Furnaces and boilers must be vented with one of the following:
Gas furnaces and boilers
- a double-walled, prefabricated metal B-vent (chimney) with an aluminum lining
- a properly sized masonry chimney lined with a clay flue tile
- a masonry chimney lined either with a B-vent or an approved stainless steel liner
Oil furnaces and boilers (except for sidewall-venting models)
- a double-walled, insulated, prefabricated metal (Type A) chimney with a stainless steel lining
- a masonry chimney lined with a clay flue tile
- a certified stainless steel liner in a masonry chimney
Routine Chimney Care
Although chimneys from gas and oil furnaces rarely need cleaning, they should be checked occasionally for signs of deterioration or corrosion.
The lower flue temperature achieved by the improved efficiency of today’s heating equipment has made it possible for flue gas condensation to damage the inside of a masonry chimney. This is especially true for chimneys located on the outside wall, where they are chilled by exposure to the outside air. Under these conditions, it is common for water vapour to be produced when oil or natural gas is burned. Human house air drawn into the chimney could also contribute to the problem.
You can check the vent simply by inserting a mirror in the cleanout opening at the bottom of the chimney on a bright day. Look for any of the following:
- a white, powdery efflorescence on the outside of the chimney
- spalling or flaking of the bricks
- crumbling mortar joints
- rusting or bending in the metal liner
- wet patches on inside walls behind the chimney
- broken or flaking flue liner or interior chimney damage (pieces of tile at the bottom of the chimney)
- water running out of the cleanout door or around the bottom of the chimney behind the furnace.
The design of the chimney itself might contribute to condensation. More efficient furnaces need smaller chimneys than the 200 mm x 200 mm (8 in. x 8 in.) flue tile that has been standard for many years. Because of this, the combustion gases, already cooled by the improved heat exchanger in the furnace, rise slowly in the cold, oversized flue, where they are sometimes cooled to the dew point of the water vapour they contain. The resulting condensation can then leak into the bricks and cause structural or water damage.
Certain types of higher efficiency systems have special venting needs that may require your attention. Check your owner’s manual or discuss this with your installer or serviceperson.
The combustion of heating fuel sends a great deal of water vapour up the chimney. If the chimney is too cool, the vapour will condense; the alternate freezing and thawing of the water, as well as the acidic corrosion from the condensate, can seriously damage masonry chimneys. This problem is particularly serious with outside chimneys, which are much cooler and more exposed to the elements.
Gas -- If your gas heating system is vented through an existing masonry chimney, you can usually avoid condensation problems by inserting an approved metal liner, either a double-walled B-vent or a single-walled, stainless steel Underwriters' Laboratories of Canada (ULC) liner. Approved liners reduce the size of the flue so that the chimney will match the requirements of the gas-fired appliances being vented. The reduced diameter of the flue allows gases to go up the vent faster with less chance of cooling down. At the same time, the inside surface of the metal liner is warmed more quickly by the flue gases escaping the chimney, reducing the likelihood of condensation. Metal liners should be used with natural gas furnaces and are required in many provinces and territories. Contact your local utility or provincial/territorial authority for specific advice.
Oil -- The condensation caused by low temperatures in the chimney can be greatly reduced if you install an insulated metal liner, such as a Type L, double-walled, stainless steel liner or a single-walled stainless steel liner surrounded by insulation, as per ULC requirements. Check with your provincial or territorial fuel safety division to find out which method it approves. Consult a standard such as CSA Standard CAN/CSA-B139-M91, “Installation Code for Oil-Burning Equipment.”