photo of colorful fall leaves

Image Source: ShutterStock

This article explains why a Fall Gas Furnace Tune-Up Should Be Performed.   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 & Replacements for every plumbing component in your home in; Frisco, TX; Allen, TX, McKinney, TX and other cities in Dallas County, TX; Tarrant County, TX; Denton County, TX & Collin County, TX.   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.

15 Reasons Why A Gas Furnace Needs A Fall Tune-Up 

The Older The Furnace — The More Important A Fall Tune-Up Becomes. 

At Some Point, A Gas Furnace’s Heat-Exchanger Often Cracks.  A Cracked Heat-Exchanger Can Allow Carbon-Monoxide Into Your Home (discussed in detail later in this article).

There are a number of Safety-Features on a gas furnace.  They need to be checked to ensure they’re working correctly and are clean


  • One Furnace Safety-Feature is the Flame Sensor (discussed in detail later). 
  • It checks that a flame exists when the gas-valve is open and gas is flowing into the burners.     
  • This sensor gets dirty during normal operation of the furnace. 
  • If it gets too dirty, it can’t detect a flame — and the furnace shuts down until the Flame Sensor is cleaned. 
  • This leaves the home without heat — and often requires an emergency service-call that costs much more than a Fall Tune-Up. 

high efficiency gas furnace with two white pipes visible

Image Source: Dreamstime


Most Furnace Tune-Ups Include An Exhaust-Gasses Analysis.  

(Al’s Tune-Up’s Include This)

It’s important to perform a combustion analysis during routine service.  A digital combustion-analyzer provides the measurements to determine; efficiency, safety, and pollution being generated by the furnace. If there is an imbalance in the exhaust-gasses measurements — the combustion-analyzer will detect it — and helps the technician find the   Combustion efficiency measures how efficiently gas is being burned — and ensures the maximum heat-output for the amount of gas used.  An improper mixture within exhaust gasses can mean poor performance and worse, a possible safety concern.

What An Exhaust-Gasses Analysis Tests *

During combustion, new chemical substances are created — and exhaust gasses are created.  A combustion gasses analyzer measures these gasses.  The tester has a probe that’s inserted into the furnace’s exhaust vent or flue.

The Tester Checks The Exhaust / Combustion Gasses For These 4 Items*

* Flue Gasses Temperature:

The temperature of the exhaust gasses is a measure of how much heat is being carried away with the exhaust gasses.   The more efficient the gas furnace is, the cooler the exhaust-gasses are.  Today’s Hi-E furnaces are up to 97% efficient — only 3% of the heat leaves with the exhaust).   As compared, many of the first furnaces were as low as 50% efficient (half the heat leaves with the exhaust).

Some of the first central-heating furnaces were called: “Gravity Feed or Octopus”.  They were installed in homes built during the late 1800’s to the early 1900’s.  They were originally fired by coal.  Over the years, many were converted to natural gas or heating oil.  They did not have a blower-fan to move the air.  Instead, the heated air was so hot that it rose out of the furnace — and the cold air sank into the furnace’s cold-air returns and onward into the furnace.    Because they had no moving parts — many of these furnaces will last forever.  There are still some of these furnaces heating homes — with a fuel cost double that of today’s highest efficiency gas furnace.

These furnaces got so hot that most were insulated with asbestos — to keep the heat in the furnace instead of moving outward into the basement.  These furnaces operate very hot — and 50% of the heat they generated went up the flue with the exhaust gasses.   If you wonder about the 375 degrees exhaust gasses number just below — that number represents gravity-feed furnaces.

Click Here To See A Gravity-Feed / Octopus Furnace:  Gravity-Feed / Octopus Furnace


Exhaust Gasses Temperature — By Furnace Efficiency.  Simply put — the more heat that goes up the flue — the hotter the exhaust gasses are. 

  • < 80% Efficient
  • Older, natural-vent:  Exhaust gasses are  +270 — 375 degrees hotter than the air going into the furnace (for combustion).
  • In older furnaces — the exhaust is hot enough to rise up the exhaust flue on its own.
  • These furnaces have a larger diameter metal exhaust vent that rises upward toward the flue.

old gas furnace

Image Source: CanStockPhoto

SHOWN:  An older (<80% efficient) natural-vent furnace.  The metal vent must be considerably larger than for an 80% efficient furnace.  The exhaust gases are hot enough to rise up the flue.


“Incomplete Combustion” Occurs When there isn’t enough oxygen for the amount of fuel being burned — so some carbon monoxide is produced.  ** 1

NOTE: 100% combustion is not realistically achievable.  Typical combustion processes produce efficiencies of 10–95%.  **1


**1 Source:

  • 80% Efficient – – the exhaust temperature should not exceed 125 degrees (f).
  • The Energy Code requires an 80% (or higher) efficiency furnace in the southern US, including DFW. *2
  • These furnaces have a metal exhaust vent.
  • The diameter of their metal exhaust vent is smaller than a natural-vent (<80%)gas furnace.
  • Their exhaust is too cool to rise on its own.  The furnace has a small fan that pushes the exhaust outside.

Click Below To See A Standard-Efficiency (80%) Furnace 

Standard Efficiency (80%) Gas Furnace


  • 90%+ Efficient: The exhaust gasses’ temperature can be as low as only slightly higher than the temperature of the air going into the furnace.
  • The Energy Code requires a 90% (or higher) efficiency furnace in the northern US. *2
  • These furnaces have a plastic exhaust vent.  The exhaust can be(and typically is) vented sideways at some point.
  • Their exhaust is warm at most.   With a Hi-E Furnace, very little heat leaves with the exhaust.
  • These furnaces have a small fan that pushes their exhaust outside.

*2 Source:

hi-e gas furnace

SHOWN: A high-efficiency gas furnace (90% or higher) — with white plastic exhaust vent.


Much Like People, When A Gas Burner Operates — It Consumes Oxygen And Produces Carbon Dioxide.

Unlike People, A Gas Burner Also Produces Carbon Monoxide.  

This Is Why The; Oxygen Level, Carbon Dixoide Level, & Carbon Monoxide Level Within Furnace Exhaust Gasses Are Measured. 

* O2 = Oxygen Level In Exhaust Gasses

Excess oxygen in exhaust gasses absorbs heat & vents the heat outside with the exhaust.  This is caused by an incorrect air-to-fuel setting.

A certain amount of oxygen is needed for combustion.   The proper amount of air is required to keep combustion efficiency at its maximum.

In a properly operating gas furnace — the oxygen level won’t rise when the blower-fan turns on.  If it does — there may be a crack in the furnace’s heat-exchanger

Oxygen (O2) levels in the exhaust gasses should be between 6 — 9% — and should remain stable shortly after the burners are lighted. *6

*CO2 = Carbon Dixoide Level In Exhaust Gasses

Carbon Dioxide is byproduct of normal combustion.

Carbon dioxide level in the flue gas provides an indication of the efficiency of the burner.  The higher the CO2 — the more efficiently the furnace is burning the gas.

If the gas were 100% burned — only carbon dioxide would be in the exhaust gasses (no oxygen),  100% combustion-efficiency is extremely rare.

The maximum exhaust gasses’ CO2 level for a natural gas furnace is 11.8%. *8


* CO = Carbon Monoxide Level In Exhaust Gasses

A high level of carbon monoxide in exhaust gasses results from incomplete combustion of the gas.   

If there’s too little air available for proper combustion — excess carbon monoxide is generated.   This is caused by an incorrect air-to-fuel setting.  It can also indicate a misaligned burner.

Building Code typically allows up to 400 parts per million (ppm) of carbon monoxide in the exhaust gases.

A normal reading for a newer gas furnace should be <150 ppm.   A reading above 150 ppm indicates the need to adjust the furnace’s air-to-fuel setting. 


** Source:

*** Source:

*5 Source:

*6 Source:

*8 Source:


Exhaust Gasses Analysis To Check The Heat-Exchanger 

 International & National Gas Codes state: “The heat exchanger must be checked on all service calls.”

photo of heat exchanger in gas furnace

Image Source: DreamsTimes

SHOWN: A Gas Furnace’s Heat Exchanger.  NOTE: The heat-exchanger is inside the furnace — and ii’s difficult to perform a complete visual inspection.


