Combustion Gases in Your HomeThings You Should
Know About Combustion Spillage
Are Combustion Gases Spilling Into Your Home?
Does your home have any of these combustion appliances?
- gas-fired furnace, boiler, or water heater
- oil-fired furnace, boiler or water heater
- or fireplace
- other fuel-burning device
If so, then combustion gases will be produced as the fuel
burns. Normally, these combustion products which can include
both visible smoke and various invisible gases should be
vented to the outdoors through a chimney or vent pipe.
Unfortunately, they may instead escape into your home, where
they could raise a variety of health and other concerns.
Combustion spillage is the term used to describe the
unwanted flow of combustion gases into your home. The
quantities involved are usually small. However, the number of
houses with potentially significant spillage is increasing,
and on occasion the results can be extremely serious.This fact
sheet provides some important information about combustion
spillage. It alerts you to some of the symptoms and outlines
practical steps you can take to reduce the risks. In short,
this fact sheet is designed to help you keep combustion
gases OUT of your home.
Why the Concern?
Because toxic elements can be present in combustion gases,
sharing your home with these gases can lead to problems
ranging from nuisance headaches to serious illness, carbon
monoxide poisoning and even death. The most likely health
effects are chronic, low-grade, sometimes difficult-to-define
ailments, and health deterioration due to long-term exposure
to the combustion gases. These effects can occur even if
concentrations are low.
Toxic and other harmful products in the combustion gases
- carbon monoxide
- polycyclic aromatic hydrocarbons (PAHs)
- sulphur dioxide
- nitrogen oxides
Carbon dioxide and water vapour, which are relatively
harmless, are often present in larger quantities.
The exact composition and characteristics of combustion
gases, and the severity of their effect on your house and its
occupants, depend on several factors. These include the type
of fuel being burned and the condition of your system.
Understanding Venting and Spillage
When Things Go Right
A typical oil or gas forced-air heating system is shown
schematically in Figure 1. When operating,
the system generates two separate air flows:
- Combustion air
Combustion is a
process in which air and fuel combine to produce heat and
various combustion products. Depending on the type of
furnace, the air required for combustion may be drawn into
the furnace from the surrounding room, or it may be ducted
directly from outside the house. Furnaces should be designed
to completely remove the resulting combustion gases from
- Circulating air
The heat generated in
the furnace, if it is to have any value, must be transferred
to the living areas of the home. In a forced-air system,
this is accomplished by circulating heated household air.
Cooler air is returned to the furnace, heated in a heat
exchanger, and returned to the house via the heating ducts.
In a properly operating forced-air furnace, the combustion
air and the circulating air both flow through the furnace as
it operates, but do not mix at all (as shown in Figure 1).
Hydronic heating systems systems that rely on water and
radiators to distribute heat don't have a circulating air
stream. They do, however, require the same supply of
combustion air and removal of combustion gases as the
Similarly, gas or oil water heaters, fireplaces, and wood
stoves all require combustion air, and all require the
combustion gases to be vented to the outdoors.
Figure 1: Basic Forced Air Heating System
When Things Go Wrong
Unfortunately, combustion systems don't always work as they
should, and combustion spillage is the result.
Sometimes this spillage is obvious for instance, if you
have a wood stove or a fireplace, you may occasionally see
smoke escaping into the room. In other cases, spillage may not
be so evident, in part because the furnace and water heater
are usually located away from the main living areas of home.
In addition, many combustion gases are hard to detect they
are invisible and have little or no odour.
Three major factors, working alone or together, can create
conditions conducive to combustion spillage in your home.
In addition to these factors, unusual winds can also
sometimes be at fault.
Factor 1: Chimney Problems
Your chimney's job is to remove combustion gases from your
home. However, your chimney won't work properly if it is
poorly designed, poorly installed or poorly maintained.
There are many causes of inadequate chimney performance or
failure. Here are some examples:
- A chimney may be improperly sizedtoo small for the job
or too large to maintain an adequate draft.
- Obstructions such as birds nests, broken bricks and ice
can block a chimney's air flow.
- Corrosion may become a problem as a result of
condensation or poor construction or installation.
