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How do we inspect a chimney for safety? Chimney components and chimney types Chimney codes and regulations How to determine the necessary flue size How to inspect chimneys Chimney cleaning advice Chimney repair advice Home inspection reporting obligations This article provides detailed instructions for visual inspection of chimneys for safety and other defects. The original version was provided by Donald V. Cohen, P.E., President, Imperial Building Inspection, Inc, Milwaukee, Wi. This article first appeared in the ASHI Technical Journal, January, 1994; edits, updates and additions by DJ Friedman periodically through February 2007. Also see the Chimneys Home Page © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

Introduction to Inspection of Chimneys and Flues

Those home inspectors who live and practice in the northern climates have developed a conditioned reflex approach to the exterior of any home, ie: look up! Check the exterior of the chimneys and look for disintegrating masonry, missing flue liners or corroded metal vents. This is the third most costly repair to the exterior of snow belt houses due to the changes in appliances since the introduction of the moderate and high efficient units. A naturally drafted chimney for natural gas or oil fuel appliances lasted many years because of the hot gases venting up through the clay flue. Changes in chimney utilization is causing many of the problems we observe which are unsatisfactory appliance operation, sluggish or inconsistent start up and more frequent need for service. And, the most critical and life threatening problem is the risk of flow reversal and spillage of combustion products into the dwelling. The photo above shows a cracked chimney top seal or "crown" and also that there is no chimney cap installed. Notice my pen [DF] sticking right into the chimney masonry at the top? Water and frost damage are risks for this flue. What else can we see on this chimney top? The flue looks pretty clean - perhaps this is a gas-fired appliance. The top clay liner looks good - at least the part we can see in the photo, but don't assume anything about the rest of the flue before it's inspected.

Unfortunately, the function of the chimney is seldom ever the concern of the occupants of a residence and never inspected. The chimney must be moisture and gas tight and convey the products of combustion to the outside atmosphere.

The 1992 (NFPA 54) National Fuel Gas Code recommends that when a new appliance is retrofit into an existing installation, or an existing appliance is removed from a common vent, the entire venting system, which may include a masonry chimney, should conform to current codes. Many houses inspected have had higher efficiency appliances installed, some direct vented, and we observe corroded vent connector pipes, corroded chimney cleanout doors and disintegrated masonry at the cleanout. It is the intent of this paper to inform and suggest to the ASHI members methods of inspecting chimneys and flues so as to provide a more complete inspection for the client.

Chimney, Flue, Vent Connector - Defined

Chimney A chimney is a structure manufactured or constructed to form and enclose one or more vertical passages through which products of combustion pass to the outside atmosphere.

Flue A pipe or shaft for the passage of smoke, hot air and gas in a chimney.

Vent Connector A vent is a manufactured product intended only to serve a specific type of appliance under narrowly defined conditions.

Components of a Chimney - Defined

Foundation or Support for Masonry Chimneys

Masonry chimneys represent a heavy concentrated load on the soil or support structure. Therefore, proper footing support is critical and is generally separated from the building footings except possibly at the exterior wall. Due to the drying of lumber and movement in structures the chimney shaft must remain free of any ties into the framing of the building. The space between the shaft and the building is or can be sealed with "fire code" [fire-rated] sheetrock or metal flashing if a fire stop is required between floors.

Manufactured chimneys

Manufactured chimneys of multiple metal walls or plastic have no appreciable weight and can be supported from the structure since they can deflect as the building moves with no problem. Some segmented cementitious manufactured chimneys can be supported with a steel bracket bolted to reinforced joists to carry the weight.

Chimney Wall

The chimney wall has two primary functions, ie: structural and draft inducing or thermal performance. Masonry walls are generally built of brick, stone or concrete masonry units. Codes dictate the thickness and mortar requirements. All concrete products must be waterproofed and all mortar joints solid through the thickness. Masonry chimneys may not be supported on structural elements of the building. They must be fully self supporting.

The thermal performance of a chimney - draft

A chimney's thermal performance provides the "draft" by maintaining a warm interior lining. The draft is the pressure difference between ambient air and the less dense flue gases within the chimney. The lighter gases are buoyant and rise to be displaced by heavier ambient air.

