Rules for Single-Wall Flue Pipe Assemblies

  • Minimum clearance from combustible material: 450 mm (18 in.).
  • The minimum clearance may be reduced by 50 percent to 225 mm (9 in.) if suitable shielding is installed either on the pipe or on the combustible surface.
  • Maximum overall length of straight pipe: 3 m
    (10 ft.)
  • Maximum unsupported horizontal length: 1 m
    (3 ft.).
  • Maximum number of 90-degree elbows: 2
  • Minimum upward slope towards the chimney:
    20 mm/m (1/4 in/ft.).
  • The crimped ends (male) of the sections must be oriented towards the appliance.
  • Each joint in the assembly must be fastened with at least three screws, including the connections at the appliance flue collar and chimney.
  • 6-, 7-, and 8-inch diameter flue pipes must be at least 24 gauge in thickness.
  • Galvanized flue pipes must not be used because the coatings vaporize at high temperatures and release dangerous gases. Use black painted flue pipes.
  • The assembly must have allowance for expansion: elbows in assemblies allow for expansion; straight assemblies should include an inspection wrap with one end unfastened, or a telescopic section.


Certified double-wall flue pipe systems are also available. These systems are tested to determine the minimum clearance at which they can be installed. The clearance information is found on the labels attached to the pipe and in the manufacturer's installation instructions. The rules for their installation may differ from the rules for single-wall flue pipes. The minimum installation clearances for certified double-wall flue pipes are much less than those for single-wall pipes. Also, the maximum length of a double-wall flue pipe assembly may be greater than is permitted for a single-wall pipe.

There are two general types of double-wall flue pipes: sealed and vented. A sealed double-wall flue pipe is effective at retaining the heat in the flue gases because the air space between the inner liner and outer shell acts as an insulator.

A sealed double-wall pipe is a good choice to maximize draft and minimize creosote deposits. Use sealed double-wall pipes if the assembly must be long or if the appliance is expected to produce low flue gas temperatures.

A vented double-wall pipe allows cooling air to pass between the inner and outer layers. Where the flue pipe assembly is very short and straight, a vented double-wall pipe can be acceptable. However, vented pipe is not a good choice for longer flue pipe assemblies or for appliances such as central heating appliances that tend to produce low flue gas temperatures.