Heating Appliance Combustion Products
Indoor Air Quality Hazards
POST a QUESTION or READ FAQs about flue gases & combustion products produced by burning home heating oil, natural or LP gas, wood or wood pellets, or coal in home heating appliances
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Hazardous products of oil or gas heating appliance fuel combustion: this article explains flue gases and particles produced by various heating appliances and their impact on safety and indoor air quality in homes.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
What are the Products of Burning Heating Oil, LP Gas, Natural Gas, What is in "Flue Gases" and What are the Hazards?
A combustion appliance is any device that burns fuel
for heating, cooking, or decorative purposes. This includes
central-heating systems, space heaters, water heaters,
ovens and cooktops, woodstoves, and fireplaces.
major pollutants associated with combustion are carbon
monoxide, nitrogen dioxide, sulfur dioxide, and particles.
Unvented space heaters and gas stoves without range
hoods dump combustion products directly into the living
space and have no place in the modern home. Vented appliances,
such as boilers, water heaters, and fireplaces, are
designed to exhaust combustion products to the outdoors,
but they are vulnerable to backdrafting in today’s tightly
built houses. When appliances are malfunctioning or out
of adjustment, they produce more pollutants, including
The combination of backdrafting and
the high production of carbon monoxide can be deadly.
Health Effects of Combustion Products. Possible
health effects from combustion products include eye and
respiratory irritation, persistent coughing, headaches,
fatigue, and dizziness. In the case of carbon monoxide,
symptoms can include nausea and confusion, and, at very
high levels, loss of consciousness and death. Effects associated
with specific pollutants are discussed below:
Carbon monoxide. CO is a colorless, odorless gas
produced by incomplete combustion. Common
sources include blocked chimneys or vents, cracked
or rusted heat exchangers, poorly adjusted appliances,
smoldering fireplaces, and auto exhaust from an
CO interferes with the blood’s ability
to deliver oxygen to the body. Low concentrations
may increase chest pain in people with heart disease.
Sustained concentrations above 70 ppm can cause
fatigue, headache, weakness, and nausea, and may
be confused with the flu or food poisoning.
infants, the elderly, and people with anemia or heart
disease are especially vulnerable. At very high
levels, CO causes confusion, loss of consciousness,
CO alarms are programmed to sound
before levels reach 100 ppm for 90 minutes,
200 ppm over 35 minutes, or 400 ppm over
see CARBON MONOXIDE - CO for details about carbon monoxide standards, exposure, testing, and remediation.
Nitrogen dioxide. NO2 is a colorless gas with an acrid
odor at high levels.
T he primary source of NO2 in
homes is unvented gas and kerosene space heaters,
gas stoves without a range hood, and stoves with
continuously burning pilot lights. Studies have shown
that homes with unvented gas appliances have
elevated NO2 levels, and there is some evidence linking
this with impaired lung function and increased respiratory
infections in children. At high levels, NO2 is
an eye, nose, and respiratory irritant.
people with asthma and other respiratory problem are
more susceptible to exposure.
Common Indoor Pollutants and Sources 293
Sulfur dioxide. SO2 is a colorless gas with a pungent
odor and is primarily associated with oil- and coal-
burning appliances. At low levels of exposure, SO2
can cause eye, nose, and respiratory tract irritation.
At high exposures, it can cause the airways to narrow,
leading to chest tightness and breathing problems.
People with asthma are particularly susceptible to
SO2 exposure. See Sulfur Dioxide Gas.
Particles. The health effects of breathing particles
depend on several factors, including the size and
chemical makeup of the particles.
suspended particles can cause eye, nose, and throat
irritation, and increased respiratory symptoms for
people with chronic lung or heart disease. In addition,
a number of pollutants, including the carcinogens
radon and benzo(a)pyrene, attach themselves to
small particles and are then inhaled and carried deep
into the lungs.
Guide to Reducing Exposure to Combustion Gases & Particles Indoors
The three main sources of combustion
products in household air are unvented appliances,
appliances or flues that are broken or poorly adjusted, and
backdrafting. To minimize exposure, follow these general
Unvented space heaters. Do not use unvented
space heaters in living spaces. If required for
temporary use, closely follow manufacturer’s
directions, open a window, and open doors to
adjoining rooms. A persistent yellow-tipped flame
is generally an indicator of poor adjustment and
Cooking. With gas ranges and cooktops, always use
a range hood vented to the exterior. Choose appliances
with electronic ignition rather than a continuously
burning pilot light. Or replace with electric
Woodstoves. Make sure doors in older woodstoves are
tight fitting with intact gaskets. New stoves should
meet EPA emissions standards. Burn only seasoned
wood. Make sure there is adequate air for combustion
and that the house is not depressurized by exhaust fans
See Backdrafting and also page 295 of Best Practices Guide to Residential Construction).
is it possible for an appliance to put out zero co emission?
Reply: typical safety limits on CO measured at gas fired heaters: 100 ppm action threshold in the exhaust flue, 0 ppm in the utility room
Assuming we are talking about a gas -fired appliance, if combustion of LP or natural gas is absolutely perfect, the results of combustion will include only CO2 (carbon dioxide) and water - no CO (carbon monoxide).
It would be no surprise to find traces of CO in the exhaust of many gas-fired appliances, especially at the start of a burner-on cycle. But there should be NO carbon monoxide (CO) released into the living area, even if there are traces of it within the exhaust flue.
Many heating companies set a threshold for CO in the exhaust and will call for appliance inspection, repair, replacement if the CO is above a specific level in PPM or percent. Some use 100 ppm as the safety threshold for finding CO in the gas boiler exhaust flue.
On a recent boiler repair job in Two Harbors MN an initial inspection by a heating technician condemned a heating boiler as unsafe because of elevated CO levels in the boiler's exhaust. He found 101 ppm in his measurement of CO in the flue and pointed to rust on the boiler bottom as the possible cause of the trouble.
Rust perforation of the heat exchanger on a steel furnace or boiler can be indeed dangerous. But we thought that rust perforation was unlikely on the Two Harbors boiler as it is a Weil McLain cast iron unit, so we decided to make a more detailed inspection together with a more-experienced heating service technician from the same company.
On that subsequent inspection we found that the chimney cleanout door at the base of the boiler flue had rusted through, admitting so much air into the flue that the boiler's draft was inadequate. Simply cleaning and repairing the seal at the chimney base was enough to fix the carbon monoxide problem.
So if you do see a high CO level be sure that its source is diagnosed with some care before condemning a boiler or furnace. The problem could be one of adjustment, inadequate combustion air, a chimney blockage, or something else.
Watch out: in all events be sure that there are properly-located, tested and working carbon monoxide and smoke detectors in the building.
Search InspectApedia for CARBON MONOXIDE to read more details. Or see these articles
Questions & answers or comments about flue gases & combustion products produced by burning home heating oil, natural or LP gas, wood or wood pellets, or coal in home heating appliances.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones