One installer for a Haman stove said if the flue is wider then 7x7" they will have to put in a liner as it's too large for a pellet stove. Why is this. Is this true? I've only heard one installer mention this to me when I have been discussing with different installers how I want to vent my pellet stove directly to my chimney replacing my wood burning stove. So is there a real concern with venting a Harman P36 in to a 2 story chimney with a 8x8" flue? And what would that problem/concern be? Thanks!