Information Sheet: Negative Pressure

What is negative pressure?

The term negative pressure is used to describe an environment that is found to have insufficient air flow to allow the correct operation of mechanical extraction devices. When this occurs there is a danger that the mechanical extraction device may draw flue gasses down an open flue and back into the property. According to Energy Safe Victoria this "means that dangerous gasses, such as carbon monoxide, can be drawn into living spaces via the gas appliance flue or chimney."

Watch the below video to learn more about negative pressure


How do gas fitters test for negative pressure?

When the gas fitter attends a property that has an open flued gas appliance they are legally obligated to test for negative pressure. This is carried out by simulating a scenario that is most likely to identify the presence of negative pressure. 

The gas fitter will start by closing all the external doors and windows in the property. All extractions devices such as range hoods and exhaust fans are then turned on. Internal doors to rooms that contain extraction devices are left open. The gas fitter will then conduct a 'smoke test' adjacent to the appliance.

In order to pass the test the smoke created needs to travel upwards or towards the flue system in a steady upwards stream. If the smoke draws away from the appliance and towards the extraction devices, then it is determined that the property is prone to negative pressure and additional ventilation is required. 

How is negative pressure rectified?

Sufficient ventilation is required to allow the pressure in the property to be equalised. The gas fitter will determine the size & location of the ventilation required and a quotation for the required work is then issued. 

Alternatively, the appliance can be replaced with a room sealed or power flued appliance that is not affected by negative pressure.

What are the property owner's obligations when negative pressure is found?

Each time negative pressure is found, the gas fitter completing the service has a legal obligation to report the finding to the Victorian Builders Authority (VBA). The VBA will then keep these properties on file and follow up with the owner to ensure the correct steps have been taken to have the problem rectified.


Related articles & additional resources:

Negative pressure environment

Fact sheet: Negative pressure

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