In modern houses (post circa 1965), chimneys are constructed using pot liners, which are generally circular. Here you can use one of our circular Chimsocs. The thing to remember in this instance is that you need to allow for a little ventilation within the chimney stack. By using a circular Chimsoc in a round chimney, you will be completely blocking the opening. It's therefore a good idea to use a small length of plastic pipe wedged between the chimney and the installed Chimsoc to allow for the ventilation.