How A Home Furnace Heat-Exchanger Works

The heat & exhaust-gasses from the burners flow inside the heat-exchanger. 

The home’s indoor air flows along the outside of the heat-exchanger.

The heat-exchanger’s purpose is to keep the heat & exhaust-gasses (made by the burners) separate from the home’s indoor air. 


Click Below For A Video Of Using A Combustion-Gases Analyzer To Check For A Cracked Heat-Exchanger.  

 Combustion-Gases Analysis Checking For Cracked Heat-Exchanger (at Time = 2:40 / 5:17).

NOTE: In This Video, The Oxygen In The Exhaust-Gasses Does NOT Increase When The Blower Turns On — Indicating The Heat Exchanger Is NOT Cracked)

NOTE:  An increase in Oxygen / O2 reading (when the blower-fan turns on)  typically indicates that the heat-exchanger has failed (typically cracked) — and indoor air is mixing with the exhaust gasses.

A Furnace With A Cracked Heat-Exchanger Is Not Safe.  If a furnace technician finds this condition — the furnace is typically “Red Tagged” — and is shut down until the furnace is repaired or replaced. *3

*3 Source:  


3-Part Heat-Exchanger Inspection To Determine Its Safety

Part 1: Watch the burners’ flames when the blower comes on.  In normal operation — the blower running doesn’t affect the burners’ flames.

1. Click For A Video That Shows The Furnace Burners’ Flames Becoming Distorted & Changing Color (from mostly blue to lots of yellow) When The Blower Fan Turns On:  

Click Here For A YouTube Video — Burners’ Flames Distort & Color Changes When The Blower-Fan Turns On

With A Cracked Heat-Exchanger — Exhaust Gasses Can Mix With The Home’s Indoor-Air. 

If this happens, it creates a dangerous environment for the home’s occupants — and can result in Carbon Monoxide Poisoning.  This poisoning will make you sick, and can be fatal in large enough quantities.

Part 2:  Perform a visual inspection of the heat exchanger.  The visual inspection is often limited by visual obstructions.

Part 3.  If concerns occur after Part 1 & 2  — Perform a chemical test on the heat exchanger.  Add a substance inside the heat exchanger.  Next, use an instrument that detects that substance on the outside of the heat-exchanger.  If the substance is detected on the outside of the heat-changer — it’s leaking

*5 Source:

Click Here For Further Details About A Cracked Heat-Exchanger: Signs Your Gas Furnace Heat Exchanger Is Cracked


These Gas Furnace Safety Devices Need To Be Inspected Annually 

If a Safety-Device Fails — The Furnace Will Shut Down To Protect Itself And You.

  • Control Board:  Newer gas furnaces have a computer type circuit-board called the: “Furnace Control Board”.  The control board instructs the furnace on what to do and when.  It also performs several safety checks during each heating cycle.  The furnace monitors for correct & safe operation the entire time it’s running.  If the control board gets a message that a something isn’t working correctly & safely — the control board shuts the furnace down.  This can prevent the furnace from; creating a gas explosion, catching on fire, or allowing carbon monoxide into your home.

Click Here To See A Gas Furnace Control Board: Photo Of Furnace Control Board


  • Air Pressure Switch:  The draft-inducer is a small fan that pushes exhaust gasses through the vent to outside.   When the draft-inducer turns on — negative air pressure indicates the exhaust gasses aren’t safely being vented outside.  If negative pressure is detected — the Air Pressure Switch shuts the furnace down.

Click Here To See A Draft Inducer Inside A Furnace: Furnace Draft Inducer


  • Flame Sensor: checks to ensure the burners are lighted while the gas valve is open If the flame sensor does not detect a flame, it shuts off the gas to the burners off to protect itself and you.

close up photo of gas furnace flames

Image Source: ShutterStock

Shown: Property Ignited & Operating Burners Will Be Mostly Blue And Burning Within A Confined Area

  Click Here To See A Gas Furnace Flame Sensor: Photo Of Gas Furnace Flame Sensor


  • Flame Rollout Switch: Checks That the Burners’ Flame Are Only Where They Are Supposed To Be.  If the flames are outside their correct location, this is called Flame Rollout — and can cause a fire.  If thee Flame Rollout Switch detects flame rollout, it shuts the furnace down.

photo of properly working gas burners in a gas furnace

Image Source: ShutterStock

  Shown: Gas Furnace Flames Correctly & Safely Within Their Proper Location

Click Here To See A Flame Rollout Occurring In A Gas Furnace: Photo Of Flame Rollout In Gas Furnace

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


  • Limit switch: This switch monitors the temperature near the burners.  If this area gets too hot — the Limit Switch shuts the furnace down.  If the furnace continued to get hotter — it could start a fire.  If the furnace gets too hot, the Gas Furnace Limit Switch shuts the furance down.

