fuel gas and fuel oil combustion by

fuel gas and fuel oil combustion by
Consulting, Resource, Education, Training, and Support Services for Home Inspectors
“A candle loses no light when it lights another candle.”
Part of an inspection of vented fuel gas-fired or fuel oil-fired appliances includes inspecting for
combustion by-product backdrafting, leakage, and spillage. Backdrafting, leakage, and spillage can
occur at:
Openings into combustion chambers
Draft hoods/diverters and at connections of vents to vent collars
Improperly assembled and/or secured vent section joints and at improper terminations of
vent systems
Damaged areas of draft hoods
Damaged areas anywhere along the developed length of the appliance vent/chimney
The causes of and contributing factors which lead to combustion by-product backdrafting, leakage, and
spillage are numerous and may include:
Insufficient combustion air
Negative air pressure in the building, room, or compartment in which the appliance is
Obstructed or damaged draft hoods/diverters
Obstructed or damaged heat exchangers; obstructed flue gas vents/chimneys
Damaged flue gas venting system components
Improper termination of flue gas venting systems
Improper assembly, sizing, insulation, or installation of flue gas venting systems
Excessively long flue gas vents
Combustion by-product flow restriction caused by too may changes in direction in the
The single greatest potential hazard associated with combustion by-product backdrafting, leakage, and
spillage is the production of CO (carbon monoxide) and its introduction into the interior environment
where it can result in both occupant exposure and CO poisoning of occupants. CO poisoning is silent
and insidious because CO is odorless, colorless, and tasteless. Just one molecule of CO for every
1999 air molecules can result in a lethal mixture. This is 500 parts per million - not very much.
CO typically enters the bloodstream through the lungs. Once in the bloodstream, CO attaches to the
blood’s hemoglobin, the protein coloring matter in blood that gives it its red color and which transports
oxygen. When this occurs it decreases the total oxygen carrying capacity of the blood – in effect,
starving the various tissues and their cells of oxygen. By attaching to the blood’s hemoglobin, CO
effectively makes a portion of the hemoglobin unusable for binding with oxygen. As if that were not bad
enough, CO also binds with hemoglobin more avidly than does oxygen. This, in turn, makes it more
difficult for oxygen to be released to the body’s tissues resulting in asphyxia – oxygen starvation.
Because hemoglobin has about a 250 times greater affinity for CO than it does for oxygen, it takes
much longer for the body to get rid of CO. In other words, hemoglobin doesn’t just prefer CO to
oxygen; it wants to hang on to CO once it binds to it.
The most common initial symptoms of CO poisoning are headache, dizziness, weakness, nausea,
vomiting, chest pain, and confusion. High levels can result in loss of consciousness and death. If
CO poisoning is not suspected as a cause of many of these symptoms, it can be difficult to diagnose
since the symptoms mimic other illnesses such as flu. Individuals who are asleep or intoxicated can die
from CO poisoning before ever waking or experiencing any symptoms.
As with other aspects of a home inspection, inspection for combustion by-product backdrafting,
leakage, and spillage from vented fuel gas and fuel oil-fired appliances is a primarily visual examination
of the visible, safely accessible, and readily accessible portions of fuel gas and fuel oil-fired appliances
and their related venting system components. While inspectors may choose to use specialized
combustion by-product detection equipment or special tools in the course of conducting a home
inspection, they are not required to do so (watch for an upcoming ProSpex article Specialized
Equipment and Tools - Raising Expectations and Liability in the “FREE Articles” area of the ProSpex
In order for fuel gases and fuel oil to burn at maximum efficiency, they must be provided with
combustion air which contains enough oxygen to permit complete combustion. Whether combustion air
sources are from the interior of the building in which appliances are located or from the exterior, all fuel
gas and fuel oil-fired appliances require some source of combustion air and all combustion air
ultimately comes from the exterior of a home. In many older homes there may be no dedicated sources
of combustion air because such homes were not constructed as tightly as homes built today. In such
older homes air infiltration provided sufficient combustion air. However, because of today’s higher
energy costs, it is not uncommon for older homes to have been modified to reduce air infiltration without
consideration being given to the effect that this may have on the combustion air supply for gas and fuel
oil-fired appliances.