The chimney must contain the hot gases and protect the surrounding materials against combustion. Residential masonry chimneys must protect the building while under exposure to 1000oF continuous flue gas temperature although most gas appliances operate with a flue gas temperature of about 300oF and oil burners with a flue gas temperature of about 500oF.

Chimney Flue Lining

Flue liners can be made of clay, ceramic or metal and must contain the products of combustion and protect the shell from heat and corrosion. Clay linings are usually square, round or rectangular sections two feet in length. They are stacked upon each other with refractory cement in the joints to form a continuous smooth walled vent from below the appliance inlet to the top. A space of one half inch to one inch must be maintained between the flue and masonry surround. Alignment of the flue tiles is secured with one walnut sized mortar deposit between the masonry surround and each of the faces of the tile.

As the flue is heated the warm liner will expand outward and lengthwise. Thus it is important that the liner not be secured to the surrounding masonry. The air space effects a reduction of temperature at the exterior of the masonry surround and helps to maintain the elevated temperature of the flue for proper draft.

[Also see: U.S. vs. Canadian Unlined Masonry Chimney Flue Requirements & Case Study -- -Editor]

Chimney Caps

The purpose of the chimney cap is to close off the space between the flue liner and chimney wall, to shed water clear of the chimney and generally prevent moisture entry.

The Brick Institute of America recommends caps of precast or cast-in- place concrete a minimum of 2" thick with a projection of 2 1/2" beyond the face of the masonry surround so that water shed from the top will not run down the face of the brick. The cap should slope away from the flue 3" per foot and not be bonded to the flue liner or top of the chimney in order to allow for thermal expansion of the liner. The space between the cap and the flue liner must be closed with a flexible sealant. Mortar caps are prone to cracks and allow water to drain over the face of the chimney masonry leading to spalling, loss of mortar and leakage to the interior spaces. Corrosion at the base cleanout doors are common to those types of chimneys.

Required Chimney Height Above Roof Surfaces or Ridges

The 1992 Vent Sizing Tables require that all Type B gas vents terminate above the roof with a listed cap or listed roof assembly in accordance with the manufacturer's instructions. Vent caps 12" and smaller may terminate a distance above the roof if 8 feet or more away from a vertical surface as follows:

ROOF PITCH -  rise/run in inches
MINIMUM HEIGHT*
Flat to 7/12   - 1.0 foot above the roof surface
7/12 to 8/12   - 1.5 feet above the roof surface
8/12 to 9/12   - 2.0 feet above the roof surface
9/12 to 10/12  - 2.5 feet above the roof surface
10/12 to 11/12 - 3.25 feet above the roof surface
11/12 to 12/12 - 4.0 feet above the roof surface
*  Continues to 21/12 pitch at 8.0 feet

[Other references provide an alternative "rule of thumb" as follows: a horizontal line drawn from the top of the chimney to the point at which the line would touch the roof surface must be 10 ft. in length, or where such a line would pass over the ridge of the roof, the chimney should extend 24" above the ridge line. see "Masonry chimneys" note below -- Ed.]

Vent caps larger than 12" must be located at least 2 feet above the highest point and 2 feet higher than any portion of the building within a horizontal distance of 10 feet.

Masonry chimneys must terminate at least 3 feet above the highest point and extend 2 feet higher than any part of the structure within 10 feet horizontally.

Chimney Cleanouts Required

All chimneys must have a cleanout at least 12" below the lowest appliance inlet opening. A fireplace is considered as access for cleaning. Cleanouts must have metal or precast concrete doors that can be secured tightly.

Chimney Thimble Requirements

The thimble is a sleeve embedded in the chimney wall designed to accept the flue connector from an appliance. They must be placed with the chimney end flush with the inside wall of the flue lining and cemented in place with the refractory mortar used in the flue tiles.

Fire stopping at Chimney Passage Through Building Floors

Chimneys that pass through several floor levels of a home may be required to be fire stopped at each penetration. Typical fire stops are sheet metal or fire code sheetrock.

