Welcome! Here are the website rules, as well as some tips for using this forum.
I just had my older (~20 year) furnace (err, boiler for radiant heat) cleaned/tested, and it passes CO testing, but the readings seem to fluctuate between 50 and 70 ppm. We tested on 2 different days, and the furnace has been running, and the two tests were consistently inconsistent (both tests went up to 70ish, back down to 50ish, and so on). The tester wasn't sure what might be causing this, so I've been searching the web for clues but haven't gotten anywhere. Any insight here?