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.
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How can you tell if a fireplace will draw properly without lighting a fire?
- Fireplace draw is dictated by the design of the fireplace, however, there are critical specifications that carry considerably more weight when you are trying to determine whether or not a fireplace will draw.
- The two most significant criteria are the height of the chimney as it relates to the roof and the size of the firebox in relation to the flue.
- 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.
- The flue must be 1/12 the size of the firebox opening, or larger. If the flue is 8'' x 12'' (approximately 96 square inches) and the firebox opening is approximately 30'' x 32'' (960 square inches), the flue is determined to be 1/10 the size of the firebox, indicating that the fireplace is very likely to draw properly.
- Although there are other important concerns, if the above 2 items are correct, the likelihood that the chimney will draw is high.
How can you tell a chimney is blocked without looking up or down the chimney?
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- If the heating appliance is operating, there will be an excessive amount of heat and moisture backing out of the draft hood (gas-fired) or draft control and view door (oil-fired). This would be obvious to a casual observer.
- If the heating appliance is not on, such as during the summer months, and the chimney is blocked, there will be rust debris at the draft hood (gas fired) or excessive soot at the draft control and view door (oil fired). If the blockage has accumulated over a long period of time, the rust build-up would be excessive.
- The exhaust from the appliance combustion is approximately 90% water, which will cause oxidation and rust at the draft hood. When the unit is operating, the relative humidity in the area of the heating appliance can reach 100%.
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.
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- Smoke stains on the outside of the firebox or below the mantle, etc.
- Smoke odors in the house
Other Problem Areas
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.
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- 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.
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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Chimneys that are one brick thick and unlined are a fire hazard and must be noted. 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.
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.
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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.
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 18² diameter-Non combustible materials-4 inches minimum.
- Exterior and Interior. Metal chimney 18² 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.
- 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.
- 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.
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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.
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(Type “C” vents will also have appliance and vent listings that will indicate the allowable clearances.)
Inadequate Wall Thickness
- 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 3 feet higher than roof structure; top at least 2 foot taller than any other component or structure within 10 feet. Known as "3-2-10 Rule"
Damper, Draft, And Flue
- 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
What To Do About Spalling
Spalling. Most people have never heard of it before. But there's a good chance that you've witnessed it on your home or on someone else's home.
The faces on bricks sometimes peel off due to water being absorbed by the bricks and then freezing. The expansion of the ice will break off the brick's face if the brick has not been fired at a high enough temperature. This is called "spalling."
One of the main causes of spalling bricks near the top portion of a chimney is a defective masonry cap. Often times the cap is cracked and it allows water to enter into the brickwork below.
The water is then absorbed by the masonry, and during cold weather it freezes. During this process, its volume expands, causing pieces of the brick face to come off. Spalling can also result from cracked and open mortar joints between the bricks.
The only way to control the problem is to stop the water from penetrating into the chimney. Damaged areas of mortar should be removed and fresh mortar installed. Some people recommend applying a waterproofing agent, such as silicone, to the chimney to prevent or reduce water absorption.
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