My friend has a pre-fab fireplace in his house - it has a 6" rigid stainless steel liner in a wooden chase on the outside of the house. He wants to install a pellet stove. What are his options? Does he need to replace the liner with a smaller one? Can it even go inside of the old liner inside the chase? Is this up to code? He was looking at the Enviro M-55 and the Quadra-fire Mt. Vernon. Any input would be helpful. Thanks.