Flue Draught Testing Recap

At the time of commissioning combustion appliances, you are verifying that the installation is safe and performing correctly. The flue draw is an important element of effective combustion to meet the design output specification of the appliance. HETAS advise that a flue draught test is performed and a record of the result documented on either the appliance manufacturer’s commissioning form located in the appliance manufacturer installation instructions, or in the absence of, the HETAS commissioning form available through the HETAS shop. Appliance manufacturers often state in their installation instructions a minimum and a maximum permitted flue draught tolerance reading, normally this would be in Pascals (Pa) and is established during the CE type test for the product. At the point of any warranty issues or trouble shooting, providing the reading at the commissioning stage will indicate whether the flue is working adequately or not with the specified appliance installed. 

A flue draught reading under the prescribed range may inevitably cause fuel to not combust efficiently due to lack of air in the fire bed. Under continued operation like this, poor combustion will lead to increased and faster build up of deposits in the flue which (without intervention) will inevitably lead to an appliance spilling products of combustion into the room. A reading higher than the scale will create excess draw resulting in a fire that is uncontrollable and fuel wastage, which in turn may cause internal parts to warp and buckle and therefore unlikely to be covered under any manufacturer’s warranty. It is also important in ensuring Part L of the Building Regulations are met, which prescribe that appliances are appropriately commissioned to ensure they use no more fuel and power than is reasonable under operation.

It is becoming more common that appliances are now being built with an integrated test point within them to aid industry best practice. In the absence of a test-point on the appliance, the installer may be required to retrofit their own test point. A way of achieving this would be to install a connecting flue pipe with a sweeps access cover plate which can be removed and a spare cover plate which has a test point already installed (see photo below) can then be fixed in place to the customer’s access plate in order to undertake the test at commissioning or servicing stages. Once the test has been completed and documented, the plates can then be swapped back as before.

If a test point is to be added to a connecting flue pipe, then the flue manufacturer’s guidance should be referenced, to stop the invalidation of any warranties associated with the flue product. With twin wall insulated flues, a flue draught test point should only be installed at the time of manufacture, the same method of using a spare cover plate could be utilised for this type of flue. 

With inset appliances, the appliance manufacturers testing procedures should be sought.

In lieu of a dedicated flue draught test point, once a suitable location has been identified to undertake a reading, the location should be noted, so that the process can be reproduced at times of service and maintenance.

Procedure for taking flue draught reading

The typical procedure is as follows:

  1. Visually inspect the draught gauge for signs of damage and/or defect.
  2. Take a reading before the appliance has been lit.
  3. Ignite a small amount of fuel in the appliance and leave to burn for 20 minutes.
  4. Position the gauge on a level base or hang upright.
  5. Ensure that readings are zero without the gauge connected to a test point.
  6. Set the appliance controls to manufacturer’s requirements.
  7. Insert metal sampling tube into test point preventing secondary air entry.
  8. Once a warm flue has been established wait approximately 30 seconds for stabilisation (when using a mechanical tester) and then record the draught reading.
  9. Once the appliance has reached its optimal output another reading should be recorded.

Please Note: Atmospheric or weather conditions may cause incorrect and/or fluctuating draught readings.


Depending upon the draught reading obtained, further investigation may be required to any problems arising from the result. Any adjustment should be made in accordance with the appliance manufacturer’s instructions and the draught reading retaken after each adjustment.

If acceptable draught conditions cannot be achieved the appliance should be taken out of service and where appropriate either an uncommissioned label filled out and attached to the appliance or the HETAS Unsafe Situations Procedure should be followed whilst the fault is rectified.

Mechanical Extraction and Ventilation

If the appliance is located within a property where mechanical ventilation is present, a flue draught interference test shall be undertaken, to ensure the extract system is not having an adverse effect on the flue. It is not uncommon for modern high-volume extract fans in different parts of a property to affect the flue draught.

  1. Ensure all doors to the room and all external doors, windows and air vents designed to be closable are closed, and all ventilation fans are switched off. 
  2. Light the appliance and allow for the optimum operating temperature to be reached 
  3. The flue draught reading should be recorded and checked that it is in the parameters specified by the appliance manufacturer. 
  4. Open any internal doors interconnecting the room in which the appliance is installed to rooms where extraction fans are present within the property 
  5. Turn on all extract fans within the property to the maximum speed setting allowed 
  6. Run the extract systems for ten minutes, and then, record the flue draught reading. The reading should not fall below the parameters specified by the appliance manufacturer. 
  7. Once the tests confirm satisfactory operation, remove the test device and seal any apertures in the flue way if required.

Please Note: If at any stage during commissioning the flue draught reading taken differs from the draught parameters specified by the manufacturer or if parameters have not been provided, appliances tested to BSEN 13240 must have a minimum of 12Pa, action should be taken to locate the cause of the discrepancy and the installation rectified before proceeding with further testing.

The extraction test is a means to verify that the manufacturer’s required flue draught during operation is met under the relevant conditions. In some cases, spillage can still occur and so it is important to carry out the prescribed smoke spillage tests detailed.

Flue Draught Stabaliser

Where a draught stabiliser is deemed necessary, extra consideration should be taken in regards to permanent ventilation referenced in ADJ Table 1.

The draught stabiliser should be installed as close as practicable above the appliance outlet.

HETAS lists Flue Gas Analysers, along with their approval status and manufacturer’s remarks, click here to view the range.

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