FIREANGEL FireAngel Carbon Monoxide Detector. Replaceable Battery Powered with the latest sensing technology, a clear loud alarm and BSI Kitemark approvals, the FireAngel Carbon Monoxide Alarm offers the highest level of CO detection for you and your family and meets the latest regulations. Replace every 10 years. The only approved method of gas testing is to use FireAngel Easy Gas in accordance with the documented test procedure in the manual. No other gas test procedure or gas is suitable.