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.

What are the Products of Burning Heating Oil, LP Gas, Natural Gas, What is in "Flue Gases" and What are the Hazards?

This article includes excerpts or adaptations from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , 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 (Steve Bliss, J Wiley & Sons) :

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:

• 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 attached garage.
• 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.
  
  Fetuses, infants, the elderly, and people with anemia or heart disease are especially vulnerable. At very high levels, CO causes confusion, loss of consciousness, and death.
  
  CO alarms are programmed to sound before levels reach 100 ppm for 90 minutes, 200 ppm over 35 minutes, or 400 ppm over 15 minutes.
  
  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.
  
  The 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.
  
  However any fuel burning appliance may release dangerous levels of NOx into a building and NOx may also be contributed to indoor air from busy roadways nearby.
  
  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. WHO reports NOx as a mutagen.
  
  Children and people with asthma and other respiratory problem are more susceptible to exposure. Common Indoor Pollutants and Sources 293
  
  see NITROGEN DIOXIDE NOx HAZARDS for details.
• 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 SO₂
• Particles. The health effects of breathing particles depend on several factors, including the size and chemical makeup of the particles.
  
  In general, 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 guidelines:

• 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 increased pollutants.
• 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 appliances.
• Combustion air:
  
  See COMBUSTION AIR REQUIREMENTS for details about the requirement for combustion air.
  and
  
  See COMBUSTION AIR for TIGHT BUILDINGS explains how to provide outside combustion air for tight buildings.
  Also
  
  See COMBUSTION GASES & PARTICLE HAZARDS for an explanation of the dangers of inadequate combustion air.
• Sealed combustion. In new construction, avoid the use of atmospherically vented boilers, furnaces, or water heaters. Instead, use power-vented appliances, preferably with sealed combustion.
• Inspections and maintenance. Have central heating systems and water heaters inspected and adjusted annually. Inspect all flues and chimneys for blockages or damage and promptly repair any problems.
  
  Blocked or leaking chimneys or flues can result in serious illness or death from carbon monoxide poisoning.
  
  See DRAFT HOODS - gas fired for gas fired heating equipment,
  and
  
  see DRAFT REGULATORS, DAMPERS, BOOSTERS for oil fired heating equipment.
  Also
  
  see GAS MEASUREMENT TOOLS for a discussion of gas leak testing instruments.
• 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 (Steve Bliss, J Wiley & Sons) ).
• Fireplaces. Fireplaces should have inserts with tight- fitting doors and a dedicated outside air supply. Fireplaces should not be used if the house is depressurized by exhaust fans
  
  See Backdrafting and also page 295 of Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , by Steven Bliss, Wiley & Sons).

- - Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

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

Tim:

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.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-02-03 by (mod) - help with neighbour's heater exaust entering my council flat

Wendy

You might get help from

- a heating service company

- your local fire department

- the utility company supplying the fuel

- the local building inspector

the concern is that the heating system must be safe both in having a properly installed chimney of proper height and proper combustion for safety

On 2020-02-03 by wendy stone

A narrow side alley separates my semi from the semi next door.

At the back of my house i have a large conserrvatory which periodically is filled with white clouds from the next door flue from their boiler with insert outflow on their wall., which is directly opposite from my conservatory.

You can see the white clouds emerging from this flue which the wind blows one way or another. Sitting in my conservatory as a senior lady I can taste bits in the air and feel the texture of this emission I am now being advised that these emissions could contain nitrogen oxides and be very harmful to me. Please can you advise how it could be inspected..

.I have tried Wandsworth Council with no success and the owner next door does not live there and all residents have been very un-helpful in the past. Thank you.

On 2019-04-15 - by (mod) - testing for CO is first line of safety defense but is not comprehensive

Thanks for your comment John.

Keep in mind that a CO detector is a first line of defense; if flue gases are leaking into a building interior building experts rely first on the detection of carbon monoxide rather than trying to scan for all gases that may be present.

Please see your comment repeated along with a detailed reply at NITROGEN DIOXIDE NOx HAZARDS where we discuss NOx and NO2

On 2019-04-15 by John

Good read BUT! We talk a lot about CO and not about NOx

So you have CO spilling into room, would you not also have Nox splilling with it?