This makes inspection for visible conditions that are consistent with combustion by-product
backdrafting, leakage and spillage, for conditions that could increase the potential for combustion byproduct backdrafting, leakage and spillage, and for combustion air sources extremely important in all
For the purposes of this discussion, we will assume a conventionally aspirated (not forced or induced
draft), atmospheric burner and conventionally vented gas-fired appliance such as a tank type water
heater or an older, gas-fired, conventionally aspirated, atmospheric burner furnace or boiler. We will
also assume that the appliance is located somewhere in the interior of the home and that it takes its
combustion air from the interior of the home. Combustion by-products proceed along a path from their
point of initial creation at the appliance burner and combustion chamber; past a heat exchanging
surface; through a draft hood, draft diverter, or barometric damper (remember, we’re assuming a
conventionally aspirated unit); into the vent connector; into the vent; and are finally discharged to the
outside atmosphere at the downstream termination of the vent on the exterior of the home.
The purpose of draft hoods, draft diverters, and barometric dampers on conventionally aspirated
combustion by-product venting systems is to ensure that a constant low draft condition is maintained in
the combustion chamber. By doing so, draft hoods, draft diverters, and barometric dampers contribute
to the stability of the air supply for the combustion process. They also reduce the potential for both
excessive chimney draft (updrafts) and downdrafts that have the potential to extinguish burners and
pilot flames.
The combustion by-products pass into the draft hood or diverter or past the barometric damper at
around 500° F where they are mixed with cooler room air (dilution air) and the temperature of the
combustion by-products drops to around 300 -350° F. At this temperature the combustion by-products
are still warm enough to rise up the vent by their own buoyancy and to prevent the acidic water vapor in
the combustion by-products from condensing out and running down the inside of the vent and into the
appliance causing corrosion damage to both.
When there is sufficient combustion air, the by-products of combustion are carbon dioxide (CO2),
nitrogen (N), water (H2O), very small amounts of carbon monoxide (CO), particulates, occasionally very
small amounts of sulfur (S) [depending on the specific chemical composition of the fuel], and, last but
not least, heat – which what the fuel is burned for in the first place.
If the appliance is equipped with a properly designed, assembled, installed, and maintained venting
system, these combustion by-products (except for a portion of the heat) are conveyed from the
appliance to the outside atmosphere through the appliance vent (also referred to as a flue, smokestack,
or exhaust). Water heaters, furnaces, and boilers all have components which transfer some of the heat
of combustion to air, water, or some other heat-retention/storage or heat-transfer medium.
If there is insufficient combustion air, the burner will be starved and the fuel will not burn efficiently.
When his happens, as the appliance continues to operate, the combustion process will produce soot,
higher levels of CO, and less heat. If the appliance combustion by-product vent is intact, properly
assembled, and properly installed, it will continue to convey the combustion by-products to the outside
atmosphere. However, the soot that’s produced as a result of incomplete combustion can build up on
the inside surfaces of burners, combustion chambers, heat exchangers, draft hoods, and vents.
This soot build-up reduces the cross sectional area of heat exchangers and vents and can eventually
block them off completely. I have observed heat exchangers with so much soot build-up on the
opposing walls of the exchanger chambers that the soot met in the center of the chambers almost
completely blocking the passage of combustion by-products through the exchanger to the vent. This
occurred because the primary air shutters for the burners were completely closed and very little
combustion air was being fed into the fuel gas to mix with it before it was ignited.
Even slight restrictions inside a vent can reduce the ability of combustion by-products to move through
the vent and can cause them to spill out at the draft hood or at the appliance vestibule area into the
interior space where the appliance is located. Combustion by-product spillage dilutes the interior air
with CO and other components of combustion resulting in a reduction of the percentage of available
oxygen in the combustion air. The reduction of available oxygen now starves the burner and, the more
the burner is starved, the more inefficient the combustion process becomes resulting in the production
of more soot and CO. Now the CO production cycle begins – more CO leads to more inefficient
combustion leads which to more soot build-up and even higher levels of CO which leads to more soot
build-up and… the picture should be clear.
In addition to heat exchanger and venting system blockage, incomplete combustion can result in
damage to the vent. Since the appliance in this example is conventionally aspirated (not an induced
draft or a forced draft appliance), it will typically have a galvanized (zinc-plated) steel metal vent.
Incomplete combustion produces less heat and produces nitrogen dioxide (NO2) and water vapor which
combine to produce nitric acid (HNO3). Nitric acid is aggressive toward steel, zinc, paint, human tissue,
and silver among other things. Its effects on silver can be seen in the tarnish on silver plated items in
homes with gas-fired cooking appliances. When less heat is produced, the potential for this acidic
water vapor component of the combustion by-products to condense out of the combustion by-products
increases. It will react with and corrode the inside surface of the metal components such as heat
exchangers, vent hoods, vent connectors, and vents.
