I've been told I have to get a chimney liner. Have an 8" opening on burner.. one company said i need a 7" liner, and another said a 6" liner... I'm concerned about safey and health. Please advise what I should do. Grateful.

You need to take the advice of the service people who are there. I have no idea what the exact circumstances are since I cannot visit your home from here. This is a item that really needs to be seen on site.