Masonry Chimneys
Fireplace flues, chimneys and vent connectors should be inspected carefully, as they impact the safety, health, welfare and comfort of the occupants.
Every fireplace must have a separate flue. Fossil fuel-burning appliances should have separate flues. There are exceptions. Multiple appliances are allowed to use the same flue as long as the vent connector and the chimney flue can handle the volume of exhaust from the appliances. The most common situation where this occurs is when a heating appliance and a water heater use the same flue. The vent connector from the water heater or appliance with the smaller burner should enter the chimney flue above the heating appliance or appliance with the larger burner.
Modern construction requires liners in all masonry chimneys. However, there are thousands of older homes that do not have lined chimneys. Generally speaking, these chimneys have 8² or thicker walls, and if there is not an excessive amount of deterioration, are satisfactory. The fact that all modern chimneys require linings is one of the reasons that chimney specialists nearly always propose new linings, whether they are needed or not. The older the chimney, the more likely it may need a liner.
Determining the condition of a chimney requires a thorough understanding of the functions of the chimney and items that may impact its condition and functionality. Items that impact condition include:
Temperature of the flue gases
Quality and condition of the brick
Quality and condition of the mortar, inside and outside of the chimney
Workmanship – such as mortar mix; the way it is connected to the masonry or frame structure; depth and design of the foundation; and the skill level of the mason
Length of the vent connector (heating system)
Items that impact functionality include:
Workmanship – such as throat and smoke-shelf; ratio sizes of the flue and firebox opening; depth of the firebox; and damper installation
Location of the chimney, as it relates to wind currents
Design of the firebox, throat and smoke chamber, etc.
Height of the chimney
Masonry chimneys in modern construction are normally lined with terra cotta liners. These liners are dependable for 50 or more years, unless condensation forms in the chimney when the attached appliances are operating.
Brick, solid block or concrete that is 4² thick requires a fireclay liner. These liners are sized for the fireplace or appliance they service and should be 5/8² thick.
Stone chimney walls should be at least 12² thick. Liners are required in modern construction, however, they were not required in older homes.
Chimneys that are only one brick thick, without a liner, can be considered a fire hazard. Chimneys that are 8² or 2 bricks thick, without a flue, are acceptable as long as the mortar is in good condition.
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Spalling brick. Spalling occurs when moisture gets into the brick and freezes. This causes the surface of the brick to fall off. The porosity and quality of the brick, and the climate have a significant impact on spalling.
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Deteriorating brick. This is usually a softer clay salmon brick that is designed for use on the inside of an 8-inch thick wall. The deterioration is more complete than the spalling brick. It is relatively easy to recognize due to its orange color.
Cutting or chiseling the problem bricks out and replacing them can repair these conditions. Placing cement stucco over the problem bricks is an option, however, this would not be recommended unless the appearance was not a concern.
