On Sun, 25 Nov 2018 14:20:21 +0000, Robin wrote:
On 25/11/2018 13:14, Peter Parry wrote:
Unfortunately fire protection by combustion detectors is one of the
fields where too many people believe that more = better and fail to
understand that if there are too many (and too many is a small number)
nuisance alarms then occupants will both ignore and later disable the
alarms. "I've put lots of sensors in so its much safer" often really
means "I've put lots of sensors in so its much less safe but I've
created a really good illusion of safety".
I agree with the message in the para. above. but wonder if some might
take it as an argument against multiple detectors per se. Eg I took the
view that smoke detectors were best placed in the usual circulation
spaces which here gave 3 (above stairs, rear landing and downstairs
gall) and in rooms where electrical kit was routinely left on and
unattended which gave another 3. Most people think that's a lot in a 3
bed terrace house. The more so as 2 are only 2m apart. But there's a
closed door between them so I reckoned it was a price worth paying for
earlier warnings. And we've had perhaps 3 nuisance alarms in 10+ years
- and that includes those from neighbours' summer barbecues 
When you are doing the installation yourself how many detectors you
use is your choice and you can add or subtract them as you wish. In
rented properties or for non-DIY installs it isn't as easy (or as
cheap).
The official guidance is still based upon presumptions from decades
ago when the two primary causes of domestic fires were chip pans and
smokers. Chip pans were usually heated on a hob and when they caught
fire the occupant often picked the burning pan up and ran towards the
door with it. After a bit they realised this hurt a lot so dropped
the pan making sure the fire and fuel were widely spread. Open pans
full of hot oil are less common these days but fires in the kitchen
are still the leading cause of domestic fires. Smokers still occupy
second place but cause far less fatalities because of the rules on
furnishing materials. The usual cause of death was Dad coming home
from the pub, sitting on the settee to light his fag and falling
asleep allowing it to drop down the side of the sofa and igniting the
sofa materials. The cyanide fumes these fires produced usually killed
the smoker before they ever realised there was a fire.
It doesn't pay to rely too heavily on smoke alarms alone, currently
about 20% of mains powered alarms will fail to detect a fire
sufficiently serious for fire service assistance to be needed. For
battery alarms the figure is about 40% (the difference being that
many battery powered detectors have no batteries fitted).
There is always going to be a balance between nuisance alarms and
detection. That balance will vary depending upon the occupants and
their activities. However, that is too complicated for simple
guidance which errs on the side of too many detectors and ignores the
fact that users will disable intrusive alarms. No simple guidance can
factor in that the occupant of one room can only sleep with ear plugs
and may miss an alarm outside their bedroom door or that another is
deaf or that another is a keen but not always effective cook.
Fortunately for DIYers the ideal balance of detectors can be achieved
on a case by case basis. Smoke dynamics and understanding that hot
air rises are easy to research at the superficial level need for
planning detector locations and an effective installation is easier
for a competent DIYer than it is for most professionals as they know
the way the house including occupants work.