A few weeks ago, at 6:15 one morning, I was in the kitchen and heart something beeping. I assumed it was my son’s alarm clock as he takes his time getting up. After a few minutes, I went to investigate. It was coming from my basement. It was my CO detector, but the problem wasn’t CO.
The pressure relief valve on my boiler had gone defective. Every few seconds, it would dump a cup of hot water onto my basement floor. The water was draining into a sump pump, so the depth was about 1/4 inch (6 mm). The water was too hot to walk thru in my bare feet. Basement temperature was hot and humid with moisture dripping from the cool walls. I shut off the furnace and turned on the house exhaust fan. The CO detector stopped.
I’m glad it went off in error. It could have taken hours to make that discovery without the beeping.
I assume it was the extremely high humidity and warm temperature that caused it to go off. When the plumber came to fix the problem, he installed a new pressure-release valve and did nothing else. If it really was CO that caused the detector to activate, I assume the boiler would have continued to emit CO after the plumber left. It didn’t.
Pressure relief valves to act like that sometimes just because they are defective, but it could also be an indication that the burner has a problem and it’s overheating, so you should have it checked before turning it again (I assume you’ll have a complete service when the relief valve is changed). If there was moisture dripping from the walls then water may have condensed inside the detector causing it to sound, and it may be time to get a new detector.
ETA: I see you didn’t have a complete service, the burner should be checked.
in the USA the method is an electrochemical method. i don’t know if humidity will cause a problem, though you had an extreme situation. high humidity can trigger a combination carbon monoxide/smoke alarm.
you should have a carbon monoxide detector on every level of the house, especially near sleeping areas. and the alarms loud enough to wake you. networked alarms, where if one alarm triggers then they all sound off, is an important safety feature.