Therefore, could you not have more NOx ppm spilling than CO? Also, unless you have some $10K analyzer, say the more economical/popular field combustion analyzers like a Testo 310, 320, 327 or Bacharach units, NOx i not considered, it's filtered out.

So you can be getting sick and not know it cause it's not something that is tested for.

On 2018-02-09 - by (mod) - rooftop heating units RTUs may leak combustion gases into a building

For other readers, Justin's "RTUs" most likely refers to "RoofTop Units" used for heating.

Some gas-fueled RTUs have been subject to complaints of combustion gas odors, and I've followed some HVACR techs who point out that some models including some Lennox RTUs were drawing combustion gases into the economizer from the above-roof exhaust.

Lennox provides a retrofit kit that "... that directs combustion gases into a B vent tee that is capped on the bottom (cap has a few small holes for condensate) Top of tee continues upwards in b vent to above top of unit. .."

This presentation

Kosar, Douglas, High-Efficiency Heating Rooftop Units (RTUs) – The Final Frontier for Condensing Gas Furnaces [PDF] at http://www.gastechnology.org/Expertise/Documents/ETP/ETP-CenterPoint-Energy-Conference-Condensing-RTU-Presentation-05-21-2014.pdf cites other defects that include leaks that could be the trouble.

A visual inspection of the RTU, especially in freezing weather, can be diagnostic even before any testing since you may see condensate freezing where there is a leak that was otherwise not obvious.

On 2018-02-07 by Justin

I work in a commercial building with may different RTU's. Upon entering the building on a cold day it smells like RTU combustion gases.

We have CO detectors but obviously you cannot smell CO so and the detectors do not go off. Issues I am personally having is extreme headaches sinus issues and watery eyes.

Any Idea what this problem can be? supposedly they replaced some heat exchanges a few weeks ago but it still smells like this exhaust fumes in here.

On 2017-11-27 - by (mod) - heater exahust can enter an open window

George

Yes that is possible.

On 2017-11-27 by George

I appreciate your response which applies common sense, if the heater is on one would presume the windows are shut.

However, let’s for sake of understanding the issue, would the flue installed on the outer wall of the My apartment have an impact on the air quality of apartment above if they had their windows open??

On 2017-11-25 0 - by (mod) - gas heater exhaust entering at closed window?

George,

I'd like to be helpful but I'm afraid I have to say I don't know. I don't understand how the presence of a window, presumably shut, would be the source of the problem that you described.

On 2017-11-25 by George

we are looking to install a gas fireplace but have had a response to the effect, “We are concerned that exhaust fumes in the immediate vicinity of windows above could pose problems. I do know that with a previous building I had, the location of a gas heater exhaust flue near open windows affected the air quality. “ Would you agree it would affect the air quality?

On 2017-08-05 by Hluka

What worst can be as you were exposed to fumes of natural gas and burning smoke of aluminium melting stove?

Question: a chemical smell is released into my home as soon as the sun hits our roof

(Oct 4, 2015) Anonymous said:
a chemical smell is released into my home as soon as the sun hits our roof.
It is only in the living room, but the smell has drifted into the kitchen.
Please help.

Reply:

I suspect an odor is coming from hot roofing materials or sealants, but you need some on-site inspection and help; I can't usefully diagnose an odor complaint from a brief e-text.

See ODORS GASES SMELLS, DIAGNOSIS & CURE - topic home, for some diagnostic suggestions.

Or click on EXPERTS DIRECTORY at the site main-links at the top of any InspectApedia page to find an expert who should be able to help out.

Question: when opening front entry door and pella storm door a strong oder is released

(Oct 11, 2015) me said:

when opening front entry door and pella storm door a strong oder is released inside our home

Reply:

Please see PLASTIC ODORS-SCREENS, SIDING

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Continue reading at INDOOR COMBUSTION PRODUCTS & IAQ or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• BACKDRAFTING HEATING EQUIPMENT
• CARBON MONOXIDE - CO
• CHIMNEY INSPECTION DIAGNOSIS REPAIR including unsafe venting and fire hazards
• COMBUSTION GASES & PARTICLE HAZARDS
• COMBUSTION PRODUCTS & IAQ for the relationship between fuel burning appliances and building indoor air quality.
• FURNACES & IAQ
• INDOOR COMBUSTION PRODUCTS & IAQ

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