What could cause a furnace to get too hot?  The most likely thing would be a failure of the furnace’s gas-pressure controller.  If gas pressure is too high — gas enters at a pressure higher than the burners are designed for, the flames will become larger and cause the furnace to overheat.


  • Plenum Thermostat: tells the furnace’s blower-fan to turn once the heat-exchanger is warm enough — and the blower to turn off at the end of a heating cycle.   If the heat exchanger gets too hot — the Peunum Thermostat shuts the burners off and shuts down the furance. .


  • Gas-Pressure Regulator.  If the furnace’s gas pressure is too high — the burners’ flames would become too large and the furnace would overheat.  If gas pressure becomes too high — the Furnace Limit Switch would sense it and shut the furnace down.

Click Here To See A Gas Furnace Limit Switch:   Photo Of Gas Furnace Limit Switch

Click Here To See A Gas Pressure Switch:  Photo Of Furnace Gas Pressure Switch


A Gas Furnace Has No Safety Device To Detect A Cracked Heat-Exchanger 

So The Furnace Continues Continues To Operate. 

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

Note: Electric Furnaces Don’t Have A Heat-Exchanger.

Why A Heat Exchanger Cracks

  • The burners heat the Heat-Exchanger.  This causes the heat exchanger’s metal to expand.
  • Once the home is warm — the furnace blower-fan runs for a short period of time and turns off.
  • During this time, there is notable cooling of the heat-exchanger. This causes the heat exchanger’s metal to contract.
  • This recurring heating & cooling of the heat-exchanger — causes recurring expanding & contracting of the heat exchanger.
  • Over the years, the metal in many furnace heat-exchangers finally cracks as result of continuous expanding & contracting.

Al’s Plumbing, Heating & A/C’s Gas Furnace Tune-Up Includes:

  • Exhaust Gases Analysis

  • Inspection:
  • Inspect furnace and test for proper operation
  • Checks for any componet about to fail.
  • Cleaning: Clean components (if needed).
  • Adjustment: Checking & tighten loose components.
  • Other minor repairs (if needed).
  • Check Thermostat
  • Inspect Furnace Control Board and Test For Proper Operation
  • Inspect Gas Supply Line & Gas Valve
  • Verify Proper Burners Ignition
  • Check & Record Temperature Rise
  • Clean Burners & Adjust For Maximum Operating Efficiency
  • Clean The Flame Sensor.  A dirty flame sensor is one of the primary reasons a furnace shuts down.  Often this occurs during peak heating season.
  • Inspect Primary Heat-Exchanger
  • Inspect Secondary Heat-Exchanger (this extracts heat from exhaust gases in a high-efficiency furnace)
  • Inspect & Test (exhaust-gases) Draft-Inducer Fan 
  • Safety Check All Controls For Proper Operation
  • Check For Gas Leaks
  • Test For Carbon Monoxide
  • Check Furnace Air Filter
  • Inspect Blower Fan Assembly Operation.  Lubricate & Adjust (if needed)
  • Check & Tighten Electrical Connections
  • Inspect & Clean Furnace Condensate Line (if needed).  This line drain any condensation the furnace creates to outside.
  • High Efficiency Gas Furnace — Inspect Combustion-Air Intake & Exhaust Vent Pipe

high efficiency gas furnace with two white pipes visible

Photo Source: Dreamstime

Shown: 2 White PVC Pipes For High-E Furnace.  One pipe brings in combustion-air from outdoors.  The other removes exhaust-gasses to outdoors.


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 in; Frisco, TX; Allen, TX, McKinney, TX and other cities in Dallas County, TX; Tarrant County, TX; Denton County, TX & Collin County, TX.  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.