I have an exterior pre-fab 30 foot chimney running up an outside chase. What is the minimum vertical stovepipe length coming off the Oslo's top flue before going horizontal? I know the Lennox Country line requires 2 feet whereas the Lopi line requires 84 inches. I would like to keep the vertical pipe less than 18 inches with a double wall stove pipe before making a 90 degree turn into the thimble. My horizontal run will be about 18 inches.