Chimney Types and Chimney Construction Materials

Masonry and Clay Tile Chimneys

Masonry chimneys for low heat (less than 1800oF) in residences shall be constructed of solid masonry, solid waterproofed modular concrete block or rubble stone laid with full push-filled head and bed mortar joints. The thickness shall be a minimum of 4" for brick and concrete block to 12" for rubble stone. Fire clay flue linings (ASTM C-315) with a 5/8" wall thickness shall line all masonry chimneys. The flue section joints shall be fully bedded in a medium duty non-water-soluble calcium aluminate refractory mortar with a smooth surface inside the flue. An air space of one half inch to one inch maximum shall separate the flue liner from the masonry with only enough mortar to be used to make a good joint and hold the tiles in position.

Notice the unlined flue at the top of this page? It is a single wythe or "one brick" thick. If those bricks are less than 4" wide the chimney is not in compliance with modern codes. Its condition is very important since loss of mortar or a chipped brick can be a fire or gas hazard.

Factory Built Chimneys

Several types of factory manufactured chimneys are in use for residential situations. They are a Precast Pumice Masonry unit that stacks together, a Form Filled Refractory Cement stack unit, a Stainless Steel Double or Triple Wall air insulated sectional unit and a Combination Air and Ceramic Fiber insulated triple wall unit.

Single-Wall Metal Pipe Chimneys or Vents

Single-wall metal shall be galvanized sheet steel not less than .0304" thick or other approved, non combustible, corrosion resistant material. Limitations require all lengths of single wall vents to be exposed from the draft hood up to the roof or wall thimble.

Double-Wall Metal, Type B and Type L Chimneys

Type B (550oF) metal vents are a pipe within a pipe with air space between the two walls. The inner wall is aluminum to resist corrosion and the outer wall is galvanized steel for strength. Type L (1000oF) metal vents use stainless steel for the inner pipe for higher temperatures.

Flue Vent Connectors

"B" or "L" Vents: A vent connector connects gas equipment to a flue or chimney. Appliances having draft hoods and installed in an attic or concealed space must use Type B or L connectors. Appliances installed in basements can use Type B or Type L or metal pipe (.0304" thick) or aluminum pipe (.012" thick). Gas fired equipment should not be connected to any flue serving solid fuel appliances.

Multiple vents into a single flue: Where two or more vent connectors enter a common gas vent, chimney flue or single wall metal pipe, the smaller connector shall enter at the highest level consistent with available headroom and clearance to combustibles. Avoid unnecessary bends and secure all joints with sheet metal screws or other approved means.

Horizontal vent length limits: The horizontal length of a vent connector to a natural draft chimney or vent serving a single draft hood appliance shall not be more that 75% of the height of the vertical portion of the chimney above the connector. [NFPA 1992 (7.10.10)].

The maximum horizontal length of vent connectors per the GAMA tables is limited to 1.5 feet per inch of diameter with a provision of a 10% reduction in capacity for each multiple of the length permitted. The venting tables must be consulted to size an adequate venting system. All draft hood vent connectors must slope upward toward the flue connection at 1/4" per foot of length.

High Temperature Plastic Chimneys and Vents for Gas Appliances

Single wall plastic pipe (450oF) is listed to be used with condensing gas appliances.

[Note: in freezing climates, the pitch of direct-vent appliance vents such as those using plastic vent lines can be critical. If condensate can be produced in the vent line, the line must be pitched to drain properly - usually inside to a drain - so as to avoid dangerous flue blockage by ice.-- Ed.]

Chimney Building Codes and Chimney, Flue, and Appliance Venting Standards

• NFPA #211-3.1 1988 - Specific to chimneys, fireplaces, vents and solid fuel burning appliances.
• NFPA # 54-7.1 1992 - Specific to venting of equipment with fan-assisted combustion systems.
• GAMA - Gas Appliance Manufacturers' Association has prepared venting tables for Category I draft hood equipped central furnaces as well as fan-assisted combustion system central furnaces.

Chimney and Fuel-burning Device Categories vs. Chimney Requirements

National standards committees organized gas appliances into four categories based upon the flue gas temperatures and pressures.

