Soot and oil burner combustion product leak (C) Daniel Friedman Heating Appliance Combustion Products
Indoor Air Quality Hazards

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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?

Debris in a stack pipe (C) Daniel FriedmanThis article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

Our photo left shows a heavy collection of soot and debris in an oil burner flue. What is in oil or gas burner exhaust or combustion products?

As stated in Best Practices Guide to Residential Construction:

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.

The major pollutants associated with combustion are carbon monoxide, nitrogen dioxide, sulfur dioxide, and particles.

See INDOOR COMBUSTION PRODUCTS & IAQ for a Table of Combustion Products & Indoor Air Quality Hazards that accompanies this article.

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 carbon monoxide.

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:

Leaky flue (C) Daniel Friedman

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 guidelines:

-- Adapted with permission from Best Practices Guide to Residential Construction.

Question: can a gas fired heater put out zero CO?

2017/02/18 Tim asked:

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


Continue reading at INDOOR COMBUSTION PRODUCTS & IAQ or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see CHIMNEY INSPECTION DIAGNOSIS REPAIR including unsafe venting and fire hazards.

Or see COMBUSTION PRODUCTS & IAQ for the relationship between fuel burning appliances and building indoor air quality.


Suggested citation for this web page

COMBUSTION GASES & PARTICLE HAZARDS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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