The recommended MINIMUM is 15'. As other have advised, you can wait to add until it gets colder, but if you would like to have better performance and run during the shoulder season, you may wish to add length. Pipe is like LEGOS, sold in 1' increments. I'd suggest a 3' section to start.I will take a sample of a few pieces today. Given my current setup, would you recommend adding height to my chimney?