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1. Scope of watermist systems in BS 8458

BS 8458 has a clearly defined scope:

‘recommendations for the design, installation, water supplies, commissioning, maintenance and testing of watermist systems with automatic nozzles installed in residential and domestic occupancies’.

It defines automatic nozzles as:

‘Automatic watermist nozzles operate at a pre-determined temperature to discharge water over a known area below. The flow of water thus initiated causes the sounding of an alarm. Only those watermist nozzles operate which are individually heated above their operating temperature by the heat from the fire.’

Electronically controlled nozzles can be included in the definition of automatic nozzles, as is the case in the American water mist standard NFPA 750:

' Electronically-Operated Automatic Water Mist Nozzles. Nozzles that are normally closed and operated by electrical energy that is initiated by fire detection and control equipment.'  

Key differences between Automist and a traditional watermist system:


Traditional watermist system



Glass bulb or fusible link
(mechanical thermal)

Electronic trigger based on smoke detection and thermal sensing

System design

Wet pipe (i.e. one that is permanently charged with water)

Non-pressurised (open) dry pipe

Nozzle location

Ceiling (or close to the ceiling) mounted

Wall mounted at 1.3m height

Discharge performance

The system should be capable of providing pressures and flow rates to permit all the watermist nozzles in the fire test room or largest compartment, whichever is the greater, up to a maximum area of operation (AMAO) of 64 m2, to operate simultaneously at not less than the greater of: a) the nozzle pressure given by the pass criteria determined by the test in Annex C if BS 8458; or b) the approval listed discharge performance.

The electronic activation ensures only one watermist nozzle activates with not less than the pressure and flow given by the pass criteria determined by the test in Annex C of BS 8458.

Minimum run time

10 minutes

30 minutes (connected to mains water)

Ceiling height

3.5m or 5.5m height depending on fire test results according to Annex C of BS 8458

3.5m height depending on fire test results according to Annex C of BS 8458. Higher ceiling heights, if supported by fire engineering justification

BS 8458 takes the form of a ‘Code of Practice’. As per BS-0:2021 9.4.1, it ‘contains recommendations and guidance, where the recommendations relevant to a given user have to be met in order to support a claim of compliance.’ ‘Users may also justify substitution of any of the recommendations in a code of practice with practices of equivalent or better outcome’. To learn more about our product's conformance to BS 8458 and details of any substitution from the code of practice please review page 14 of our Design Installation and Operation Manual (DIOM). Here we evidence and explain the improved performance obtained by substitution.

Read more - Does Automist comply with the requirements of BS 8458?