Difference between precast and prefabricated flues

Am planning to put in a new gas fire, or possibly stove and all the brochures and other bumpf seem to distinguish between Precast and Prefabricated flues. Several manufactures seem to rule out Precast flues yet allow prefabricated ones.
Prefabricated usually means 5in (internal) twinwall (with or without insulation twixt the walls) - i.e. cross section 16,500 sq. mm
Precast seems to come in two sizes and two configurations 1. Pre 1986?? Cross section 12,500 sq. mm and only 63 mm deep. 2. Post 1986 ?? Cross section 16,650 sq. mm (90 * 185).
Where a Baxi backboiler was fitted there is usually no flue box, there is normally a 45 degree entry block about a metre above the top of the boiler with section of asbestos/cement (if it was pre 86) pipe with a 45 degree angle connector on top joining the two. The question is, would a modern (16,650) Precast flue be as effective as a 5" prefab 'twinwall' flue
--
Andrew

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