As if it were not bad enough to have elevated levels of nitric acid in the combustion by-products,
consider that, because incomplete combustion produces less heat, the combustion by-products will not
be as buoyant. That is, they will not be as light in weight compared to the ambient air as they would be
if there was complete combustion. Therefore, they will not rise through the vent as rapidly. This means
that the corrosive components in the combustion by-products will remain in the vent longer and can do
more damage to the vent.
If the corrosive components of the combustion by-products corrode through the sheet metal of the vent
or through the walls of a heat exchanger, the combustion by-products can vent or leak directly into the
interior of the home. Now, there is another source of CO and other combustion by-products and further
reduction of oxygen for combustion. Again, the CO cycle is initiated and/or accelerated.
Add to this a vent connector that has too little slope, a cold outside temperature that can cause
temporary cold air blockage in the vent when the appliance isn’t operating and no heat is moving
through the vent, and components which contribute to depressurizing the house such as a properly
vented clothes dryer in the house, a couple of bathroom exhaust fans, a solid fuel appliance (fireplace
or wood stove), and an exterior vented kitchen exhaust fan. Any of these, alone or in combination, can
contribute to further starving the gas-fired appliance of combustion air and to the potential for CO
poisoning of the home’s occupants.
While this example may represent a kind of worst-case scenario, it serves to point out some of the
ways combustion by-products from fuel gas and fuel oil-fired appliances can find their way into the
interior of a home. As an inspector, you are certainly going to examine all of the visible, safely
accessible, and readily accessible portions of the vent connector and the vent for proper materials,
proper assembly, proper support, proper minimum slope, clearance to combustible materials, proper
termination, and a storm cap. Good! But you are not done yet. You want to determine the source(s) of
combustion air and you want to check for backdrafting of combustion by-products.
Corrosion and/or scorching on the exterior surface or paint of a draft hood or holes through draft hoods,
draft diverters, and vents are common signs that combustion by-product spillage or backdrafting have
occurred. Make sure that all gas-fired appliance burners are off. Then, turn on the bath fans, the
kitchen exhaust fan (if it vents to the exterior), and the clothes dryer. Now, go to all of the gas-fired
appliance controls and energize the appliances so that their burners ignite. Wait about sixty seconds
and then place your mirror next to each appliance’s draft hood or diverter. If your mirror fogs up, that is
an indication that combustion by-products are spilling out of the vent system and into the house air at
the appliance. Place your hand near the draft hood or diverter, if you feel moist air flowing out of the
hood or diverter, you have spillage.
If you can use your mirror and flashlight to view the interior of metal vent connectors or vents and you
see a white, almost chalky substance on the interior surfaces, you are seeing nitric acid corrosion of the
sheet metal of the vents. Excessive or atypical rust and corrosion on the interior and/or exterior
surfaces of a furnace, boiler, or water heater draft hood are signs of improper venting and of
combustion by-product spillage or backdrafting.
It is not difficult. All that are necessary are your hand and a mirror and a flashlight, both tools that all
inspectors should carry. Remember to turn off the exhaust fans and clothes dryer and to set the gasfired appliance controls back to their original settings after you have completed your examination for
spillage and backdrafting.
Be sure to carefully examine the portions of the vent in the attic and above the roof line whenever
possible regardless of whether the appliance is venting through a conventional vent or through a
masonry chimney. Inspect the condition of the vent above the roof line and its storm cap. If the vent
connector vents into a masonry chimney, remember to look for a cleanout below the point where the
vent connector enters the chimney. Open the cleanout and use your flashlight and mirror to examine
the interior of the chimney for deterioration, any liner, obstructions or interior damage, and to determine
if the vent connector extends past the interior surface of the flue. If more than one appliance vents into
the chimney, be sure to determine that the vents are offset and don’t enter the chimney at the same
level and that smaller vents always connect to the chimney above (downstream of) larger vents.
Gas or oil-fired appliances venting into an unlined masonry chimney should be flagged in the written
inspection report as a condition for which immediate action is recommended. The moisture in the
combustion by-products will eventually cause deterioration of both any mortar lining and masonry joint
mortar. This will result in weakening of and damage to the chimney structure as well as in the potential
for leakage of combustion by-products into the interior of the house.
