You can direct vent the Koker for far less money than a liner (stainless or cast in place), or a "Class A" chimney. Ideally you would want to run it into a chimney, but I have installed Keystoker boilers and 2 Keystoker 90s with the direct vent, and they operate just fine with minimal maintenance. Try to keep your horizontal run to a minimum and install a Tee section, to allow access to the horizontal run for easy cleaning. Remember, the longer the horizontal run, the more maintenance. I would try to keep it under 3 feet.