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WInter 2018 energysafe issue 51
05
Understanding the negative pressure risk
By Enzo Alfonsetti , Gas Technical Regulators Committee ( GTRC ) Chairman
All open flue gas appliances , by their very nature , draw air for combustion from the environment in which they are installed .
It is for this reason that the importance of adequate ventilation and the impact of the operation of exhaust systems , such as kitchen range hoods and bathroom exhaust fans , must be considered when installing or servicing an open flue gas appliance .
A negative pressure environment can result in make-up air along with combustion products being drawn into living spaces through the flue or chimney of an open flue gas appliance . The obvious safety implication is carbon monoxide ( CO ), which can be fatal .
Open flue gas space heaters with draught diverters
Efficient open flue gas space heaters carry the greatest risk in a negative pressure environment for the following reasons .
1 . A more efficient flued appliance will have lower flue gas temperatures , and hence , the buoyancy effect within the flue or chimney will more likely be compromised in a negative pressure environment .
2 . Usage patterns are such that these appliances are commonly operated for long periods of time , and in many cases left unattended e . g . leaving the heater operating overnight .
Type 1 open flue gas decorative effect appliances
Type 1 open flue gas decorative effect appliances are appliances without an enclosure that are designed to be installed in an existing fireplace with a chimney .
These appliances carry less risk than open flued gas space heaters with draught diverters in negative pressure environments for the following reasons .
1 . They require an oxygen depletion system ( ODS ) that will shut off the gas supply before the concentration of CO exceeds 200ppm and the oxygen concentration reduces to less than 15 %.
2 . They require permanent ventilation in accordance with AS / NZS5601.1 irrespective of the gas consumption of the appliance .
3 . They are an inefficient appliance so the buoyancy effect within the chimney will less likely be compromised in a negative pressure environment .
Type 2 open flued gas decorative effect appliances
Type 2 open flue gas decorative effect appliances are appliances with an enclosure designed for connection to a flue that vents the flue gases to outside .
These appliances carry less risk than open flue gas space heaters with draught diverters in negative pressure environments for the following reasons .
1 . These appliances are either supplied with a fabricated flue or the installation instructions indicate that a fabricated flue must be used when installed in a masonry chimney . Fabricated ( e . g . metal ) flues will heat quicker than masonry chimneys so the buoyancy effect within the flue will less likely be compromised in a negative pressure environment .
2 . They require permanent ventilation in accordance with AS / NZS5601.1 irrespective of the gas consumption of the appliance .
3 . They are an inefficient appliance so the buoyancy effect within the chimney will less likely be compromised in a negative pressure environment .
Indoor open flue continuous flow gas water heaters with draught diverters
Indoor open flue continuous flow gas water heaters with draught diverters carry less risk than open flue gas space heaters with draught diverters in negative pressure environments for the following reasons .
1 . They are prohibited from installation in confined spaces such as bedrooms , bathrooms , toilets and combined living / sleeping areas .
2 . Usage patterns are such that these appliances only operate when hot water is being drawn .
3 . They are installed with a fabricated ( e . g . metal ) flue , which will heat up quicker than masonry chimneys so the buoyancy effect within the flue will be compromised to a lesser degree in a negative pressure environment .
Indoor open flue storage flow water heaters with draught diverters
Indoor open flue storage gas water heaters with draught diverters carry less risk than open flue gas space heaters with draught diverters in negative pressure environments for the following reasons .
1 . Usage patterns are such that these appliances only operate when hot water is being drawn or when the water in the storage tank cools through heat loss .
2 . They are installed with a fabricated ( e . g . metal ) flue , which will heat up quicker than masonry chimneys so the buoyancy effect within the flue will be compromised to a lesser degree in a negative pressure environment .
Indoor open flue ducted heaters with draught diverters
Indoor open flue gas ducted heaters with draught diverters carry less risk than open flue gas space heaters with draught diverters in negative pressure environments for the following reasons .
1 . They are typically installed in roof spaces where there is little or no negative pressure .
2 . They are installed with a fabricated ( e . g . metal ) flue , which will heat up quicker than masonry chimneys so the buoyancy effect within the flue will be compromised to a lesser degree in a negative pressure environment .
Open flue gas ducted heaters with draught diverters designed for installation in cupboards can be affected by negative pressure environments , including negative pressure generated if return air plenums are not adequately sealed and separated from the appliance . At the time of writing this article , it is ESV ’ s understanding that there are no current certifications for open flue gas ducted heaters with draught diverters designed for installation in cupboards .
Check testing of open flue gas space heaters and Type 2 decorative effect appliances with draught diverters
As a result of recent incidents and taking into consideration the above mentioned risk factors , ESV considers that open flue gas space heaters with draught diverters , and to a lesser extent Type 2 decorative effect appliances , could pose a risk to public safety if non-compliant with Australian standards and if subjected to a negative pressure .
ESV has contacted all recognised certification bodies and has asked that a review be undertaken of all current certifications for open flue gas space heaters with draught diverters and Type 2 decorative effect appliances .
This review would consist of laboratory testing against relevant standards and in particular would focus on compliance with combustion requirements during downdraught , blocked flue and overload combustion testing .
The results of this testing will indicate to ESV whether there are any compliance issues with any currently certified open flue gas space heaters with draught diverters and Type 2 decorative effect appliances , and if so , whether any of the non-compliances could compromise public safety .
ESV will ensure that stakeholders and the general public are kept informed of any safety related outcomes that might arise .
Due to the incompatibility between open flued gas appliances and modern energy efficient housing caused by the lack of ventilation , ESV will be commissioning on behalf of the Gas Technical Regulators Committee ( GTRC ), a Regulatory Impact Statement ( RIS ) that will explore the future of various types of open flued appliances in modern energy efficient housing .
For more information on open flue gas heaters visit : www . esv . vic . gov . au