Rank: Forum user 
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Looking for advice on whether there should be a detector unit on each floor of an open three floor service riser.
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Rank: Super forum user 
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It depends on what you want the detection to do. That might sound stupid, but there are broadly two main categories of fire detection
'L' (Life safety) systems which aim to raise the alarm to warn occupants in enough time that they will be able to move to a place of safety
'P' (Property protection) which - as it says on the tin- is for building/asset protection
Generally, P systems have more coverage/heads than L as L systems mostly need to maintain coverage for escape routes. But some L systems may require riser coverage in certain circumstances where the alarm needs to be raised as early as possible (Sleeping risks, vulnerable occupants with slow escape times etc).
You need advice from a competent person who has sight of the building plans, understands the premises' use and knows occupancy details. It's not wise to take advice 'blind' over the net
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Rank: Super forum user
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Scott
An open service riser means there is no fire stopping intended to prevent spread of fire or smoke. The biggest risk will be spread of smoke as the riser will act like a chimney, it has natural air currents already but hot gases and smoke will greatly increase dispersion. That said, smoke and gas will rise to the top of the riser first so a single smoke detector at the top of the riser would suffice although the maximum mounting height will vary for different detector types, 10.5 M for single point detectors, 25M for beam detectors, etc.
Hope that helps a little, Jonty.
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Rank: Forum user
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Agree with Jonty, one head at the top as hot air rises. If you have a sprinkler system in the building, then there should be a sprinkler head in the top of the riser shaft.
Peter
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