Photograph of a cracked chimney top crown and no cap installed. Chimney Types & Materials - an Inspection Guide
     


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Guide to types of chimneys found on buildings: this article names and describes the various types of chimneys used on buildings. Readers may also want to see the basic chimney definitions at CHIMNEY DEFINITIONS.

These articles on chimneys and chimney safety provide 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. Our page top photo shows collapsing brick masonry chimneys taken by the author in Los Angeles, CA following the Northridge Meadows earthquake in 1994.

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Chimney Types and Chimney Construction Materials

Description & Performance Specifications of Types of Masonry and Clay Tile Chimneys

Photograph of a cracked chimney top crown and no cap installed.Masonry chimneys (brick, masonry block, concrete block, cinderblock, concrete or stone 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.

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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.

Below we define the basic types of chimneys used to vent various types of heating appliances and devices. For more detail about each chimney type, construction, inspection, fire clearances, rooftop height requirements, etc. See the links listed at the "More Reading" links at the bottom of this article or at the end of this section of text.

Definitions & Specifications of Types of Factory Built Chimneys

Collapsing metalbestos insulated chimney (C) Daniel FriedmanSeveral types of factory manufactured chimneys are in use for residential situations.

They are a Pre cast 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.

List of Manufactured Metal Chimney Types

Here are links to detailed articles about each of the principal types of manufactured metal chimneys. We name each of these types in text below this list, and discuss each of these in the articles listed. Readers may also want to see the basic chimney definitions at CHIMNEY DEFINITIONS.

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. See FIRE CLEARANCES, FLUE VENT CONNECTORS.

Wood framed chimney chase surrounding factory built metal chimneys (C) Daniel FriedmanDouble-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. See TYPE B-VENT CHIMNEYS and TYPE L VENT CHIMNEYS.

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.]

See PLASTIC PLEXVENT ULTRAVENT RECALL and PLASTIC VENTS Goodman HTPV RECALL.

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.

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 dew point and thus reduces chances of corrosion.

The dew point 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 dew point 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 dew point 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.

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