If you have a gas or oil-fired furnace, boiler or water heater, or a wood stove or fireplace, you have combustion gases being produced as the fuel burns. And while these appliances normally vent outdoors through a chimney or vent pipe, the gases could escape into your home, potentially causing a variety of concerns.

Combustion spillage is the unwanted flow of combustion gases into your home. While the quantities are usually small, the results can still be serious. Unfortunately, combustion systems don’t always work as they should and combustion spillage is the result. Occasionally you may see smoke escaping into the room from a woodstove or fireplace – this combustion spillage is obvious. However, many combustion gases are invisible with little or no odour, making them hard to detect. Also, the furnace and water heater are usually located away from the main living areas of the home, making spillage less noticeable.

The toxic compounds found in combustion gases can cause problems ranging from nuisance headaches to serious illness, carbon monoxide (CO) poisoning and even death. Even if concentrations are low, long-term exposure can cause health effects such as chronic, low-grade, sometimes difficult-to-define ailments and health deterioration.

How to prevent combustion spillage

Inspect and maintain your combustion appliances – Hire professionals to do annual maintenance checks. In particular, have them check for heat exchanger leakage, evidence of start-up spillage and condensation in chimneys. Regular maintenance should also include a tune-up, especially since a properly tuned combustion appliance rarely produces CO, the most serious threat.

Inspect and maintain your chimney – Your chimney’s job is to remove combustion gases from the home, but the combustion gases may not vent if the chimney is compromised. A professional can check to ensure your chimney is not cracked and is clear of obstacles such as ice, dead birds or broken brick.

Upgrade your chimney – Chimney professionals can help you on improve your chimney’s performance. If you are renovating or building, try to have the new chimney located on an inside wall.

Watch for backdrafting – In the winter, we keep doors and windows closed to keep our homes warm and, at the same time, use exhaust fans and other devices to pump air out of the house. Many appliances, fireplaces in particular, exhaust a considerable amount of air even when they are not operating. When a house is sufficiently depressurized, backdrafting can occur – air is sucked in through the chimney, flowing down rather than up. Reducing indoor and outdoor pressure differences can help to minimize backdrafting:

• Avoid operating several powerful exhaust devices at the same time
• If you install a powerful range hood exhaust fan, get expert advice on how to balance the air supply
• Allow air to move freely between the furnace room and the rest of the house. If your furnace or water heater is in a small room, louvred doors will provide more air flow
• Make sure the blower door on your furnace is always in place. If you have a forced-air heating system, ensure return air is not being drawn from the immediate vicinity of the combustion appliances

Take care with fireplaces – As fireplaces can be a significant combustion spillage threat, make sure chimney maintenance is up to date, warning devices are in place and potential backdrafting conditions are avoided. Always provide an outdoor air source while the fireplace is burning strongly or smouldering, particularly since smouldering embers of a dying fire can release high concentration of CO, a colourless, odourless and extremely toxic gas. When a fire is burning down, little heat is being released – the chimney draft may be very weak and the CO can easily spill into the home, sometimes after the family has gone to bed. Tightly shutting fireplace doors can help reduce the potential for spillage as the fire burns down.

Detecting combustion spillage problems – Even with a good maintenance and prevention program, always be on the lookout for combustion gas spillage. Warning signs include:

• Repeated headaches, skin and throat irritations, and other low grade illnesses
• Combustion odours anywhere in the house
• Hot and muggy air around the furnace
• Soot stains around any combustion appliance, or unusual rumbling sounds when it is operating

Make sure effective warning devices are in place to alert you to potential danger from combustion gas. Standard smoke alarms detect combustion spillage from oil and wood furnaces and boilers. CO alarms are recommended with gas or oil furnaces and water heaters, and with fireplaces.

CO alarms can be purchased from hardware and electronic stores. Look for ones that are certified to UL2034 or CAN/CGA6.19 standards. Electric-powered units should also bear the CSA logo. Install CO alarms close to the combustion appliance to be monitored, as well as the bedrooms in your home.

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