- An uninsulated chimney on an exterior wall is a
particular concern because it can become very cold when
combustion gases are not present.This can lead to
condensation of moisture from the air. When the chimney
first fills with moist combustion gases, the condensation
may increase, at least until the chimney warms up.
Condensation can result in damage to the materials in the
chimney and ice formation.This in turn leads to problems
such as crumbling bricks, cracks and leaks, blockages, and
Factor 2: Equipment Problems
Your home's combustion appliances are made up of several
components. Like chimneys, they should be well designed,
properly installed, and regularly maintained. Otherwise,
mechanical problems may prevent combustion gases from venting
As an example, your furnace may be causing a spillage
problem if the heat exchanger is corroded or cracked.This
would allow crossover of circulating air into the combustion
chamber or of combustion gases into the circulating air
stream. Either way, combustion gases will end up being
distributed through your home, as shown in Figure 2.
Figure 2: Combustion Spillage Due To A
Cracked Heat Exchanger
Factor 3. Pressure Problems
In the winter, we close up our homes. At the same time, we
run exhaust fans and numerous other devices that pump air out
of the house. (In fact, many appliances, particularly
fireplaces, exhaust a considerable amount of air even when not
operating.) As a result, the air pressure indoors falls below
the air pressure outdoors, and the house becomes
depressurized. Pressure is balanced as fresh outdoor air is
drawn into the house through available openings, such as
cracks and gaps around windows, doors, and small openings in
the building structure.
If your house is sufficiently depressurized, air may be
sucked in through the chimney. When this happens, air flows
down the chimney, rather than up a condition known as
backdrafting. If you have ever opened the damper before
lighting your fireplace and felt the big wash of cold air come
into the living room, you have encountered backdrafting.
Backdrafting is most common during the "off" cycle of the
combustion appliance. If the appliance starts up while
backdrafting is occurring, the downward airflow in the chimney
may be difficult to reverse. Combustion gas spillage could
persist for as long after start up as it takes for the
backdrafting to be reversed. In houses where the "on" cycle is
short and the chimney is not insulated, this type of start up
spillage may occur frequently, since the chimney has little
opportunity to heat up and establish a good draft. Because the
combustion products during start-up are particularly dirty,
even minor spillage of this type should be considered
undesirable. In some circumstances, backdrafting can also take
place while the combustion appliance is operating for
instance, in a fireplace with a smouldering fire. (See "What About
Figure 3: Backdrafting Due To
Can We Control Combustion Gas
By reading this fact sheet, you have already made a start
toward controlling your combustion gas problems, because
increased awareness is the foundation for action.You can build
on this foundation by taking measures to prevent,
detect, and correct combustion spillage
If you follow the recommendations below, you are unlikely
to experience hazardous levels of combustion gases in your
home. Some of the actions have costs but that is a small
price to pay for improving the quality of the air in your home
and for ensuring your health and safety.
Preventing Combustion Spillage
As the saying goes, prevention is the best cure. Some of
the actions described below will be easier to implement if you
are building, renovating or replacing existing equipment. Even
if you are not, there is still a great deal you can do.
Maintain Your Combustion Appliances
Start an annual maintenance routine for all your combustion
appliances. Get professional assistance to do this. The
service person should check for heat exchanger leakage,
evidence of start up spillage, and condensation in chimneys.
Maintenance should include a tune-up a properly tuned
combustion appliance rarely produces carbon monoxide, the most
serious threat. If necessary, have your furnace adjusted so
that it operates on cycles that are six minutes or longer (to
minimize start up spillage). Remember that a thorough
maintenance check may cost a little more than a simple
cleaning, but it is money well spent.Inspect and
Maintain Your Chimney
A blocked chimney will not vent your furnace's combustion
gases. Have a professional check that your chimney is clear of
obstacles, such as pieces of broken brick, or ice, or dead
birds, and is not cracked.This check should be done routinely
as part of an annual or bi-annual service call.Upgrade
Talk to chimney professionals to find out how your
chimney's performance can be improved. If you are building or
renovating, try to have the new chimney located on an inside
Have a specialist assess the air supply for your combustion
appliances. Remember that even a properly designed combustion
air duct will not, on its own, solve spillage or backdrafting
problems; chimney problems and depressurization should also be
When replacing existing equipment or buying new equipment,
invest in appliances that are less prone to spillage. Forced
draft appliances, which rely on a fan to establish positive
venting of combustion gases, are often resistant to spillage.