Finally, multiple gas-fired appliances can share a common vent and multiple oil-fired appliances can
share a common vent as long as the common vents for both meets certain criteria pertaining to the
minimum cross sectional area of the common vent and the points at which the appliance vent
connectors join the common vent, but neither gas-fired or fuel oil-fired appliances can share a common
vent with the other or with a solid fuel appliance.
Each solid fuel appliance is to have its own separate vent or flue –multiple solid fuel appliances should
never vent into a common vent or flue. If large enough, a single chimney structure can contain multiple
flues/vents for solid fuel, gas-fired, and oil-fired appliances. However, while multiple gas-fired
appliances can share a common vent and multiple oil-fired appliances can also share a common vent,
appliances which burn different fuels cannot be vented in common and multiple solid fuel appliances
cannot share a common vent or flue.
Additional information can be found in Code Check and Code Check Mechanical.
If you want to make a recommendation regarding the installation of a CO detector in homes with gas or
oil-fired appliances, you may want to consider placing such a recommendation as an elective
Other Sources of CO Poisoning
There are other potential sources of CO poisoning that should be considered and evaluated in the
course of conducting a home inspection – these are gas-fired clothes dryers and vehicle garages. Be
sure to inspect the vent and vent terminal of gas-fired clothes dryers for proper materials, damage,
proper installation and termination, proper assembly, and for obstructions. Screws or other mechanical
fastening devices that can project into the interior of any clothes dryer vent should not be used to
secure sections of the vent. Such fasteners can trap lint resulting in the obstruction of the vent,
combustion by-product spillage, overheating of the dryer, and an increased potential for both CO
production and fire. Cloth and plastic type duct tape are also inappropriate for securing dryer vent
sections at their joint connections. Such tapes are not designed to be subjected to the high
temperatures generated in a dryer vent. These tapes and their adhesives will quickly become dry and
brittle and will fail resulting in the potential for dryer exhaust leakage and separation of vent sections at
joints. The hot dryer exhaust will also reduce the combustion temperature of these tapes through a
process known as “pyrolysis.” Dryer vent section joints should be secured using an appropriate,
adhesive-backed metal tape or other approved methods and materials.
Flexible plastic clothes dryer vents should not be used with any types of clothes dryers because they
are vulnerable to physical damage. Clothes dryer lint is extremely flammable and a tear in a flexible
plastic vent can create a lint build-up behind and inside of any dryer, gas or electric. This can increase
the potential for ignition of the accumulated lint. In addition, in the case of a gas-fired clothes dryer, the
vent is conveying the combustion by-products of the burning gas to the exterior. A damaged vent can
permit those by-products to leak or spill into the interior of the home resulting in the production of and
exposure of occupants to CO.
NOTE: For additional information regarding clothes dryer exhaust vent systems see
Clothes Dryer Venting in the “FREE Articles” section of the ProSpex website at:
For additional information regarding on offset clothes dryer transition ducts go to
On the menu on left side of the page place your cursor on “Laundry” and when a drop
down menu appears immediately to the right of “Laundry,” move your cursor to “Dryer
Venting” and double click. This site provides a lot of information and photos of
specialized offset transition ducts.
The installation of self-closing devices on doors between attached garages and interiors of homes
serves to help maintain the fire separation between attached garages and the interior of homes as well
as reducing the potential for gas and oil-fired appliance combustion by-products and vehicle exhaust
fumes to enter the home.
Several years ago in a northeastern Colorado town an entire family, three children and both parents,
died from CO poisoning when one of the children started a car in a closed garage and went back to
bed. His parents had been allowing him to start the car on cold mornings so they would have a warm
car for their ride to school. Unfortunately, on this particular morning, their son had awakened earlier
than usual, before anyone else was awake. After he started the car, he failed to close the door
between the garage and the house and he went back to bed. When the children didn’t show up for
school, the father didn’t show up at work, and no one answered the telephone, someone went to the
home and found the entire family dead in their beds and the car still running. This should plainly
illustrate why currently, in many states, the installation of CO detectors is required when a home is sold
and why discussing the installation and location of CO detectors, the installation of self-closing devices
on doors between garages and the interior of homes, and documenting breaches such as pet doors,
holes, or other damage to walls, ceilings, and doors between attached garages and the interior of
homes is an important part of a home inspection.
Regarding inspection of self-closing devices on doors between attached garages and the interior of
homes, if they are in place, they should be evaluated for proper operation. If a self-closing device is
damaged, not operating properly, or if there are clear indications that such a device has been installed
but is no longer present, it should be noted in the written report.
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