Chimney Issues - Masonry Chimneys Pre-Fabricated Metal Chimneys

Chimney Issues - Masonry Chimneys Pre-Fabricated Metal Chimneys.

(Note: The information below was provided to me to use on my website and has been authorized by Amerispec home inspection services of Mississauga)

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Chimney Issues

I have noticed that there are damaged bricks on my chimney above the roof line. What has caused this and what needs to be done to correct it?
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. Post 1940’s a clay tile liner was usually installed inside the masonry to protect the brick and mortar from the exhaust gases. Flaking sometimes occurs on a masonry chimney due to the moisture in the exhaust gases that is 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 and 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. As well, 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 at the top.

Once flaking has occurred, the brick requires replacement to prevent further deterioration. This requires the services of a qualified mason.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.

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 routinely found during home inspections.

Masonary 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.
In most cases, a prefabricated metal liner is required to protect the clay flue liner and surrounding brick or concrete block from the moisture in the exhaust gases. As well, the chimney flue may require resizing to a smaller size for venting newer, more efficient appliances. If the flue is too large, the exhaust gases will not rise quickly enough, leading to the formation of condensation or soot/creosote deposits in the case of wood and oil burning appliances, and potential damage to the chimney. The original clay tile flues are usually too large which is acceptable in terms of size when older, inefficient appliances are vented, however a liner is typically required when newer, more efficient appliances are installed to protect the chimney from the exhaust gases.
Wood burning fireplaces generally do not require a metal liner, however wood burning stoves and fireplace inserts do require that a stainless steel liner be installed because of the potential for chimney fires. A clay tile liner is not capable of withstanding the heat of a chimney fire like a stainless steel liner is, so most wood stove and fireplace insert manufacturers make the chimney liner a requirement of proper installation.
As discussed above, flaking brick is a common occurrence on brick chimneys of almost any age and should be addressed to prevent further deterioration.

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.
This type of chimney should not extend up the side of the building un-insulated 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.
Rusting and corrosion is typical on this type of chimney. Having the chimney professionally painted with a proper metal paint can reduce the rate of deterioration. If breaches (holes) are present, the affected section of the chimney should be replaced.
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 doubled 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.
Factory Built Type ‘A’ chimneys were introduced in the 1950’s and were initially used to vent oil burning appliances. Many were used for wood burning appliances until it was discovered that they could not withstand the heat of a chimney fire. They are round or square metal chimneys that are usually installed on the exterior of the home and sometimes have a painted brick façade on the exterior. This type of chimney is still acceptable for venting oil burning appliances because of the lower exhaust gas temperatures of these appliances. However, if this type of chimney is being used to vent a wood burning appliance, it should be replaced to ensure safety.

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