Category I is non-condensing, negative flue pressure, draft hood, AFUE 65% - 83%, vents of Type B, single-wall metal or lined masonry chimney. Mid- efficiency, fan assisted appliances are also in this category.
• Category II have negative flue pressure with a lower flue gas temperature and require a corrosion resistant vent.
• Category III is non-condensing, positive flue pressure, AFUE 78% - 83%, vents of air tight high temperature plastic or air tight single wall metal.
• Category IV is condensing, positive flue pressure, AFUE 90%+, vents of air tight high temperature plastic, PVC or CPVC.

Wet time and Corrosion in Chimneys and Vents

Recommended venting systems with draft hood appliances were developed in the 50's for atmospheric aspirated appliances. The efficiencies were low and approximately 35% of the energy input was wasted.

Newer mid-efficiency Category I appliances have a much lower flue gas temperature and reduced off-cycle losses. These new furnace designs alter the vent gas temperature, dewpoint temperature, flow rate and dilution air in the vent. Greater potential for condensation and corrosion exists and vent systems are critical to the life of the appliance.

The Gas Research Institute Venting Project reported on the following venting occurrences:

1. 1. The reduced dilution air increases the chances of more condensate to form in the vent.
2. Reduced flue gas temperatures result in oversized vents not heating quickly which contribute to increased condensation.
3. Reduced dilution air increases the maximum capacity of a vent.

Wettime or "wet time" is the time measured after start up that the flue wall temperature is below the dewpoint. The "Wettime" of a mid-efficiency appliance is double that of a draft hood appliance. Therefore, during the flue heat up time condensation forms inside the connector or flue. This moisture can dislodge soot products inside the flue which will fall to the bottom of the chimney or connector and become a corrosive media.

New Vent Requirements

Minimum vent sizes for fan assisted appliances to limit condensation and maximum vent sizes to avoid positive vent pressures for fan-assisted appliances are design features of the venting tables.

Draft Hood Appliances

Hot gases rise and draft upward into the chimney. The draft hood allows dilution air to vent and mix with the flue gas which reduces the humidity or dewpoint and thus reduces chances of corrosion. The dewpoint in a gas vent is about 90oF to 130oF.

Mid-Efficiency Appliances

A fan assisted system reduces dilution air in the vent with the following effects on vent performance.

1. Vent gas dewpoint temperatures (or humidity) are higher.
2. Less gases flow through the vent.
3. There is less airflow through the appliance when the unit is off.

Higher dewpoints requires that the vent warm up above the dewpoint to stop condensation. However, the lower volume of gases flowing into the vent will make it more difficult to raise the temperature. This results in more condensation in the vent system. Oversized flues, especially on exterior chimneys, never warm up in cold weather.

High Efficiency Oil or Gas Burning Appliances

These systems have positive pressure sealed vent/combustion air control. The vent connectors are plastic pipes usually vented through the sidewalls of a basement or utility room with design limits on the length and number of elbows. Installation requires reverse pitch on the vents so that condensation can flow back to the float trap drain.

Inside Chimneys

All chimneys internal to the structure up to the roof line are considered inside chimneys.

Outside Chimneys

Chimneys with three walls exposed are considered outside chimneys. Vents may experience continued condensation. A "Type B" vent or a listed chimney lining system passing through an unused masonry chimney flue is not considered to be exposed to the outdoors.

Sizing of flues - Category I Draft Hood and Fan Assisted Appliances

The old sheet metal workers rule of thumb was to size the chimney flue by taking the square root of the sum of the squares of the appliance draft hood diameters, ie: a 3" plus a 4" appliance vent will require a 5" flue. ( 32 + 42 = 25. The square root of 25 = 5.)

NFPA 1992 (7.5.3) states that draft hood appliances, Category I appliances and other appliances that use Type B vents must have a chimney flue area not less than the area of the largest vent outlet plus 50% of the area of the additional flue outlet. The following calculation works out to be the same answer as the "Rule of Thumb" sizing, ie: (4/22 x 3.14 = 12.56) + (3/22 x 3.14/2 = 3.53) = 16.09 / 3.14 = 5.1251/2 = 2.26 x 2 = 4.52" or 5".

