What fouls the window glass?

What makes the glass dirty? One of the reasons is quality of fuel: damp wood and softwood. This however depends on the user. What does not depend on him, is the burner construction facilitating considerable CO emission. Therefore it is good to check the CO output before purchase. The standards request the manufacturer to disclose the CO emissions reported in testing. You should seek for system featuring CO emission below 0,15%. But many inserts now in the market output as much as 0,3 or even 0,4 % of CO. So check it unless you want to have obscured view after 10 minutes of burning