Blocked chimney found venting into attic (C) Daniel Friedman Chimney Inspection Procedure Summary
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  • CHIMNEY INSPECTION PROCEDURE SUMMARY - CONTENTS: How to inspect chimneys and flues - a detailed guide to all aspects of chimney inspection, diagnosis, safety, repair, and specifications for venting fireplaces and heating appliances
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Chimney inspection procedure summary:

This article summarizes a number of chimney inspection points as well as noting appropriate chimney codes for building owners and inspectors. The article includes links to more detailed chimney inspection procedures for each stage of a thorough building chimney inspection, troubleshooting, or repair procedure.

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.

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Inspecting Chimneys and Flues

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

- Donald V. Cohen, P.E., President, Imperial Building Inspection, Inc, Milwaukee, WI

An earlier version of this article was published in the ASHI Techincal Journal, January 1994


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.

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 & Chimney Component Definitions

Definition of 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.

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

Definition of 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

Chimnney Foundation or Support

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 firecode sheetrock or metal flashing if a firestop is required between floors.

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 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.

Chimney Cap

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.

Chimney Height Requirements

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:


  • Flat to 7/12 1.0 foot
  • 7/12 to 8/12 1.5 feet
  • 8/12 to 9/12 2.0 feet
  • 9/12 to 10/12 2.5 feet
  • 10/12 to 11/12 3.25 feet
  • 11/12 to 12/12 4.0 feet

* Continues to 21/12 pitch at 8.0 feet

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 Cleanout

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

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.

Chimney Fire stopping

Chimneys that pass through several floor levels of a home may be required to be firestopped at each penetration. Typical firestops are sheet metal or firecode sheetrock.

Chimney Types and Materials

Masonry and Clay Tile Chimney

Masonry chimneys for low heat (< 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 nonwater-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.

Factory Built Chimney

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 Chimney

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 Chimney

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.

Chimney Vent Connectors

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. 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.

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 Chimney

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

Chimney Codes and 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.
  • Categories 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.

Wettime and Corrosion in Chimneys

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. 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 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 Chimney 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 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.

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.

Visual Inspection of Chimneys

The condition of the top exterior section of the chimney and the exposed flue liner will generally reveal the soundness of the chimney and this can be seen from the ground level. A roof inspection allows inspecting the flue lining, cap and roof flashing. Interior basement and attic inspections will reveal corrosion at the cleanout door, connector problems and flashing leaks.

Use of a mirror inside the cleanout access will allow viewing the flue from below. Metal 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 staightness and for blockage.

Repair of Chimneys

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 Inspector when Inspecting Chimneys

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 tradespeople and do not want to open the door to a liability action if an improper or delayed repair is made.


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

Chimney Inspection Articles


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