Flue and Chimney Testing

Testing applications existing flues

Flues in existing chimneys can suffer from obstructions and can also leak as a result of holes or cracks. The following should be carried out to check the state of the flue

  1. sweeping – this will free any obstructions and enable better visual inspection and testing. Any debris should be examined for lining or brick pieces, which would indicate a need for further repairs
  2. visual inspection – this should identify the following:
  • deterioration in the structure, connection or linings. This should include the interior of the flue and exterior of the chimney including the roof space. The presence of smoke or tar stains are signs of leaks and possible damage
  • modifications made to the flue such as a ventilator terminal which may be incompatible with the intended use
  • correct type and size of lining for the new application
  • checks to determine any restrictions, either visually (if full length visible) or by a coring ball test
  • operation and gas-tightness by carrying out a smoke test

New masonry and flueblock chimneys

  • operation and gas-tightness by carrying out a smoke test

New factory made metal chimneys

A visual inspection should be carried out following the checklist given in BS 7566-3:1992 (1998) Section 10 plus additional checks advised by the manufacturer. Following this a smoke test should be done to check gas-tightness.

Relined flues

All doors in the room should be closed. The flue should be warmed to establish a draught (eg by blowtorch or electric heater). A number of smoke pellets are placed at the base of the flue and ignited. When smoke starts to form the base should be sealed so smoke can only enter the flue (eg by boarding off and sealing the recess opening or closing all doors and vents to the appliance).

Smoke should be seen emitting from the flue outlet. When this occurs the top of the flue is sealed. The full length of the flue should be checked that there is no significant leakage over a minimum test period of 5 minutes.

  • flues for gas appliances

    All doors and windows should be closed. The flue should be warmed as above and a smoke pellet ignited at the base of the flue, or intended position of the appliance, so that smoke is drawn in by the rising draught. If the pellets are placed in a recess at the flue base, the opening between the recess and the room should be partially closed, such as with a board, to leave a bottom gap of about 25mm.

    Smoke should be seen emitting from the flue outlet and not to spill back into the room. There should be no significant leakage from the length of the chimney inside or outside the building.

Notes in relation to smoke testing:

  1. where the flue is to be warmed this may take more than 10 minutes for large or cold flues
  2. appliances, where fitted, should not be under fire when testing
  3. smoke should not emerge from any other flue as this indicates leakage between flues
  4. when checking for smoke leakage it should be remembered that smoke can emerge some distance from the original fault, and can, for example, occur at barge overhangs or window reveals
  5. smoke pellets create a pressure difference significantly higher than the pressure required in the product standards for natural draught chimneys and for flues having a gas-tightness designation of N1. Flues to this designation are permitted to have an air leakage rate of up to 2 litres/s/m2. Some leakage may therefore be seen during testing and it can be a matter of expert judgement of whether leakage indicates failure
  6. wisps of smoke visible on the outside of the chimney or near joints do not necessarily indicate a fault. However, if forceful plumes or large volumes of smoke are seen this could indicate a major fault such as an incorrectly made connection or damaged section of chimney, which will require further investigation and remedial action, followed by a repeat of the test.



The above guidance is not exhaustive and reference should also be made to the guidance given in.

Guidance Notes in PDF format Flue and Chimney Testing (PDF, 147KB)

Combustion Installation Application Form (PDF, 243KB)