The 1992 GAMA vent sizing tables for single-wall metal vent connectors attached to a tile lined masonry chimney uses Table 8. The result of those calculation using a 38,000 BTU water heater with a 3" draft hood and a 37,500 BTU boiler with a 4" draft hood connected to a 20' high chimney is to use a common flue with an area of 28 square inches or a 6" flue vs a 5" flue in the previous examples.

These tables also indicate that a 3" vent is not capable of venting the 38,000 BTU water heater. A 4" would be required. Also the flow area of the chimney must not be more that 7 times the area of the smallest vent area. Therefore, 7 x 7.065 = 49.455 vs 28, a 6 inch flue pipe is ok. The maximum size flue that could be used is 8 inch at 50.3.

Visual Inspection of Chimneys

Inspecting chimneys from ground level

The condition of the top exterior section of the chimney and the exposed flue liner will generally reveal the soundness of the chimney above the roofline. The exterior of the upper portion of a chimneyt can be seen from the ground level though not in detail. The first photo here shows even from a distance that the chimney is curved, so we'd be alert for flue damage or for evidence that the chimney has separated from the building. Any movement in a masonry chimney risks damage to its interior liner and could be a safety concern. The second photo above shows a chimney to viewed from the ground, with our camera zoomed, and photographed during a rainstorm - not an unusual inspection condtion. We can see some incomplete repairs to a brick at the chimney top and other spalled, loose bricks. This chimney needs further inspection and probably some repairs, as well as a rain cap.

Chimney Movement detected outdoors: movement such as curving or leaning or separation from the building, especially in masonry chimneys, can cause damage to the flue liner or openings that might admit sparks (fire risk) or gases (carbon monoxide hazards) into the building structure or even into occupied spaces. The photographs here show a chimney which probably lacked a good footing, possibly aggravated in its lean by roof spillage onto the area where a footing should have heald the chimney steady. As the chimney leans away from the house we might find several problems: Damaged, unsafe chimney liner Damaged, unsafe fireplaces where the chimney has pulled the firebox away from the building or created dangerous openings around the fireplace into the building structure, risking both sparks (and fire) or air leaks (and inability to control the draft). Leaks into the building walls The usual repair is to remove and replace the chimney, though in some cases it may be possible to jack an intact masonry chimney back to level and repair its connections into the building.

Attempts to hide chimney movement can be dangerous since if there is a safety problem the building owner or inspector may not pick up its clues. The fresh and thick band of caulk between the chimney and the wall as shown in this photograph were traced to a chimney separation that had been "repaired" simply by more caulking at the wall. Sometimes we see a chimney that has been "repaired" in this manner several times, with several generations of concrete or caulk or wood trim strips. Such chimneys should be inspected by an expert to assure that they're safe. If I see such a chimney that already has been re-lined I speculate that it may have been inspected and repaired but we'd still want to know just what was done. If the chimney moved further after the liner was installed, connections between vented appliances or a woodstove and the chimney flue liner could have opened and thus might be unsafe.

Inspecting chimneys from on the roof

An on-roof inspection of the chimney allows inspecting the upper portion of the flue lining, cap and roof flashing, chimey top seal or "crown", and it permits a closer inspection of masonry chimneys for cracking or other damage that might not be visible from a more remote "from ground" inspection.

Chimney to Roof Clearance: from the ground you can guess and from on-roof access you can measure to determine if a chimney is simply too short for fire safety and code compliance. The top of the flue should be at or above the height of a horizontal line draw from the chimney to the point at which that line touches the roof surface. Or the chimney should be at least two feet above the ridge. This photo is slightly incorrect (I didn't want to lose my tape down the flue while taking the picture). Measure from the edge of the chimney flue closest to the roof, horizontally, until you touch the roof surface to get this clearance distance.

Check the chimney top for damaged masonry (or rusted metal), a missing cap, damaged, craked, or missing top seal or crown on the top of a masonry flue, and here, an important discovery (at least in some jursidictions) is whether or not the chimney is single wythe or thicker masonry and whether or not the chimney has (or perhaps needs) a chimney liner.

