I have a FireAngel CO-808 which I located in the room where my gas boiler is located ( spare unused bedroom) Detector is about 1.3 m from the boiler, recommended distance is 1 to 3m and above door level
Its been there over a year and about 10 weeks ago went off about 4am, Gas boiler was off since about 10:30pm previous evening.
I was a bit concerned but suspicious as no boiler operating for a long time before it went off, rather than call emergency numbers, I reset the alarm, and went back to bed, No more alarms, but I decided to buy a 2nd detector next day, This one is a Kiddie with a digital readout, so even if it detects CO under the minimum alarm level (70ppm I think), it will not alarm, but still give a readout.
This one recommends placement 5-15ft from the potential CO source and to "avoid nuisance alarms 15ft is recommended". It also recommends placement at eye level. The room where the boiler is, is very small so in order to get it 15ft away I placed it in the next room.
Then 4 weeks ago, The FireAngel alarmed again about 7am, this time boiler was running but the Kiddie was showing 0 ppm. I normally have my boiler serviced every 2 years ( I know 1 year is recommended), it was serviced last year, but I decided I better get it serviced again this year, by someone different to last year.
They are so busy this time of year, it only finally got done today, I explained the Alarm issue to the engineer, he said he would do a CO test.
He found no issues with the boiler, said in fact it was running as well and as efficiently as possible, gave me the CO test printout from his analyser showing 0ppm.
So as a result of all this, I'm still wondering why the FireAngel is alarming occasionally, My guess is either it is faulty or despite the placement recommendations, its possibly a bit too close to the boiler. Googling CO detector placement, many sites recommend not placing to close to boiler as a small amount of CO can be given off on boiler startup.
For now, I have decided to leave the FireAngel in place with the Kiddie as a backup.
So lessons from all this,
(1) I would recommend getting an alarm with a digital readout like the Kiddie, It may give you an extra level of certainty if an alarm happens.
(2) Consider having a 2nd alarm as a check, Not sure about everyone else, but I don't want to be calling out Emergency services if its a false alarm.
(3) When you get your boiler serviced ask them to do a CO check and give you a copy of the readout if possible, it gives an extra level of peace of mind