You notice the your boiler is up off the floor, this is probalby to satisfy some rules about flame at least 24" up from the floor in a garage. This becomes awkward with coal boilers gettign coal higher and so on. You need a separate combustion gas system, chiminy with ceramic liners is the best, I have one of those metalbestos stacks, with no problems, but I burn 24/7/365, if you don't the ash and water in the air will combine to eat the stainless. Not sure how you place is layed out, maybe block chiminy on back end of garage, red brick above roof line. To me these problems have variable solutions and take the most thinking, and signifigant money, the coal boiler system is just money, but even that can be helped with rebuilt units. I don't know much about power vents but they certinly are quick, but require maintenace.