Good job having the manometer on hand!! I used some extra copper tubing from a kit for running a water line to a freezer that I had left over. i think it is 3/16" or 1/4". a compression union transitions to the plastic tubing. The fitting on the flue pipe has threads on one side screwed into the pipe and compression on the other side for the tubing. I don't remember the size of that allen bolt on the front of the Harman, but the tubing slid into it on mine.
Keeping the CO monitors at 0 is a good thing!!
Oh dang... I didn't even think about just sliding the tubing in there for a minute or so.. because once I get the correct reading on the Menometer, I can just slip it back out. Good call
And when checking the flue, I was just gonna do that real quick, but I need to get a decent sized plug to fill the hole. My flue pipe is literally a 20 foot straight up 6" pipe that goes right out of the roof.. and when we were installing it, it was sucking the air pretty good... and I live on the top of a mountain so the wind always gives me a good draft.