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College of Agriculture | College of Engineering | Iowa State University
Carbon Monoxide Poisoning - Checking for Complete Combustion
ABE Home
ISU Extension Pub # AEN-175
Author: Thomas H. Greiner, Extension Agricultural Engineer
Department of Agricultural and Biosystems Engineering, Iowa State
University.
September, 1997
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CARBON MONOXIDE POISONING
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Checking for Complete Combustion
Fossil fuels contain carbon (C) and hydrogen (H). During complete
combustion carbon and hydrogen combine with oxygen (O2) to produ
carbon dioxide (CO2) and water (H2O). During incomplete combustion
of the carbon is not completely oxidized producing soot or carbon mon
(CO). Incomplete combustion uses fuel inefficiently and the carbon
monoxide produced is a health hazard.
A properly designed, adjusted, and maintained gas flame produces on
small amounts of carbon monoxide, with 400 parts per million (ppm)
the maximum allowed in flue products. Most burners produce much le
with between 0 and 50 ppm being typical. During incomplete combust
carbon monoxide concentrations may reach levels above 7,000 ppm. E
small amount of spillage into occupied structures from appliances pro
large amounts of CO is a health risk and can be a threat to life.
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Incomplete combustion occurs because of:
* Insufficient mixing of air and fuel.
* Insufficient air supply to the flame.
* Insufficient time to burn.
*Cooling of the flame temperature before combustion is complete.
Typical reasons for incomplete combustion in residential heating appli
include:
* Blocked vent systems.
* Blocked flue passages in heating appliances.
* Air shutter on burner not opened sufficiently.
* Gas orifices too large or too small (usually too large).
* Manifold gas pressure too high or low (usually too high).
* Rust, scale, or soot on burner.
* Burner installed incorrectly.
* Flame impinging on cold surface.
* Insufficient combustion air to appliance.
* Downdrafting of vent system.
* Failure of power-vent or induced draft blowers.
* Loss of integrity of the heat exchanger (cracked, rusted, or holes in
exchanger).
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* Physical disturbance of the flame (i.e., strong air currents blowing o
flame.)
To reduce the risks of CO poisoning, obtain and follow carefully all the
manufacturers' recommendations for inspection and maintenance. The
following are recommended additional steps for servicing and inspecti
heating appliances:
1. Visually inspect the burner and flame, looking for rust, soot, discolo
and abnormal flame color or pattern.
2. Visually check the heating appliance for evidence of flame roll-out,
downdrafting, and spillage. Burned wires, soot, rust, scale, and "track
combustion products are signs of problems.
3. Check the vent system for proper design, integrity, and draft.
4. Check adequacy of combustion air and make-up air.
5. Verify that vent system functions under "worst-case" depressurizat
the dwelling.
6. Check flue passages in appliance for blockage or restriction-- clean
necessary.
7. Visually inspect heat exchanger for integrity.
8. Check manifold gas pressures using a manometer and adjust if nece
9. Check gas flow rate to appliance (on metered appliances).
10. Measure CO in flue products using an electronic CO analyzer with d
display.
Additional steps which help determine the cause of a heating applianc
carbon monoxide problem include:
11. Verify proper combustion using a combustion analyzer with capab
measure carbon monoxide and oxygen content.
12. Determine leakage areas and pressure differences in the structure
vent system using a blower door and micromanometer.
13. Check for proper gas orifice size.
14. Continuously monitor carbon monoxide concentrations in the struc
VISUAL INSPECTION OF THE FLAME
Visual inspection of the burner will reveal obvious problems including
scale, or soot. Obvious flame pattern disruptions or improper color ind
a problem with combustion. Unfortunately, visual inspection is NOT
sufficient to verify proper combustion. Burners producing EXTREMELY
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concentrations of carbon monoxide can burn blue. Conversely, burner
producing little carbon monoxide can burn yellow.
Carbon monoxide is a colorless, odorless, tasteless, non-irritating and
poisonous gas at low concentrations. Because it is not possible to relia
determine if a burner is producing excessive carbon monoxide visually
use of an electronic carbon monoxide analyzer with digital display is
recommended. The American Society of Heating, Refrigerating, and Ai
Conditioning Engineers, Inc. states "It is desirable through the use of
suitable indicators to determine whether or not carbon monoxide is pr
in flue gases" (ASHRAE Handbook, pg 26.3). Use of an instrument to
measure carbon monoxide concentrations outdoors, inside the structu
and in the flue products is critical to insure the heating appliance is
operating safely.
Blue Flame Burner
Flame Color: Blue
Can be either high or low CO, no reliable visual method of determining
production.
Flame Color: Yellow
Usually indicates high CO production. One cause-- insufficient primary
opening.
Yellow Flame Burner
Flame Color: Blue
Combustion not as designed, Can be either high or low CO, no reliable
method of determining CO production.
