January, 2016
Inspection News and Views from the American Society of Home Inspectors

Metal Chimneys


In this article, we only scratch the surface on metal chimneys and we do not focus on related topics such as vent connectors, for example. Please keep in mind that there are many other considerations and topics related to venting combustion appliances.

Metal chimneys or vents perform the same function as masonry chimneys. The goal is to get the products of combustion out of the building quickly and safely. It is important to keep the exhaust temperatures high to minimize condensation.

Chimneys should be correctly sized and should be warm. The key to a successful chimney or vent is to have it sized correctly and to keep it as warm as possible. Typically, sizing is determined by the heating system manufacturer. Some chimneys have integral insulation to keep the flue gases warm and the exterior cool.

Is it a vent or a chimney? One common convention is to call it a vent if it serves an oil or gas appliance and to call it a chimney if it handles wood.

Types of Metal Chimneys and Vents
There are several types of metal chimneys and vents and they are generally characterized by the fuel that they exhaust.

Type B Vents
Gas appliances. Type B vents are designed for natural gas or propane appliances such as furnaces, boilers and water heaters. They are typically double-wall with an air space between. The inner wall may be aluminum or steel and the outer wall is typically galvanized steel. Typically, they run from their point of connection to a single-wall vent connector through the building, terminating above the roof with a rain cap.

Furnace vent connectors attached to a B vent chimney.

Not exterior, usually. These units are not designed to be exterior over their entire length unless they are specially approved for such an application. If you see this, you should verify approval or recommend further evaluation by a specialist.

350°F normal temperature. B vents typically see exhaust temperatures of 350°F. They are usually tested for up to roughly 500°F.

BW Vents. Oval vents used for wall furnaces may be referred to as B vents or BW vents. Their construction and function are the same as B vents. The oval shape allows them to fit inside a stud cavity.

Type C Vents
C vents are single-wall vents, most often used as vent connectors in residential construction. In some regions, people don’t use the term “C vents,” but refer to these vents simply as single-wall vents, flues or vent connectors. There are many considerations with respect to vent connectors that are beyond the scope of this article (for example, rust, support, slope, combustible clearances, materials, manifolding, connections, length and an excessive number of elbows).

Typical Type C vent used for a domestic hot water heater. Note: The streaking below the chimney cleanout may indicate problems.

Type L Vents
Gas or oil burners. Type L vents are used most often with oil burners, but they also are suitable for use with gas. In most cases, L vents are used with oil. An L vent is typically a double-wall vent with an air space between. The inner wall is typically stainless steel and the outer wall is galvanized. Type L vents normally see temperatures of less than 750°F. Type L vents are tested for temperatures up to 1000°F, which is a much higher temperature than the temperatures seen with B vents.

L vent used for an oil boiler. Note: Combustible materials may be too close to this furnace.

Indoor Use Only. B vents and L vents can be used indoors only unless they are labeled for exterior use.

Class A Vents or Chimneys
Double-wall insulated. Class A is a designation for chimneys designed for use with oil and some wood-burning appliances. They can be insulated double-wall chimneys with a stainless steel inner wall and a stainless steel, aluminum or galvanized outer wall. Various insulation materials are used in these chimneys.

Triple-wall uninsulated. Class A chimneys also can be triple-wall uninsulated chimneys that allow outdoor air to pass between the walls. This cooling air prevents overheating of the chimney and nearby combustibles.

Appliances must be approved for class A chimneys. These chimneys are typically acceptable only for use with the appliances with which they have been tested. A zero-clearance fireplace, for example, should only use a Class A chimney that has been approved for use with that specific fireplace. Class A chimneys should not be used with heating appliances like wood stoves, furnaces or boilers that are in continuous use.

Class A chimneys are rated for 1000°F continuous use and intermittent temperatures up to 1400°F.

A triple-wall metal chimney that allows cold air in and exhaust gases out.

Factory-Built Chimneys
Other names. Factory-built chimneys may be referred to as UL 103 or UL 103HT (HT stands for high temperature) or super chimneys. In Canada, they are also known as 650°C chimneys, or 629 chimneys, referencing the standards ULC S629.

One-inch to two-inch thick walls. These chimneys are designed for solid fuels and are heavier duty than class A chimneys. The total wall thickness can be one or two inches, with stainless steel often used as both outer and inner walls. They benefit from a stronger inner liner and more insulation.

Replaced class A chimneys. These chimneys are tested for up to 2100°F and are intended to operate at up to 1200°F continuously. In general, these are required for wood stoves and furnaces, as well as some factory-built fireplaces.

A factory-built chimney used with a wood stove. Note: Chimney flashing is improperly installed.

Multiple Appliances
B vents. B vents and L vents can serve multiple appliances using similar fuels. For example, several gas appliances can use a single type B vent. They can be on different stories within a building.

L vents. More than one oil-fired appliance can share an L vent, although they must be on the same floor level.

Wood and oil. For the most part, chimneys serving solid fuel (wood-burning) appliances each should have their own flue, although there are some exceptions for which a chimney serving a wood-burning stove also can serve an oil-burning appliance. Again, these have to be on the same floor level, and the oil-burning appliance vent connector must enter the chimney above the wood-burning appliance.

Rules Vary
Manufacturers’ recommendations vary widely and local practices also vary. You should find out who the authorities are in your area and speak to them about their requirements. Ask these people what defects they most often find in the field.

Furnaces and water heaters sharing the same metal chimney is very common.

Connections of Vent Pieces
Twist-lock or locking bands. Sections of metal vents and chimneys (usually 2 or 3 feet long) are mechanically attached either through a twist-lock mechanism (in effect, the sections are screwed into each other) or locking bands. Locking bands are considered more positive and commonly are used on newer systems. Either type of connection can work loose.

Some common problems with metal chimneys and vents:

  • Not labeled for application
  • Sections not well-secured
  • Chimney or vent not well-supported
  • Inadequate clearance from combustibles
  • Inadequate fire stopping
  • No cap, wrong cap or obstructed cap
  • Warped, buckled or twisted chimney walls
  • Rusted or pitting
  • Inadequate chimney height or size
  • Creosote buildup
  • Excessive offset from vertical
  • Not continuous through roof

In this article, we’ve introduced various types of metal chimneys and listed common problems associated with them. In the ASHI@HOME Training Program, we explain these conditions in detail, discussing causes, implications and strategies for inspection.

Thanks to Roger Hankey and Kevin O’Hornett for their valuable input for this article.