Our photo at our "Introduction" and repeated here shows a chimney with a cracked, damaged, leaky top seal and missing rain cap on the flue.

Our photo at the very top of this page and repeated here shows an unlined chimney flue which, by luck of illumination from our flash camera, also showed us that a common flue was being used to vent two fireplaces - a fire code violation in many jurisdictions.

Inspecting chimney flue interiors using remote cameras and lighting

Peering up into a chimney flue from below or down into a flue from a rooftop access point, even with a bright light, is pretty much staring into a black abyss. It is difficult to see all of the interior chimney surfaces. Quite a few chimney cleaning companies offer a camera service which is in essence, a fisheye lens attached to a video camera which is lowered through the flue.

Chimney inspection cameras permit a more careful inspection of the condition of the entire flue interior. Especially on older masonry flues of unknown condition, this step can be quite valuable in assessing the safety and security of the flue against fire, spark, or gas leaks.

Inspecting chimneys from inside a building

Interior basement and attic inspections will reveal corrosion at the cleanout door, connector problems and flashing leaks.

Inspect exterior the chimney while you are inside the building. Attic access to a chimney or basement access, may provide key inspection points since there the chimney is likely to be directly accessible and there may be cleanout or other access ports to check the condition of the chimney interior. In this photo the exterior of this chimney, visible in an attic, was in very poor condition, with soft spalling bricks. Worse, there was an immediate fire and gas hazard since there were actual openings at the base of this chimney where it passes through the attic floor framing. This is a serious fire hazard and a flue gas leak hazard. This flue is unsafe. Inspect the entire route of the chimney through all building areas. Even if the chimney is not directly visibile you may find evidence of leaks, movement, or old chimney penetrations, say for a woodstove, that may not have been safely closed.

Missing chimney bricks: this photo shows a homeowner "repair" involving putting aluminum sheet metal over a hole in their chimney to make it "safe" - removing the metal disclosed a nice hole where bricks were missing. We asked the homeowner where s/he thought those bricks had gone? This was an unsafe chimney. The second photo shows the chimney hole with the sheet metal removed. The bricks had fallen down and blocked this flue, creating a carbon monoxide hazard.

Chimney Movement detected indoors in masonry chimneys can cause damage to the flue liner or openings that might admit sparks (fire risk) or gases (carbon monoxide hazards) into the building structure or even into occupied spaces. The photograph here shows a leaning, corbelled brick chimney in a pre-1900 home. When inspecting an old chimney which was corbelled over at an angle to come out at the center of the building roof's ridge (done for aesthetic reasons) inspect the chimney carefully at the attic floor where the chimney changes from angled (corbelled) to vertical. If the corbelled flue has sagged along with the roof, the chimney may be cracked and open at this point even though other cracks were not seen.

Abandoned chimneys in an attic can be a big surprise. We often wonder what's holding up all this weight. Someone may have eliminated a fireplace or an entire chimney on the lower floors, but neglected to remove the chimney from the attic out through the roof, perhaps because they didn't want to repair the ensuing hole in the roof left if the chimney were removed. Point loads from unanticipated weight or even a sudden collapse can be a real hazard if chimney bricks suddenly come through an upper floor bedroom ceiling.

Inspect the chimney interior

Missing chimney cleanout doors: In this photo the chimney cleanout is easy to find but isn't that door missing? A missing chimney cleanout door means that there is a fire safety hazard (sparks or ashes falling out onto the basement floor) and also that it is impossible to control the draft in the flue. I've found heating boilers with draft problems and "repair attempts" like removing the boiler's barometric damper, when all the while the root problem of bad chimney draft was that the cleanout door was open or missing. Where you can find a chimney cleanout or can remove a flue vent connector to look into a flue, use of a mirror inside the flue to fiew the flue from below. This is by no means a thorough and accurate view of all of the chimney flue interior. But if the chimney was constructed as a straight flue up through the building roof, you should see daylight - otherwise the flue has become blocked, perhaps with nesting animals or collapsing debris.

