Photograph of  damaged single wythe brick chimney flue, owner repair by propped up board and alumnum, serious carbon monoxide hazards in the home, heated by a gas fired boiler Chimneys and Flues Home Page
Chimney Construction, Inspection, Diagnosis, Safety & Repairs

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Chimney, Fireplace & Woodstove information home page:

This article series on chimneys, flues, and vents and on fireplaces & woodstoves explains how to inspect & detect defects and hazards in these heating systems & components. We also provide chimney & fireplace repair advice along with chimney repair alternatives such as relining vs. replacement.

Chimney and flue safety hazards such as carbon monoxide gas leaks, fire hazards, and chimney inspection and testing are addressed.

Guide to chimney inspections: this series of detailed chimney articles provides detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects.

Chimney inspection methods and chimney repair methods are also discussed.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Chimneys and Flues: Construction & Inspection Methods, Diagnosis, Safety, Repairs

Blocked chimney found venting into attic (C) Daniel FriedmanIn our page top photo, a single wythe brick flue had lost some of its bricks. The owner "fixed" this condition by propping a board against a piece of aluminum flashing to cover the hole in the chimney.

At left we show what the chimney looked like when we moved the propped-up aluminum flashing: this chimney was venting dangerous flue gases into the building, the under-side of the roof decking was charred, and the chimney top itself had been blocked by a piece of slate.

This chimney inspection article series describes and illustrates chimney inspection procedures and critical chimney defects which can be observed from outdoors at ground level, from the rooftop, from inside the building, and finally, by inspecting the interior of the chimney flue itself.

Where do you think the missing bricks were?

They were not on the attic floor. I had been called to the building to investigate a basement water entry problem.

This is what we found: a single wythe brick chimney in a tall two story house with full basement. The water on basement walls was condensate from flue gas spillage from the gas fired heating boiler whose flue was totally blocked by the fallen bricks and debris.

This was a serious carbon monoxide hazard and a dangerous chimney.

The details of this case are presented at UNLINED FLUE INSPECTIONS. Our photo at left shows a client joining us in beginning to investigate the separation of a chimney from the house gable end wall.

In the photo just below the client is considering the amount of separation between the three-sided brick chimney and the gable end wall of the home. Even small signs of movement in a chimney can indicate a dangerous condition as movement, particularly in masonry flues, can cause cracks that present fire hazards or that can leak potentially fatal flue gases into the building.

Article Contents

A complete index to chimney construction, inspection, diagnosis & repair articles is found at ARTICLE INDEX to CHIMNEYS & FLUES.

Inspection, Troubleshooting & Repair Procedures for Chimneys and Flues

Single wall  metal fireplace chimney, too short © D Friedman at

We begin with some basic definitions of chimneys and chimney types, followed by an outdoor chimney inspection beginning at ground level. the detection of chimney movement, its causes, its symptoms.

Our metal chimney photo (left) illustrates a single-wall 1960's vintage fireplace chimney that was set into a solid masonry fireplace.

But this installation is no longer safe to use - it's too close to the second floor addition wall, and too short.

A chimney must be moisture and gas tight and convey the products of combustion to the outside atmosphere.

It sounds like a simple job, but it may not be simple, nor inexpensive.

Because chimneys are exposed to weather, high and low temperatures, water and moisture, squirrels, nest-building hornets and birds, corrosive gases, occasional incompetent builders or installers, combustible products, potentially fatal flue gases, and even sparks that could cause a fire, and because proper venting of flue gases is necessary for safe and efficient heating system operation, a periodic and careful chimney inspection is important.

Question: My neighbors noticed sparks coming from my chimney.

(Jan 17, 2018) Anonymous said:

My neighbors noticed sparks coming from my chimney. I have an oil boiler. I assume this is not normal. Any ideas?

This Q&A were posted originally at OIL BURNER NOISE SMOKE ODORS


(Jan 17, 2018) danjoefriedman (mod) said:

I agree, Anon, that sparks coming from an oil burner flue ( chimney) is not normal and perhaps unsafe, risking a chimney fire, as well as loss of heat.

Your heating service tech (whom you should call right now) will check burner operation and she may need you to have the chimney inspected and cleaned - depending on where the problem is found to lie.

Watch out: If you suspect a fire, such as a chimney fire (roaring noise, plumes of smoke) turn off the heat, get out of the house, and call the fire department.

What makes for a reliable chimney inspection for safety and function?