Flame Color: Yellow
Can be either high or low CO, depends on adequate secondary air for
complete combustion, no reliable visual method of determining CO
production.
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PRODUCTION OF CARBON MONOXIDE
Burner and combustion chamber design affects the amount of carbon
monoxide produced. Methods of reducing carbon monoxide production
include providing excess air, increasing burner chamber temperature,
providing a large combustion zone. Unburned combustion products wi
continue to be oxidized in a hot combustion chamber with excess air.
Unfortunately, excess air, high temperatures, and large combustion zo
reduce efficiency and increase costs.
Simple actions, such as placing a cold pan on a kitchen stove top burn
blowing air from a fan across an unvented burner can increase carbon
monoxide production. Oxidation of the combustion gases stops when t
gases impinge on the cold pan or are cooled by the air from the fan. A
carbon monoxide not yet oxidized will be released into the room.
Adjustment of the burner affects the amount of CO produced. Insuffic
primary burner air increases CO production. Restricted air inlets often
produce a noticeable disruption of the flame and a change from blue t
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yellow.
Another cause of insufficient primary air and incomplete combustion i
excess gas flow to the burner. Excess gas flow seldom produces a not
disruption of the flame or change of color. Excess gas flow can be cau
excess gas pressure or by oversized gas orifices. Gas regulators can fa
out of adjustment, not be correctly field-derated for altitude, or be adj
upward by field technicians to increase heat output. Overfiring is espe
dangerous because:
* High CO production can occur.
* Excess heat is produced which can damage heat exchangers.
* Excess combustion products are produced which can exceed capacit
heating appliance flue vents and vent systems.
* The flame can continue to burn blue, giving no obvious visual indica
the problem.
Typically warm-air furnace manufacturers require checking of the gas
and gas pressure upon initial start-up and allow no over-firing. To find
correct problems caused by excess gas flow it is important to check ga
flow, gas pressure, orifice sizing, and carbon monoxide concentrations
combustion products.
HOLES IN HEAT EXCHANGER
Large holes in a heat exchanger are dangerous. Air forced through hol
the circulating fan increases the production of CO by disrupting the fla
The air flow increases the amount of combustion products spilling into
living quarters by disrupting the air flow through the appliance vent fl
most natural draft and induced draft furnaces, high pressure on the fu
blower side of the heat exchanger prevents combustion products from
flowing directly into the circulation air. Usually combustion products e
the house because of vent failure or spillage. In power vent and pulse
combustion furnaces high pressures on the combustion side can force
combustion products directly through holes into the circulating air and
house. Combustion products contain large amounts of carbon monoxid
water vapor, which can smother flames or lead to increased rust and
corrosion of the heat exchanger. Flame roll-out occurs in severe cases
Holes in heat exchangers can be found by direct observation, using eit
mirrors or by disassembling the unit. Some direct vent sealed combus
units can be tested by sealing the inlet and outlet and pressure testing
Smoke bombs, odor tracing and salt sprays have been used, but have
been totally acceptable. The American Gas Association has developed
exchanger test method using tracer gases which they have determine
more reliable and accurate. Observation of flame disruption when the
furnace blower turns on will reveal large holes. Measurement of carbo
monoxide in flue products using CO analyzers reveals incomplete
combustion caused by air flow through holes.
Combustion gases should be sampled before draft hood dilution. In na
draft appliances the flue passages (chambers) at the top of the applia
can usually be reached, with a probe, through the draft diverter or hoo
equipment with several burners and separate flue passages (chamber
each burner must be checked separately by inserting the probe into th
of each chamber. It is important to insert the probe a sufficient distan
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the flue chambers to ensure undiluted combustion products are being
sampled. Sealed combustion units can often be sampled most easily a
outdoor exhaust outlet. Locations that will not give an accurate readin
combustion gases include: the perimeter of the draft hood (this will
represent room air), directly above the flame (sampling here may give
erroneous high reading), and at furnace air outlets (which contain a m
sample).
CARBON MONOXIDE ANALYZERS AND DETECTORS
There are many potential causes of carbon monoxide production and s
of combustion products into living areas. Many causes are accidental a
unpredictable; i.e.; holes in heat exchangers develop or enlarge after
inspection, vent systems fail under strong wind conditions, pressure
regulators stick, or air inlets and vents freeze.
To protect against accidental carbon monoxide poisoning, the U.S. Con
Product Safety Commission, the Iowa Department of Public Health, an
State University Extension recommend that every home have at least
L listed carbon monoxide detector on every floor with sleeping quarte
Heating appliances should be inspected and maintained yearly by a qu
heating contractor. Because flame color is not a reliable indicator of ca
monoxide production from a burner the heating contractor must use a
carbon monoxide analyzer to inspect and maintain burners.
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