exhaust release in a direct vent stove MUST be from the exhaust blower connection through the pipes , a leak anywhere "upstream" towards the fire would suck air in making for a bad burn but not for leakage out as the stove operates under negative presure, you are on the right track with the damper, you want to slowly adjust it closed to find the "sweet spot" where you have adequate combustion air and do not load up but are still capturing the max amount of heat. stack velocity can rob heat but makes for a pretty burn, you have to hit that happy medium to get optimal performance