A combination of these three things:

  1. A chimney inspection procedure. Having an inspection procedure helps assure that no critical topic or chimney condition is ignored
  2. Chimney & Venting Alertness: the inspector cannot rely on a checklist or procedure to guarantee that a chimney inspection is complete and accurate. While checklists and procedures help this process, no checklist and no procedure can list every possible chimney defect or unsafe condition.

    Alertness includes attending to construction methods and materials or site conditions that might raise a concern about a chimney and might justify additional investigation, even when no problem is immediately visible.
  3. Good knowledge of chimney construction, safety, venting requirements, various chimney materials and construction methods and what tends to go wrong with each method. This helps the inspector to recognize a chimney defect or safety hazard even if seeing it for the first time.
Photograph of a cracked chimney top crown and no cap installed.

This chimney information home page catalogs and gives details of chimney problems that can be observed from outside and from inside of a building. 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 the way chimneys are used, including changes from coal, wood, or oil heat to natural gas adds still more chimney problems, some of which are dangerous.

Outside, by attending to even minor clues we might detect improper or unsafe heating system operation, collapse hazards, building leaks and water damage, and other concerns.

Of these, most critical and life threatening problem for which a home inspector or homeowner should be alert is the risk of leaks of combustion products into the dwelling - gases which could contain the sparks of a fire or the threat of fatal carbon monoxide poisoning.

The photo at above left shows a cracked chimney top seal or "crown" and also that there is no chimney cap installed. Notice my green 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.

See the individual chimney inspection procedures and defects described at CHIMNEY INSPECTION from GROUND and at CHIMNEY INSPECTION at ROOFTOP.

Inside the building we may spot abandoned chimneys, unsafe chimneys, fire hazards and flue gas hazards. During the chimney inspection we may see little clues which point to potentially serious concerns, like broken clay flue tile liner parts in the bottom of a chimney cleanout opening.


Chimney Components & Definitions

Photograph of a damaged masonry chimney.

When do Chimney Codes Apply to Existing Chimneys?

NFPA 54, the National Fuel Gas Code, recommends that when a new appliance is retrofitted 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.

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

A masonry chimney needs to have a sound foundation to prevent settling and movement, and must be soundly constructed so as not to leak combustion gases as well as to prevent setting the building on fire.


In Carson Dunlop Associates' sketch just below, the lower end of a masonry chimney structure is visible at the left of the photo.

Definition of Flue or Chimney Flue: A pipe or shaft for the passage of smoke, hot air and gas in a chimney. A single masonry chimney may contain more than one flue.

[Click to enlarge any image]

Flue vent into chimney flue blocked by debris(C) Carson Dunlop AssociatesThere are important safety regulations about the construction, separation, and use of chimney flues within a chimney.

In general chimney flues are not shared among devices on different floors, and sharing of flues among devices on a single floor is limited to certain heating equipment combinations.

See CHIMNEY INSPECTION, FLUE INTERIOR for details. Also see our Q&A below that defines chimney flue again and explains flutes that appear on pipe organs - nothing to do with heating appliance chimneys.

Carson Dunlop Associates' sketch at left shows a horizontal flue vent connector inserted into the chimney flue. The flue or chimney interior is shown in cross-section. In this example debris has clogged and blocked the chimney - producing a dangerous condition.

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

For example, the thin-walled metal pipe, typically 6" in diameter or larger and used to connect an oil-fired heating boiler or a gas-fired furnace to a metal or masonry chimney is properly called the flue vent connector. Lots of people call this component the "flue pipe" or "stack pipe".

There are important safety regulations about the components, installation, fire clearances, and fire ratings of flue vent connectors and their component parts. See FLUE VENT CONNECTORS, HEATING EQUIPMENT for details about the inspection, installation, hazards, and repairs of flue vent connectors.

Chimney Types: Summary

A detailed guide to different types of chimney types and materials is found at CHIMNEY TYPES & MATERIALS. We can describe at least the following categories of chimneys and flues:

The articles in this series also describe types of chimney defects, approaches to chimney construction, chimney inspection methods and fire or safety hazards associated with chimney defects such as cracks, movement, leaks, fire clearance hazards, obstructions, or inadequate draft. Chimney defects and hazards also can be categorized by chimney fuel, construction, location, and materials.

Inside Chimneys

All chimneys whose construction is entirely internal to the building structure up to the roof line are considered inside chimneys.

Outside Chimneys

Chimneys with three walls exposed to the outdoors 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, but a type B-vent or other metal chimney passing through an exterior wall chimney chase would also be considered an outside chimney.

