often more than can be justified.  Additionally, for older furnaces,  the heat-exchanger is no longer available, or will take an extended period of time to arrive — while you have no heat.

Depending on its age, your gas furnace’s heat-exchanger may still be under warranty.   The warranty covers only the part — you are required to pay for installation labor charges.

Heat-Exchanger warranties are longer than for the rest of the gas furnace.    To know if you qualify for a replacement heat exchanger, check the warranty paperwork provided with the furnace.  If you don’t have the paperwork, do an internet search: “Heat Exchanger Warranty for (Brand) (Serial Number)”.   You can also call Manufacturer’s Customer Service.

EXAMPLE: The Warranty For A New American Standard Gas Furnace (at the time of this post): 

• Heat Exchanger – twenty (20) years
• All Other Parts – five (5) years 
• Parts Installation Labor Not Included. 

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

 BLOWER

Note: There are two fans you will hear run as your gas furnace starts the heating cycle:

• The first one is the Induced-Draft Motor (shown next).  This fan moves exhaust gases out of the furnace and through a vent pipe to outside the home.  This fan will run during the entire time the burners are running, and will shut off once the burners shut off.

• The second motor is the Blower (photo just below).  It moves the household air through the furnace.  The blower is notably louder.

Blower

Induced-Draft / Draft Inducer

INDUCED-DRAFT MOTOR (below):  This fan moves exhaust gases out of the furnace through venting outside the home.  This fan will run during the entire time the burners are running, and will shut off once the burners shut off.

SHOWN: Induced-Draft/ Draft Inducer Unit

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Each gas furnace required a unique draft inducer unit.  The one required is based on: Furnace Brand & Model And BT Heating Capacity.