Check the install manual page 16. there is a section about combustion air supply.The reason I asked is because I had my basement rim joists spray foamed and it seems to have thrown off the drafting/combustion. I've had my basement window open several inches for a few days now and the boiler seems to be performing better. It looks like I may have to pipe in some external air. It's a small matter but one I need to address before the start of the next heating season.
"The area of the free minimum cross-section must be 2.5 cm2 per kW of the boiler‘s nominal total output."