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Initial lighting can give you wayward CO readings on many boilers in my experience. As long as it settles to within the requirements I wouldn't be too fussed personally 🙂
The fact that you're reading minute amounts of co in the air intake could indicate a poor joint and stray POC's as you say. On older boilers the flue connections weren't necessarily as hermetically sealed as modern condensing boilers need to be 🙂
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