Metal chimney flue liners have to be inspected by removing the vent connector from the chimney breach to check for corrosion. Sight up the liner with a mirror to check straightness and for blockage.

Chimney Inspection Tips and Tricks

A good field investigator uses all available information in assessing the condition of building components, not just the obvious. Here are a few indirect observations that might be important clues about hidden chimney safety defects at a property:

• Dead-end flues, chimneys whose flue stops right at the point of entry of a thimble for a woodstove or heating appliance are inherently more dangerous than a conventional flue which extends several feet further past the thimble. Dead end flues are quickly and easily blocked by any debris that might fall down a chimney. If a chimneys venting a gas-fired appliance becomes partly or totally blocked, the applicance is likely to rapidly produce very dangerous, potentially fatal carbon monoxide (CO). Be sure that chimneys of this type are inspected and cleaned annually and be sure that you have working CO detectors as well as smoke detectors in the building.
• Masonry fragments: if you open a chimney cleanout-door or pull the flue vent connector from a dead end chimney flue, in either case you are looking at the very bottom of the chimney flue. If you are inspecting a masonry chimney, be alert for discovery of masonry fragments when inspecting or cleaning the bottom of the flue. If a piece of brick, masonry block, concrete, or clay flue tile liner is pulled out of your chimney, ask "where do you suppose this came from?" If the masonry scrap fell during construction of the chimney it may mean nothing. But if it fell because the chimney has been damaged, perhaps by water, frost, or during cleaning, then you probably have an unsafe chimney flue - more investigation is in order, promptly. Certainly if you believe that the masonry scrap found at the bottom of a flue fell during original construction, and if you remove it during cleaning, you should never find another piece in the flue bottom. If you do, the flue has been damaged anew and it is unsafe.
• Chimney leak history: leak stains inside the building attic or on any floor, if they are traceable to leaks at a chimney, are reason to be concerned for possible water damage to the chimney as well as to the building. Further investigation may be warranted.
• Chimney cap history: if a chimney has spent part of its life with no cap installed, there is extra risk of water damage to the flue interior. In a masonry chimmey damage may appear as frost cracking of the upper flue liners or masonry. In any chimney, there may also be water damage to the heating appliances being vented by that chimney.

More Problems with Chimneys

Moisture is the major cause of chimney corrosion and disintegration in both masonry and metal chimneys. The flue gases are acidic in nature and if allowed to condense and saturate the masonry or joints in metal flues the destructive results will soon be apparent. Mechanical problems in the construction or settlement after construction and overfiring or flue fires will also contribute to the problems.

External moisture enters the chimney through cracked caps, porous masonry, poor mortar joints and improperly designed and installed roof flashings. Internal moisture (condensation) collects in cracked or separated flue tiles, blocked flues and chimney caps.

Masonry chimneys subjected to moisture damage can have efflorescent salt stains, spalled bricks, eroded mortar joints, flaked cracks in the ceramic flue liner and cracked caps.

Metal components of a vent system can have rust and white acid stains at joints, corrosion holes along the bottom of horizontal connectors and corroded chimney cleanout doors at the base of the flue.

A broken or short thimble can allow combustion gases to rise in the air space between the flue liner and the masonry surround. Condensation stains will often appear in the mortar joints and as streaks running down the exterior face of the chimney. Wood or oil burners leave soot that leaches out as black streaks.

Chimney Cleaning Advice

Cleaning chimneys by arbitrary rule of thumb such as "once every 50 fires in the fireplace" is better than never cleaning your chimney, but in fact what actually determines the need for cleaning is how clean or dirty the chimney flue is.

If you burn a lot of fires, but in particular, if you are burning fires using green wood, or slow cool fires, or worst, slow cool small fires with green wood, you can clog up the chimney with creosote which, later when it catches on fire, can burn the whole house down.

• never use a chimney for any purpose if its safety condition is unknown. Serious fire or carbon monoxide hazards could be present
• look up into the flue myself with a bright light to see if I could see thick coated-on creosote or debris
• pay for a thorough cleaning, I would ask the cleaner how clean or dirty the flue actually was (they may fib about it),
• pay for a thorough inspection using their "chimscan" or similar device which is basically a fisheye camera on a string. Using the chimscan can tell you more than just how clean the flue is, it'll tell you if it has been damaged in some way that could make it unsafe, such as cracked or missing flue tile liners or leaks into the interior of the building.

