Ask the Inspector: Chimney

A chimney is designed to exhaust products of combustion from a fuel burning appliance to the exterior of the building. A masonry chimney is typically built on its own footing and foundation with brick or concrete block on the exterior. Flaking sometimes occurs on a masonry chimney due to the moisture in the exhaust gases that are a product of combustion. The moisture gets absorbed into the brick and freezes in the winter when the warm exhaust gases stop flowing out the chimney. When water freezes, it expands, and over time this freeze-thaw cycle leads to deterioration of the brick, clay tile, and mortar. The damage is usually located at the top portion of the chimney for many reasons. The top portion of the chimney is the most exposed area and therefore the most vulnerable to weather. Moisture may also enter the top portion of the chimney from the exterior if the cap is cracked or missing. Once the exhaust gases reach the top of the chimney, they may have cooled sufficiently for the moisture to condense, making the concentration of moisture higher.

Once flaking has occurred, the brick requires replacement to prevent further deterioration. This requires the services of a qualified mason. Several quotes should be obtained, including a detailed description of proposed repairs. Depending on the level of deterioration, the repairs may be costly, since the chimney may have to be completely rebuilt from the roof line, up. To prevent further deterioration once the chimney is repaired, the cap should be maintained in a properly sealed condition and a metal liner should be installed though the entire chimney flue to protect the chimney from exhaust gas moisture.

Chimneys and Their Venting

Different combustion appliances require venting through different types of chimneys. The following provides a description and characteristics of different types of chimneys and the types of combustion appliances they can vent, as well as some typical problems that are found during home inspections.

Masonry Chimneys

  • Masonry chimneys can be used to vent almost any type of appliance, including natural gas, propane, and oil burning appliances, as well as wood burning fireplaces, fireplace inserts, and stoves. Pre-Fabricated Metal Chimneys
  • Type "B" vents/chimneys are used to vent natural gas or propane burning appliances.
  • A type "L" vent can be used to vent natural gas, propane, or oil burning appliances, but is most commonly used for oil appliances.
  • These are the metal chimneys that usually run through the interior of the home and extend above the roof. This type of chimney should not extend up the side of the building uninsulated, since the exhaust gases can cool sufficiently on the exterior of the home during the colder months of the year to condense significant amounts of moisture, potentially resulting in damage to the base of the chimney or the appliance heat exchanger.

Factory-built Chimneys

  • Factory-built (or 650°C) chimneys are used to vent wood burning stoves. Their metal exterior is much larger in diameter than type "B" or "L" vents. They are built to a standard which requires the chimney to withstand flue gas input temperatures of 650°C, hence their name. This type of chimney is typically double-walled and insulated, and therefore can be installed inside or on the exterior of a home, however if installed on the exterior, an opening for removing soot after cleaning should be present at the base of the chimney.


  • Any chimney venting a wood burning appliance should be cleaned annually, or every 60 fires, by a qualified chimney sweep. This prevents creosote from building up and potentially causing a chimney fire.
  • In specific cases, a masonry chimney flue or a pre-fabricated chimney can be connected to more than one appliance, as long as the chimney or vent is correctly sized, based on the heat output capacity of the chimney/ vent. A flue venting a wood burning fireplace cannot be used for any other purpose. Because the rules for venting wood burning appliances are complicated, it is always best to check with a qualified Wood Energy Technology Trained (WETT) contractor regarding the venting of wood burning appliances.
  • Some other common deficiencies that are usually identified during a home inspection with respect to chimneys include inadequate clearance to combustibles around chimneys and flue vents (the required clearance varies, depending on the chimney, its location, and what it is venting), missing spark arrester/rain cap on masonry chimney flues (These prevent moisture or pests from entering the chimney flue), inadequate or improperly sealed roof flashing around the chimney, and inadequate chimney height (A general rule is that a chimney should be 1 m above the roof line and 60 cm above anything within 3.18 m of the chimney).

Areas that are not visually accessible during the inspection cannot be inspected, such as proper clearance to combustibles behind walls and/or the condition of the interior chimney flue. Verifying that the flue is properly sized for the appliance(s) being vented is outside of the scope of the inspection since the output capacities of the appliances are required to perform the sizing calculations and these values are not typically readily available. The home inspection focuses primarily on safety issues, and not code issues, so commenting on the chimney's code compliance is also outside of the scope of the home inspection.

Ask the Inspector: Chimney

Written by Seth Pfaehler, Posted in For Buyers, For Sellers

About the Author

Seth Pfaehler

Seth Pfaehler

Real Estate Consultant

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