Ed Sirett wrote:
I have read through the replies... um ... not a lot of light....

CO is very much the same density as air, obviously hot CO will convect
upwards but essentially once mixed with air it stays mixed.
The N2 and O2 components of air don't separate out by themselves and
neither will CO.

CO is extremely toxic just 0.4% is fatal in minutes and even 0.1% can be
fatal with a long exposure time.

CO is produced when fuels containing Carbon have insufficient air(oxygen)
to properly burn. If any flame is yellow like a candle, be it from coal,
coke, oil, gas, paper, wax, wood or garbage then soot (carbon) and CO are
being produced.

There is really no reason to install a Carbon Monoxide detectors in the
home, but there again there is no harm. You can get simple non-electronic
orange spot cards "dark spot is danger" for much less cost and to much the
same purpose.

Open flued and flueless gas appliances, and even an open fire grate all
carry a CO poisoning risk. This risk can be brought to a safe level (i.e.
where other aspects of life are much more risky) by
1) Never blocking any vents provided for gas appliances and open fires.
2) Having the gas appliance regularly checked.

The spot cards don't give an audible alarm. Unless you happen to notice
them, they're of limited use.
CO alarms will work if an appliance develops a fault or it's air supply
becomes unexpectedly restricted by whatever means. They are also being
used to detect fires I'm told. I would always favour a smoke alarm for
this though.
I have heard the detection ability of domestic CO alarms questioned. Of
course the questioner was a sales person for a more sophisticated
comercial variety but I do wonder how good the really cheap ones are
especially when it's so difficult for a householder to test them.
I agree that having gas appliances checked is always advisable but a CO
alarm is a valuable second defence.

John