Oh, it can still be a problem. See how closely the burner is to the opening? That's a lot of heat right under the smoke shield, which is right under the combustible header. If it doesn't vent 100% up the stack, it could still be a problem. This would apply to a vented set that was oversized or installed too far forwards. These units are tested with a 100mbh burner placed near the rest of the firebox--not near the mouth.