Free standing fireplace ventilation

[res0tzob]

Can you reduce a 8 inch stack pipe on a free standing fireplace down to 6 inch double wall at the ceiling and work?

[KARL]

Assuming that you actually mean that you propose to replace an 8 inch (internal) diameter vertical pipe with two 6 inch pipes (i.e. twin 6 inch), then the answer is likely to be, yes you can. The point is that what matters is the cross-sectional area (proportional to the square of the radius of a circular section pipe) of the pathway in your flue and since the combined cross section of your twin pipe is larger than your original single there are unlikely to be any problems. What you must ensure is that the joints in the flue above and below the new section are absolutely leak free and that the transitions from single to twin (and perhaps back to single again) are smooth so they do not lead to hindrance of the flow. A good DIY person supported by a good supplier should encounter nothing more difficult than the usual creation of adequate access to the work and safe installation conditions.