I have run my stove two years without OAK, but I am also always on the lookout for ways to improve the heating efficiency of home. My pellet stove is installed next to a 40 in cement wall. My heat exhaust pipe goes up above and out at about 5 feet. In order to install OAK, that also has to go up over the cement wall. How far away from the exhaust does the OAK need to be when mounted on the same wall? Can it be lower or does it need to be higher than the exhaust? Most all these OAK sets are metal. Any reason why I can't use PVC from outside to in, and then go to metal pipe to the stove?