Re installing an open gas log set into a existing masonry fireplace. Per 10.6.2.1 gas logs shall be installed per manufactures instructions, which they all give the min free area of the flue. Per 10.6.2.3 the flue shall be equal or greater than specified in table 10.6.2.3. The question is (example) have an existing fireplace with a 13X13 clay liner . Which is correctly sized for the fireplace opening, fireplace functions perfectly. The chimney is on the exterior of the house. The client wants to put in a gas log rated at 90,000 BTUs, which requires per table a min of 51" of free area which is a 8" dia flue. A 13X13 flue has an effective area of 127 sq inches, which is 150% larger than what is required. My concern is the expansion of the flue gases going into a large flue, loss of buoyancy and condensation. Not to mention deterioration of the clay flue liner. The client feels I am up selling by recommending installing a stainless liner (8" dia) to be insulated . I cannot cite a code requirement for this issue re open gas logs. There is code language re wood stove to flue size requirements.