What is negative pressure?

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Safety advice for using open flued gas space heaters

Negative pressure can occur when there is not enough ventilation in a home and an exhaust fan or range hood is operating. If an open flued gas space heater is operating at the same time as an exhaust fan, it may cause the gas heater to malfunction.

An open flued heater draws air from the room to feed the fire and combustion products are then moved outside via a flue. However, inadequate ventilation and use of exhaust fans can draw carbon monoxide (and other exhaust gases) back into the room in certain circumstances.

The exhaust fan causes a slight reduction in air pressure within the room where the heater is running and can cause a partial or full reversal of the flow in the appliance’s flue, essentially drawing the products of combustion back into the room.

This may cause high levels of carbon monoxide to build up in the room. Carbon monoxide is a colourless, odourless gas which can cause serious health issues and in some cases, may be fatal.

With houses becoming more energy efficient and therefore airtight and increasing use of powerful exhaust fans, the circumstances leading to negative pressure are becoming more common.

negative pressure environment
negative pressure environment, by Gas

Simple steps to resolve negative pressure

If you have an open flued gas space heater, there are some simple steps you can take that will keep you safe:

  • Get your gas heater serviced by a licensed gas fitter or service agent: this should be done in accordance with the manufacturer’s instructions or at least every two years. If the appliance is over ten years old, it should be checked annually. Make sure the service includes a check for negative pressure.
  • Do not operate exhaust fans at the same time as your heater: your range hood, toilet or bathroom fan can create a negative pressure situation, where carbon monoxide is drawn into the room.
  • Ensure you have adequate ventilation: check that you have permanent ventilation openings in the room where the heater is used and that they are not blocked.
  • Do not leave your gas heater on for an extended period: ensure you turn the heater off when you don’t need it.
  • Install a carbon monoxide alarm: carbon monoxide alarms can be a useful back-up precaution, but should not be considered a substitute for proper installation and maintenance.
  • Never use outdoor heating appliances indoors: portable outdoor appliances, such as patio heaters and barbecues are ideal for use outdoors, however they can present a danger of carbon monoxide poisoning if used in areas with insufficient ventilation.


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