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Carbon monoxide (CO) is an odourless, colourless, tasteless gas which is very toxic. Exposure to CO can cause symptoms such as headaches, dizziness, weakness, nausea, vomiting, or loss of muscle control. However, there could be many other causes for these symptoms and that is why carbon monoxide poisoning is so difficult to diagnose. If inhaled, carbon monoxide crowds out life-sustaining oxygen from red blood cells: prolonged exposure to high concentrations of CO can lead to unconsciousness, brain damage, or death.

Gasoline-powered vehicles and lawn mowers, kerosene stoves, charcoal grills, coal stoves or heaters, and wood burned in a fireplace or wood stoves produce some carbon monoxide. Tobacco smoking also contributes CO to the air you breathe but only in very small amounts.

When properly installed and maintained, your natural gas furnace and hot water heater do not pollute your air space with carbon monoxide. The main ingredient of marketed natural gas is methane, which is not poisonous. Natural gas is known as a 11 clean burning" fuel because under correct operating conditions, the combustion products are water vapour and carbon dioxide, which do not damage the environment. (Carbon dioxide (C02) is also present in the air we exhale and is necessary for plant life.) The products of combustion are exhausted from furnaces and water heaters to the outside by means of a flue duct or chimney.

But correct operation of natural gas, or indeed any fuel burning heating equipment requires two key conditions:

  1. There must be an adequate supply of air for complete combustion and
  2. venting of the products of combustion from the furnace through the chimney to the outside must be effective.

Incomplete or faulty combustion (burning) of any fuel, including natural gas, can produce carbon monoxide.
If this condition is combined with ineffective venting of the furnace a serious hazard to your health may exist.



  1. Dirt and Blockage: For fuel burning equipment to operate safely and efficiently, secondary air openings, vents, exhaust ducts or flues, and chimneys must be kept clean and free of obstructions. But some householders create potential hazards by not keeping their equipment free of lint, dust, and trash. Others, who may be conservation-minded individuals tamper with fuel-burning equipment and unwittingly create hazards. For example, one overzealous energy saver stuffed insulation into the draft hood of his furnace, thereby interfering with normal combustion, and creating a spillover of pollutants, including carbon monoxide, into his home. Even in well ventilated homes, dirt or obstructions can prevent the intake of air needed for safe combustion and exhausting of combustion products.

  2. Using other equipment which consumes or exhausts household air. An open fireplace with a roaring fire may use up five or ten times as much air as your furnace. Prolonged running of kitchen and bathroom exhaust fans and use of power attic vents can remove as much as a houseful of air in twenty minutes. It is possible for such situations to starve your furnace of air. This can lead to incomplete combustion and backing up of wastes such as carbon monoxide into the home.

  3. Confining or enclosing gas-fired equipment. In order to create a separate workshop or family entertainment room, some householders enclose or partition off their furnace and hot water heater from the rest of the basement or utility room. Enclosing equipment in a tight alcove or closet is a sure way to starve your furnace or water heater of air.


  1. Dirt and Blockage
    • Never insulate or try to seal up a draft hood, wind cap or exhaust vent on any gas appliance (furnace, hot water heater, range, dryer or space heater). Keep your equipment area clean. Don't store anything that could restrict air circulation close to equipment.
    • It is absolutely essential to your safety that panels and grills on the furnace are kept in place and that the fan compartment door is closed when the furnace is operating.
    • If you have a gas water heater: make sure that combustion air openings at the bottom of the tank and the opening below the draft diverter (on top of the tank next to the flue duct) remain unblocked.
    • If you have a gas dryer: the exhaust duct must be vented to the outside and have a hood at the end. Check that the exhaust system is not blocked by lint or debris and that the flapper in the hood moves freely,
    • For all fuel-burning equipment: make sure that vent hoods and pipes are securely in place and that external vents and chimneys are not blocked by insulation, leaves, or bird nests.
  2. Using Other Equipment Which Consumes or Exhausts Household Air If you use exhaust fans and a fireplace or other fuel-burning heaters or stoves:
    • Run exhaust fans for just a minute or two at a time - prolonged use could remove too much air and also wastes heat.
    • Don't run power attic vents during the winter or when your furnace is on.
    • When your fireplace, coal or wood stove is operating, open a window and close off warm air registers in the room, or install a fresh air duct directly to the fireplace or stove so that it won't steal air from your furnace.
  3. Confining or Enclosing Gas Fired Equipment If you've partitioned off your furnace and water heater, you may need additional ventilation. The following examples were developed by Gas Company engineers and are based on a common size of furnace and hot water heater. However, many different factors affect ventilation conditions. Before you make changes, consult a qualified heating contractor.
    • Example A: The home has not been tightly weatherized or sealed. There is no fireplace or solid fuel-burning stove in use, and exhaust fans are not in constant use. The furnace and hot water heater have been partitioned off from the rest of the basement, but they are not tightly confined. Suggestion: Add louvers to the top and bottom of the furnace room door, The open area of the top louver should be about 250 cm2 (about 38 square inches) and that of the bottom louver about 1000 cm2 (about 150 square inches).
    • Example B The same house as in Example A, except that the owners have caulked and weatherized. Suggestion: Air ducts which bring in air from outside the home should be considered.
    • Whether or not your furnace is partitioned off, if it is located in a basement, make sure the basement door does not seal too tightly. A space about 3 cm wide (about 1.2 inches) between the bottom of the door and the floor will aid air circulation. Even better, a louver about 1000 cm2 could be added to the door.


As you caulk and weatherize, smaller amounts of cold fresh air will enter the house and there will be less heat loss. These conservation methods save energy and money and are usually not the main cause of inadequate combustion or inadequate ventilation. Fresh air also enters a home through normal activities, such as opening outside doors to enter and exit. But if your home is tightly caulked and weatherized and does not have much fresh air circulation, you may benefit from the use of an air-to-air heat exchanger.


Stuffy, stale or smelly air and back drafts and soot from a fireplace or furnace chimney usually mean your home needs more air for proper combustion and healthy living. For gas fired equipment, mostly yellow (rather than clear blue) burner flames, a pilot light that keeps going out, or a smell of gas indicate trouble. Turn off the equipment and contact your Gas Company emergency service.


  • Have your fuel-burning equipment checked periodically for safety and efficiency by a qualified service technician.
  • If you're adding a wood or coal burning stove to a home, make sure that the stove is properly installed and vented. Check with the Building and Inspections Department of your local municipality or consult a heating contractor before installation. If you've already installed a wood or coal stove without a building permit or inspection, consult your local municipal building authority. Some "do-it-yourselfers" have unknowingly created dangerous conditions. Once you file for a permit, a qualified inspector will check your installation and explain how to rectify any mistakes.
  • Don't expose yourself to carbon monoxide through carelessness: Never operate a gasolinepowered engine in a confined or enclosed space such as a garage or tool shed. Never use a kerosene stove or charcoal grill in a confined space such as a closed garage or recreational van.
  • If you have a forced warm air furnace, ensure that there is no cold air return register in a small enclosed furnace room.
  • On masonry chimneys inspect the cleanout regularly to ensure that the chimney is free and clear of debris.
  • Regardless of the fuel your furnace, fireplace or stove uses, your chimney should be inspected from time to time by a competent chimney contractor.
  • Following sensible maintenance and safety procedures in the home will give you the fuel savings without endangering your health.

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