CO alarms....

[Stephen H]

Hi, I am wiring up my house for some interlinked mains lithium battery backed up CO alarms.

I am putting in at least 3, one in the lounge which has a wood burner, the second on in the main bedroom above the lounge (due to the chimney stack in the bedroom) and a third one in the loft (due to the chimney stack in the loft).

Now I have a boiler in the kitchen and a gas hob with 8 rings.

I would like to put a CO alarm in the kitchen due to the boiler but am concerned that the gas hob may cause nuisance alarms?

The gas hob and the boiler are at opposite ends of the kitchen if that is helpful as I could position the detector nearer the boiler rather than the gas hob.

There is a cooker hood above the gas hob but that is currently recirculating back into the kitchen but I can convert this to extract to outside. (that will help with cooking smells and stop kitchen steaming up, and of course take away products of combustion)

What do you all think?

Stephen.

[Andy Bartlett]

Get a canary.

[Dave Liquorice]

Possibly a bit OTT (again...)

Sensible and I think mandatory. Check with the installation instructions where it needs to be located. CO is heavier than air unlike smoke...

Sort of sensible but has the chimney had a liner fitted with the wood burner? If so I don't think there is much point.

Why? CO isn't a fire risk and again if the chimeny has been lined proabably a waste of effort.

A hob shouldn't generate CO. What sort of boiler is it, room sealed? One near that might not be a bad idea but again check where it needs to be installed to be effective.

Do it, irespective of any any alarms. We had a recirulating hood here when we moved in waste of time. Vented it to outside and the windows and walls no longer poured with condensation unless one was really boiling the pasta in an open pan hard, even then it was much less than with the stupid recirc.

[Tim Watts]

CO is slightly lighter than O2 and the same (as good as) density as N2:

(by atomic weights) CO = 28 N2 = 28 O2 = 32

(invoking Avogadro's Law)

[Peter Parry]

Way over the top.

CO is fractionally lighter than air (28.01 grams per mole compared with 29.96 for air).

There isn't any point if the fire has been correctly installed.

Indeed.

Any gas hob will generate some CO, but very little (insignificantly little) if it is burning correctly. Certainly not enough to trigger an alarm.

The risk from CO is usually caused by protracted failure of something, either a vent or burner, being able to measure CO content and detect variation over time is more useful than a single set point alarm.

Definitely agree with that - it will also vent the trivial amount of CO produced by the hob.

[Dave Liquorice]

What ever. B-) The point is CO alarms are a little more fussy about their location than heat/smoke alarms. In an occupied room that may be subject to CO they ought to be at the same level as ones normal head height.

[Peter Parry]

Actually they are quite a bit less fussy, but this is more to do with the different ways CO and fire present a threat. CO invariably builds up over time. Even if a catastrophic failure of a chimney liner caused a burner malfunction the rate of build up of CO would be measured in tens of minutes to hours rather than seconds. In the more common case of build up of soot or similar increasing CO output the increase is measured in days to weeks.

Fire on the other hand can develop within a few minutes. The CO alarm will work more or less anywhere (although I agree head height is best if only because it makes it easier to see the reading (and I wouldn't get one that didn't display the CO level)).

Products of combustion detectors on the other hand must be sited to catch the first flow of combustion gasses within seconds so siting is rather more critical. On a ceiling away from walls works well, on a ceiling near a wall doesn't as the initial flow of gasses won't go there.

[Andrew Gabriel]

There have been a couple of CO deaths in the news recently where the source of CO was not an appliance in the house, but actually a neighbour's faulty appliance. So it's probably a good idea to not be too blinkered by just your own appliances, but also to consider sleeping areas where you can also be vulnerable to someone else's CO.

Agree. Might not like the temperature extremes in a loft either. If you must have another, put it in another bedroom.

The grill on a gas cooker is likely to generate most (mainly because it's usually the most powerful burner, but also because of the nature of the burner design to throw a horizontal flame some distance. I believe the room ventilation requirements mean it shouldn't be able to produce more than 50ppm CO in the room, and a CO detector shouldn't trigger at that level for hours, and you don't use a grill for hours.

Again, don't just consider your own gas appliances as sources, but also neighbours'.