Open flue inspection and testing

Gas Safe Register’s Technical Helpdesk often receives questions from engineers about testing open flues. This would appear to be due to the declining number of conventionally flued appliances that are still in operation.

This article is written to support more recently qualified gas engineers who may not have had a lot of experience of testing open-flued appliances. But engineers with all levels of experience may benefit from refreshing themselves on the basics.

A flue flow and spillage check are imperative safety tests when checking the safety of open flued appliances.

Visual inspection
A flue test will always begin with a visual check:

  • Any ventilation requirements for the appliance being tested are in place, the vent is adequately sized, free from obstruction, sleeved through the wall and is not blocked in any way.
  • Check that the chimney serves only one room and is continuous throughout its length.
  • Check for restrictions such as flue dampers and, if applicable, check that the catchment space is correct.
  • Check that any flue pipe is installed in accordance with the flue pipe installation instructions, jointed correctly and is adequately supported throughout its length.
  • Check that the room in which the appliance is located is suitable for a conventionally flued appliance, ie, not a bathroom or bedroom. It is also important to remember that if the room has been converted to sleeping accommodation, whether temporary or permanent, the appliance must be suitable for that purpose. The appliance installation instructions must state that this is acceptable and you should follow the guidance in Technical Bulletin 105.
  • This is provided that all the relevant tests have been completed and have been deemed satisfactory. Please be mindful that fire and back boiler central heating units will be one of the main contenders in this area and you should refer to Technical Bulletin 105 for further guidance.

Flue flow test
Before you start any testing, prepare your site. Make sure that you can clearly see the flue terminal and the flue throughout its entire length. The test may involve going into the roof space and upper rooms of the property and, in some properties, there may be another property above the one in which you are working. If this property has the flue you are testing running through it, you will need to gain access wherever possible to complete a full visual inspection.

Visually check the chimney/flue for the correct material and look out for signs of damage and possible fume leakage. Visually check up inside the chimney/flue as far as is reasonably practicable.

Close all doors and windows into the room. Before checking with a full smoke pellet, it’s good practice to test with a smoke match to ensure you have a reasonable draw on the chimney.

If the smoke from the match doesn’t get fully pulled into the flue, you may need to warm the flue at this point using the appliance or a blow lamp.

A good measure would be five minutes of warm-up time on the chimney. After warming the flue, re-test using a smoke match. If the smoke pulls in, then continue with the flue flow check. If more heat is required, you may heat the flue for a further 10 minutes.

Once there is sufficient pull on the flue, place your smoke pellet on a fireproof tray and insert it into the flue space. The smoke pellet should be capable of producing 5m3 of smoke volume in a 30-second burn time.

Ignite the smoke pellet and visually check the chimney throughout its entire length. You don’t need to use a specific route but you must check all sections of the chimney. When carrying out this test, you should be looking for no significant escape of smoke from the appliance position, no seepage of smoke over the entire length of the chimney, and a discharge of smoke from only the correct terminal.

If the above requirements are not met, the chimney/flue has failed the test and no gas appliance should be left connected to the gas supply until further investigation has been carried out.

If all the above have been satisfied, you can proceed to reconnect the appliance and move on to the spillage test.

The spillage test
Close all windows and doors in the room containing the appliance to be tested. Close all adjustable ventilators and switch off mechanical ventilation unless it is mechanical combustion ventilation.

With the appliance running at its maximum heat input, allow the appliance to warm up for five minutes (or in accordance with the manufacturer’s instructions), but for no longer than 15 minutes in total. Check that the appliance clears the products of combustion. If spillage is detected, switch off the appliance, disconnect and rectify the fault.

Where the manufacturer’s instructions do not give specific guidance on testing for spillage, you can use the following procedure. Close all windows, doors and adjustable ventilators. Close all mechanical ventilation, apart from any that supplies combustion air to the appliance. With the appliance in operation at maximum heat input rate, carry out a spillage test.

Check by applying a smoke device to the edge of the draught diverter or gas fire canopy. This should be done within five minutes of lighting the appliance. Apart from an occasional wisp, which may be discounted, all the smoke should be drawn into the chimney and evacuated to the outside air.

If spillage occurs, leave the appliance in operation for a further 10 minutes and re-test. If spillage still occurs, switch off and disconnect the appliance and rectify the fault.

If there are fans elsewhere in the building, the tests must be completed with the interconnecting doors to all other rooms that contain fans open, but keep all external doors and windows closed.

If there are paddle fans in the room, or adjacent rooms, the tests must be completed with the fans in every speed and direction.

  • Check with kitchen cooker hoods switched on every speed.
  • Check normal extract fans switched on every speed.
  • Check with vented tumble dryers in operation.
  • Check with fan warm air heaters in operation.

If, at any time, an extract fan causes the spillage test to fail, you may open a window until the spillage stops. Measure the gap you have created: this should represent how much ventilation would be required to rectify the problem.

If a ceiling paddle fan is causing a spillage fault, then there is no cure for this scenario. You should follow the Gas Industry Unsafe Situations Procedure (IGEM/G/11) and, with the customer’s permission, terminate the gas fire, issue a gas warning notice and label until such time as the ceiling fan is removed.

Common chimney problems that cause poor flue performance

  • Poorly terminated chimneys
  • Several chimneys with different heights terminating on one chimney stack
  • Older-type terminals and chimney pots
  • Gaps within the builder’s opening
  • Pre-cast flue size (small cross-sectional area)
  • Pre-cast flue termination (high resistance type) open only on two sides not four sides
  • Twin-wall metal flue pipe located in colder areas such as roof spaces and outside the property.


Further reading

  • Technical Bulletin 105: Non room-sealed gas appliances located in sleeping accommodation.
  • BS 5440-1:2008 Flueing and ventilation for gas appliances of rated input not exceeding 70kW net (1st, 2nd and 3rd family gases) – Part 1: Specification for installation of gas appliances and for maintenance of chimneys.
  • BS5440-2:2009 Flueing and ventilation for gas appliances of rated input not exceeding 70kW net (1st, 2nd and 3rd family gases) – Part 2: Specification for installation of gas appliances and for maintenance of ventilation.
  • Combustion appliances and fuel storage systems: Approved Document J.

This article was originally published in the January/February 2024 issue of Registered Gas Engineer.

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