Fireplaces, chimneys and flues

6.8.12Gas – outlets and terminals

Outlets and terminals shall be adequately separated from combustible material and other parts of the home, and prevent the ingress of damp.

Table 5: Minimum separation distances for gas outlets (mm)


1 An opening here means an openable element, such as an openable window, or a fixed opening, such as an air vent. However, in addition, the 1 outlet should not be nearer than 150mm (fanned draught) or 300mm (natural draught) to an opening into the building fabric formed for the purpose of accommodating a built-in element, such as a window frame.
2 Boundary as defined in paragraph 0.4. of Approved Document J: smaller separations to the boundary may be acceptable for appliances that 2 have been shown to operate safely with such separations from surfaces adjacent to, or opposite, the flue outlet.
3 Should not be used.
4 This dimension may be reduced to 75mm for appliances of up to 5kW input (net).

Where a flue outlet is not serving a balanced flue appliance, it should be:

  • situated at roof level, so that air can pass freely across it at all times
  • a minimum of 600mm from openings
  • fitted with a flue terminal where the flue diameter is less than 170mm (larger diameter flues should be fitted with a terminal where required by Building Regulations).

Precautions should be taken, where appropriate, to prevent damp penetration in accordance with the requirements for resistance to frost attack and weathering for solid fuel appliances (see Clause 6.8.6).

Balanced flues which bridge the cavity of an external wall should have a means of preventing moisture crossing the cavity, e.g. a moisture drip collar set in the centre of the cavity.