Once you've done this thorough job, future cleanings can be scheduled based on how many fires you've had, more often if your're burning green wood, and before use at all if you see clogging in the chimney or thick creosote therein on your own inspection.

Also don't forget to inspect the flue before lighting the first fire of the season- since an animal or insects could have obstructed the flue over the summer. If you have a properly screened cap on your chimney top you'll keep out the animals but not the insects.

Keep me posted, take photos of the whole cleaning process, send along notes of what happened and what you learned, and I can use that info to update and improve my chimney article online - full credit given to contributors.

Repair of Chimneys - How to Make Safe Chimney Repairs

Masonry chimneys without flue tiles must be lined before a new appliance can be installed. Typically, a metal type liner is inserted into the flue space and secured with various design techniques ranging from a cap plate at the top to pressure grouting between the masonry surround and the new liner. Poured cement or a two step cement fill and ceramic glaze can be used.

Spalled brickwork can be replaced brick by brick or the chimney can be rebuilt after tearing it down to a sound level. This allows for new flue tiles to be installed if needed.

Metal components can be replaced and single-wall flue connectors, if corroded on the bottom of horizontal sections, can be replaced with Type B or L flues which will maintain the flue gas temperature and minimize condensation.

Selection of the liner system depends on the configuration of the flue. Straight flues are not difficult whereas offset flues will require a flexible liner system or the removing of brick work at the offsets so that angle fittings can be installed. Both flexible and single wall rigid metal liners can be insulated to further avoid condensation. This is very important in cold climates and for high chimneys.

Responsibility of an ASHI Home Inspectors Regarding Chimney Inspections

The ASHI Standards require reporting on the exterior condition of a chimney, the cap, the flue, the roof flashings and the vent connectors from the appliances.

This basic visual inspection can be expanded to provide a more complete understanding of the components with concurrent recommendations to the client for the necessary repairs. The author's check list for a chimney inspection includes the following items:

1. Exterior materials for the full height
2. Cap
3. Flue
4. Roof Flashings
5. Interior materials in attic and basement
6. Cleanout Doors
7. Vent Connectors
8. Abandoned vent closures
9. Clearance to combustibles
10. Mirror view inside flue when possible

Appropriate repairs are recommended based upon the items presented in this paper. The inspector has to be very careful about how he/she states the reason for the recommendation since we are seldom code authorities or trades people and do not want to open the door to a liability action if an improper or delayed repair is made.

More Information on Chimney Safety, Chimney Inspection, Chimney Testing, & Building Inspections

• Chimneys, flues, wood stoves & fireplaces: chimney inspection, defects, safety concerns
• U.S. vs. Canadian Unlined Masonry Chimney Flue Requirements & Case Study
• Metal Chimney Safety Alert - CPSC Document #5047.
• Chimney Safety Alert for Wood Burning Appliances - US CPSC Alert Document 5017, wood stoves, fireplace inserts
• "Chimneys and Flues materials, safety standards, requirements," D. Cohen, ASHI Technical Journal, with edits, updates, and additional text by Daniel Friedman.
• NFPA 211 - 3-1.10 - Relining
• NFPA 211 - 3-2 - Construction of Masonry Chimneys
• NFPA 211 - 3-3 - Termination Height
• NFPA 211 - 3-4 - Clearance from Combustible Material
• NFPA 54 - 7-1 - Venting of Equipment
• Brick Institute of America - Flashing Chimneys
  Brick Institute of America - Proper Chimney Crowns
  Brick Institute of America - Moisture Resistance of Brick
• American Gas Association - New Vent Sizing Tables, April, 1992
• Chimney Safety Institute of America - Chimney Fires: Causes, Effects, Evaluation
• National Chimney Sweep Guild - Yellow Pages of Suppliers
• SaverSystems, Div. of Meredith - Water Repellent Technologies

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