Outside chimneys, because they are exposed to colder temperatures than a chimney that passes through the building interior, may have different draft and performance properties and in some circumstances may not provide adequate draft. We discuss examples of this concern at CREOSOTE FIRE HAZARDS.

Three Sided Chimneys

A three-sided or "three walled chimney" is one which does not provide full masonry thickness or fire protection around all sides of the chimney flue.

Three sided chimneys can be a very serious fire risk because the chimney has been built close to or even directly against combustible building materials without the necessary fire clearance and masonry fire protection needed. See Three-Sided Chimneys: Outdoors for details.

The Functions of a Chimney Wall

Unsupported chimney (C) Daniel FriedmanThe chimney wall has two primary functions: structural and draft inducing or thermal performance. Masonry chimney 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.

Masonry chimneys must be fully self supporting. See BRACKET CHIMNEYS below for an example of chimneys that are not self-supporting.Our photo (left) shows an unsupported chimney in the top floor of a pre-1900 home.

This chimney has it all (bad): the masonry chimney rests on floorboards between floor joists - it does not support its own weight.

The chimney is cracked, damaged, and has evidence of a fire. There are other defects as well. Notice the glass chemical fire extinguisher hanging from the ceiling? Will that be effective against a fire at the chimney? (And are its contents toxic?)

Abandoned chimneys that have been partially removed may also be structures that are no longer self supporting. Surprising to some people is the discover that the lower portion of an internal masonry chimney has been removed in a building, leaving the inadequately-supported weight of remaining chimney sections in an attic or on upper building floors.


Choices for Re-Lining Masonry Chimney Flues

This article will discuss when and why chimney flues are re-lined and lists a few of the chimney repair or relining alternatives.

Selection of the chimney 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. See RE-LINING CHOICES for MASONRY CHIMNEYS for details.

Damaged Chimney flues: cracks, holes, spalling

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. Cracks need to be evaluated to identify the cause - a crack may indicate serious chimney movement, structural damage, risk of collapse, flue gas and fire hazards, or improper construction leading to thermal cracking.


Metal Chimney Component Replacement

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.


Critical Chimney, Vent, & Flue Defects

Metal flue clearance requirements (C) Carson Dunlop AssociatesLife Safety Hazards at Chimneys such as visual evidence of unsafe chimneys, fire hazards, flue gas and carbon monoxide hazards, missing or damaged safety devices (relief valves, emergency shutoffs), and visual evidence of dangerous overheating or leaks. Life safety hazards also include collapsing chimneys.

Responsibly conducted, an inspection of a gas-fired furnace, for example, might discover that the furnace has been exposed to severe rust-producing conditions which risk an unsafe heat exchanger and a carbon monoxide hazard which could be fatal to building occupants.

Even though the interior of a chimney or flue is not fully visible, the contextual clues around the chimney, if they are visually obvious, should be translated into a level of concern by the inspector, and where appropriate, translated into a recommendation for action.

This does not mean "failing" every chimney to be on the "safe side", it means responsible inspecting and reporting.

Watch out: at UNLINED FLUE INSPECTIONS we describe a scary story that happened when we identified an unsafe chimney and advised our client to have a chimney expert inspect and repair the condition. She called a chimney company listed in her local telephone book.

But the "expert" was so poorly informed about chimney safety that his "repairs" came close to killing the occupants of the house. See Chimney Cleaning Advice, Procedures.

Chimney Scam Warnings - Consumer Warning Notice

Watch out for consumer fraud and scam operations that promise low-priced specials on chimney inspection, cleaning, re-lining, or repairs.

Readers have informed us of a variety of common chimney rip offs involving professional criminals who combine information about new home buyers and local business names with a telephone promise of various chimney services such as chimney cleaning for $39.95.

Homeowners attracted to this chimney deal may encounter scammers who arrive with a ladder, take a superficial look at a chimney, and claim that the chimney is unsafe, needs re-lining, or other treatment.

Details about chimney repair frauds and chimney cleaning scams are at CHIMNEY CLEANING FRAUD

Index to Chimney Inspection & Troubleshooting Articles

Here is an alphabetical list of chimney codes, chimney construction, chimney cleaning, chimney diagnosis & chimney repair articles found at

Our complete index to chimney & flue construction, inspection, diagnosis & repair is at ARTICLE INDEX to CHIMNEYS & FLUES.

Also try the SEARCH box found at the upper right of each of our pages or at the end of each article.

Sorting Out Names of Chimney Parts

See these articles defining parts of chimneys:


Continue reading at CHIMNEY TYPES & MATERIALS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see these

Chimney Inspection Articles

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