CO detectors and ‘false alarms’

By Dr Bernard Thomas, Kidde Safety Europe

Carbon Monoxide (Chemical Symbol is ‘CO’) is a deadly gas, combining three characteristics which make it hard to detect: it is odorless, colourless and has no taste.

In domestic situations it can be generated by any fuel burning appliance. Gas boilers, gas fires, wood burning stoves are just three examples. The generation of carbon monoxide increases if vents are poorly sited, if inadequate ventilation exists, if vents or flues are blocked or if the appliance is burning inefficiently. It is therefore vital that all your fuel burning appliances are regularly serviced. This should be done by a qualified person with “ Gas Safe” accreditation. You should ensure that the Engineer can measure CO levels coming from your appliance. Not all engineers have such instruments.

We recommend that households install CO detectors. Only choose CO alarms that have been tested to the European standard EN50291. The certification to this standard has to be displayed on the packaging by law. Do not be fooled by alarms that state “ Designed to comply with…” as these are usually not independently tested. Alarms properly accredited to the standard have amongst many safety critical features an audible alarm and LED indicators.

The EN50291 standard requires that the printed alarm manual contains information about the dangers of CO, what to do when the alarm sounds and who to contact. Make sure you read the manual when you install the CO alarm and keep it in case the alarm sounds.

What happens if your alarm sounds and what should you do?

If the instrument has gone into alarm you must assume that you have an CO event – remember you are dealing with a gas that you cannot see, smell or taste. An alarm is a very loud sound. If the alarm is just beeping softly at 30 second intervals it is a different warning, such as a low battery warning. (See your manual for details).

All Kidde CO alarms are 100% tested in CO gas at the factory and independently tested by BSI so we stand by the reliability and quality of our products.

People’s reaction to an alarm vary.

Some ignore it, take out the battery and ring our help desk the next day.

The conversation often goes as follows…

Customer: “ Hello, my CO alarm went off last night and I had nothing on so it is faulty

Helpdesk: “Have you got the alarm with you”

Customer: “ Yes”

Helpdesk: “ Please put the batteries in and press the test button”

Usually the alarm will test OK which our helpdesk are able to hear on the telephone.

Of course if the alarm beeps we may conclude low battery or probably the end-of-life indicator. We will advise accordingly.

If the CO alarm is doing something out of the ordinary we will replace the alarm, if in warranty, within 5 working days.

Let us go back to the CO alarm that actually went into alarm but the next morning is testing OK.

The helpdesk will conclude that there has been a CO event and will talk through where the CO detector it is sited, what appliances were in use and whether the appliances have been serviced by a qualified Engineer.

What can cause a false alarm?

Of course the alarm may have developed  a fault – and this should show with the fault indicator – a combination of flashing LEDs or an amber LED flashing. In this case we will replace the alarm if in warranty.

People ask if anything else can set the CO alarm off. It is unlikely that in domestic situations this will happen. Hydrogen gas can set it off, as can some chemicals not likely to be used in the home.

In a case we were recently involved with, the CO alarm sounded and the caller claimed it was false alarming. When we talked through the circumstances we discovered he was stripping down his motorcycle engine in the room where the alarm was situated using many concentrated solvents , including petrol, to clean the parts.

The manual gives more information on solvents that can affect the alarm. Most of these would not be widely known!

Another real case – CO alarms sounding apparently falsely. We found that they had been installed in a warehouse where gas powered fork lift trucks were operating and the exhaust fumes were setting them off. (A lesson not to fit CO alarms in your garage ).

Another case in a remote Scottish Croft:

The owner said his alarm was sounding regularly. The alarms appeared on examination to be fine. On further investigation  it was found that he had a back-up power system of lead/accumulator batteries. As the batteries charge they produce hydrogen gas which can trigger CO alarms. This issue also happens regularly on boats where batteries are charged.

So, why might your CO alarm have gone off?

At a level of 50ppm (parts per million) of CO the alarm will go off after 90 minutes. You may have been cooking, generating low levels  of CO gas (perhaps your stove flame was slightly blocked and burning inefficiently), turned the stove off, closed the kitchen door and therefore sealed the gas in, leaving the alarm to sound an hour later.

In another case CO gas was entering a neighbouring house when the flue of an application was faulty, unfortunately resulting in a fatality.

When their CO alarms go off responsible customers will switch off the appliances, ventilate the house and ring the gas board. They will rapidly receive a visit from an Engineer. The following text is an extract from a typical letter we received following a callout:

“….the alarm went off so I called XXXXX. They arrived and checked my boiler and said it was working correctly. The Engineer said that there was a fault with your alarm….”

We would make a number of comments about this:

– If CO had been present then the ventilation will have cleared it.

– Did the engineer test for CO in the flue gas of the boiler? Not all Engineers carry such instruments. We know of one Engineer who used our alarm to check the boiler. Other engineers have wrongly used LPG gas detectors to detect carbon monoxide, confusing the issue with leaks of piped gas.

We made a study of such events. We requested the alarms to be returned to us and tested them in our laboratory. In all cases we found that the alarms were working perfectly.


–          Make sure that you understand from the manual what any bleeps from your CO alarm mean. They could indicate a low battery, the end of life in some alarms or a fault indication. In the latter case contact the supplier for a replacement.

–          Understand thatcarbon monoxide can come from a number of appliances including your neighbour’s and that it can build up and set off an alarm after you have gone to bed.

–          Make sure your appliances are serviced by an accredited engineer and that the engineer has the equipment to measure for CO

–          In the case of an alarm follow the instructions in your manual.

–          Fit only CO alarms certified to the EN standard by an independent test body ( eg BSI, LPCB )

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