Sealed combustion appliances isolate the combustion air and
combustion gases from the living areas.This further restricts
the possibility of spillage. Ask the salesperson for
advice.Avoid Conditions that Lead to Backdrafting
With a little care, conditions that might lead to
backdrafting can be minimized by reducing indoor and outdoor
pressure differences. For instance:
- Be wary of operating several powerful exhaust devices
- If you install a new range-top barbecue with a powerful
exhaust fan, get expert advice on how to balance this on the
air supply side.
- Avoid combinations of appliances that are likely to
create depressurized conditions for instance, a
natural draft furnace with a range-top barbecue exhaust fan.
- If your furnace or water heater is enclosed in a small
separate room, allow air to move freely between the furnace
room and the rest of the house. Louvred doors may be the
- If you have a forced-air heating system, be sure you are
not drawing return air from the immediate vicinity of your
combustion appliances. Make sure the blower door on your
furnace is in place.
What About Fireplaces?
Fireplaces can be a significant combustion spillage threat
and should be treated with great respect. Most people with a
fireplace have experienced small puffs of smoke when the fire
is lit. They may not know that the smouldering members of a
dying fire can release monoxide (CO), a colourless, odourless
and extremely toxic gas. This happens because when a fire is
burning down, little heat is being released; the chimney draft
may be very weak and the CO easily spills into the home,
sometimes after the family has gone to bed.
Fireplace safety measures include chimney maintenance,
warning devices and avoiding conditions that are conducive to
backdrafting. Extra air from outdoors should always be
provided while the fireplace is burning strongly or
smouldering. Keeping fireplace doors tightly shut as the fire
burns down can also help reduce the potential for spillage.
Consider adding tight-fitting doors if there are none or,
better still, install an energy-efficient fireplace
Figure 4: Chimney Flow Test
Chimney Flow Test
- Hold a smoke indicator (such as an incense stick) near
the draft hood of a gas furnace or water heater, or near the
barometric damper of an oil furnace when your furnace is
operating. Watch the direction of the smoke.
- Now switch on all exhaust fans and other exhaust
equipment. Check again for smoke movement at the draft hood
- If the smoke moves into the house, you may have a
spillage problem.You should immediately call an experienced
professional heating contractor for a thorough inspection.
Be Careful with Unvented Appliances
If you have been unvented gas range in your home, be sure
to use your range hood, and provide extra ventilation whenever
the appliance is operating.
Unvented portable space heaters should not be used
except in heat emergencies, and then only with windows open to
allow combustion gases to escape.
Detecting Combustion Spillage Problems
Even with a good prevention program, you should be on the
lookout for combustion gas spillage.
Watch for warning signs such as:
- repeated headaches, skin and throat irritations, and
other low grade illnesses
- combustion odours anywhere in the house
- hot and muggy air around the furnace
- soot stains around any combustion appliance, or unusual
rumbling sounds when it is operating.
Do the Chimney
Flow Test, a quick and simple procedure that will give you
an indication of how well your chimney is working. (This test
is not suitable for sealed combustion appliances.)
Install warning devices. Standard smoke alarms are suitable
for detecting combustion spillage from oil and wood furnaces
Carbon monoxide (CO) alarms should be used with gas
furnaces and water heaters, and with fireplaces. CO alarms are
sold in hardware and electronic stores. Buy one certified to a
UL or CGA standard.These devices can be installed close to the
combustion appliance being monitored. Having a CO detector
close to bedrooms is also a good idea.
Correcting Combustion Spillage Problems
If you have a combustion spillage problem, it is important
to deal with it. Often, solutions to existing problems and
prevention of future problems require similar strategies. Once
you have determined that you have a problem and have
identified a cause, consider the relevant actions described
Combustion Spillage. Ensure that all necessary
repairs or improvements are done as quickly as possible, and
by experienced professionals.
If you are unsure about your options, consult the Yellow
Pages to find professionals who specialize in, for
example, ducting, building inspection, indoor air quality,
chimneys and heating equipment. Your fuel supply company
should also be able to provide assistance.