Flue shape, round or rectangular?

[8obbuilder]

I`m having problem with installation of fireplace, more to the point flue liner. For some reason existing chimney flue has unusual shape , internal measurement 70mm x 370mm which adds up to 259cm2, I need equivalent of 7" round flue liner, if my calculation is correct it should be 240cm2. For various reasons I cannot increase this flue to fit 7". My question: is there any rule against using rectangular flue liner (mine is made of 4mm steel plate)?
Thanks

 

[swbjackson]

70 x 370 sounds like a pre cast flue. Not all appliances will fit this type of flue. The appliance needs to be specifically designed.

As for a flue liner made of steel plate, I wouldn't as the flue gasses are corrosive and will cause corrosion and possible failure. I can also see problems with joining the flue sections together.

You need to discuss flue material and design with the manufacturers of the appliance that you intend to fit/have fitted.