Sizing an insulated stainless steel rigid liner

I have a 25 year old wood-fired boiler with a 7" flue outlet. It is currently vented into a similarly sized flue of a 20 foot interior chimney, except for the 6 feet above the roof.. It has excellent draft but I am having a problem with creosote.
I would like to install a 5 1/2 X 5 !/2" stainless 316 rigid flue liner, surrounded by 1/2" of Super Wrap insulation, which will fit inside my existing clay flue liner.
Because the new liner would be the equivelent of a 6" liner and my boiler outlet is 7", will there be a problem with code. I don't anticipate any problem with draft. Otherwise, I will have to remove my existing clay flue liner.
Please advise.
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Burn seasoned hardwood, or buy a gas or oil fired boiler

Its not *just* the code but also what the manufacturers installation instructions call for.

Call your local *competent*, licensed, insured, professionally trained, HVAC technician to install a new oil or gas fired boiler.
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Thanks. The professional installer would be 'me'. I just need a little help with a flue sizing. Hoping for some advice because back in the '80s the advice was "just put up an 8x8 chimney", but today we have insulated SS liners with outer wraps, etc. and I'd like to see what's going on out there.
I've been burning wood for 30 years without a single incident.
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You may have missed the point. Are you a Professional installer or Are you a "professional" installer? Bubba
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