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Many homes in the United States use fireplaces as a primary and secondary source of heat. It is important that such items and associated flues and chimneys operate properly and are well maintained, as they may pose a fire hazard. Every fireplace, and every fossil fuel-burning appliance for that matter, must have a separate flue. The exception is that a heating plant and a water heater can use the same flue, provided they use the same fuel and the water heater stove pipe enters above the flue of the heating plant.

The following details the issues surrounding chimneys and provides some helpful tips that you may wish to consider and apply in your own home.

Height of Chimneys

Chimneys must extend at least 2 feet higher than any portion of the roof or any structure within 10 feet, but must be not less than 3 feet above the point where the chimney passes through the roof.

Masonry chimneys in modern construction are normally lined with terra cotta liners. Terra cotta liners deteriorate over time. If the flue needs cleaning, it may be appropriate to have the flue re-evaluated after cleaning.

Brick Chimneys

Chimneys that are one brick thick and unlined are a fire hazard and must be reported. A two-brick chimney, without a flue liner is acceptable as long as the mortar and bricks are in good condition.
Metal flues for gas appliances such as furnaces must have a cap on them to prevent weather and animals from entering. Caps on masonry chimneys are cement/mortar and have a tendency to crack.

(Additional brick chimney conditions and concerns are outlined under masonry chimneys.)

Brick Problems in Chimneys

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.

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. Cost may be $30.00 to $40.00 per brick with a $300.00 minimum. Placing cement stucco over the problem bricks is an option, however, this would not be recommended unless the appearance was not a concern.

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.

Metal Chimneys

Single wall metal chimneys are simply unacceptable. Metal chimneys serving appliances with flue gas temperatures at the entrance to the chimney below 350 degrees should be lined with an acid and condensate resistant metal or refractory material.


  • Metal chimneys should have enough clearance to avoid heating combustible materials to a temperature in excess of 90 degrees above the ambient temperature.
  • Exterior and Interior. Wood frame or other combustible material—18 inches minimum.
  • Exterior and Interior. Metal chimney over 18˛ diameter—Non combustible materials—4 inches minimum.
  • Exterior and Interior. Metal chimney 18˛ diameter or less—Non combustible materials—2 inches minimum.
  • Interior. Metal chimneys must be enclosed in a continuous enclosure and maintain the integrity of fire separations—1 hour rating in buildings up to 4 stories.
  • Fire stops at floors, walls and roofs must be made of entirely non-combustible materials. 9 inches is the typical clearance, however, it is 18 inches if galvanized steel or approved corrosion-resistant metal is not used.

Specific Metal Chimney Concerns

Metal chimneys will dissipate heat much quicker than masonry chimneys, even insulated metal chimneys, however, to a lesser degree. This causes two issues, and both are related to the draft.

  1. The draft in metal chimneys is more difficult to get started. The colder air in the chimney reduces the ability of the smoke to rise, because it is heavier than the smoke. Once the fire is going, the chimney is warmed; there is no problem until the fire starts to go out and the air in the chimney cools.
  2. When the fire goes out, and only smoking embers remain, the cooler air starts to accumulate in the top of the chimney. If the outside temperature is low enough, the cold air may cause the last 30 to 45 minutes of smoke to back into the house.

This condition is worse when the outside temperature is below 20 degrees Fahrenheit, and when the chimney is on the outside of the house. When the outside temperature is above 30 degrees Fahrenheit, this is generally not a concern. There is also less concern when the metal chimney goes through the interior of the house.


Signs of backdrafting include:

  • Smoke stains on the outside of the firebox or below the mantle, etc.
  • Smoke odors in the house

There are other conditions that, while they don’t identify a problem, should raise your level of awareness and maybe prompt you to ask some questions.

  • If a fireplace has not been used in a number of years, it may be that the owners do not care about the fireplace or that possibly there was a problem. If the owner/occupant is home, ask if there was a problem with the fireplace.
  • If the chimney height is marginal, check the draft with a match or ask if there were problems in the past.
  • The presence of a smoke shield should prompt a question. A smoke shield does not shield smoke, it is a piece of metal about 4 inches deep and the width of the firebox opening. It is installed across the top of the firebox and reduces the size of the firebox, which improves the flue to firebox ratio.
  • If the throat above the damper is not relatively smooth, it may also be an indication of a problem. Consider asking the question.

Vent Connectors: From Heating Appliance to Chimney


Vent connectors are metal pipes that carry exhaust gases from an appliance to a chimney flue. They should be made of non-combustible, non-corrosive material that can withstand flue gas condensate and temperatures.


Vent connectors should rise at least 1/4 inch per foot. They should be supported with wire or strapping every 4 feet and the joints must be screwed together with sheet metal screws.

The joints should be aligned so that they will be liquid-tight or of a design that liquid will drain to the interior of the connector.

The horizontal run of the vent connector should be as short as possible and never more than 75% of the height. Longer vent connectors allow heat to dissipate, condensation to form and corrosion to accelerate.

Types of Vent Connectors

There are generally 3 types of vent connectors used in residential systems, but the overwhelming majority of them are single wall metal connectors.

Single wall metal connectors. Depending on the appliance and installation, clearance to combustibles should be 9 to 18 inches. If the listing of an appliance specifies a different clearance, the listed clearance applies.

Double wall, uninsulated, or “B” vent. Clearance to combustibles is typically 6 inches. “B” and “L” vents may be in accordance with the appliance and vent listing.

Double wall, insulated. Clearance to combustibles should be 1 or 2 inches, or as the appliance or vent listing allows.

(Type “C” vents will also have appliance and vent listings that will indicate the allowable clearances.)

Common Defects

Inadequate wall thickness:

  • Minimum thickness of firebrick is 2 inches
  • Minimum thickness of unlined chimney is 8 inches
  • Minimum thickness of lined chimneys is 4 inches
  • Poor liner or firebox joints
  • Separation of fireplace face and firebox

Improper chimney height should be at least 15 feet high; top at least 2 foot taller than any structure within 10 feet.

  • Flue area should never be less than 1/12 of fireplace opening
  • Damper installed too low
  • Damper missing
  • Fireplace too large for flue
  • Inadequate draft
  • Lack of combustion air