Calibration tests of a large-scale fire room with inert walls have been conducted. A gas burner giving constant heat input was placed at the rear wall simulating an ignition source. Gas flows, temperature, radiation and C02 and CO concentrations were thoroughly measured particularly in the doorway. Theoretical analysis with available room fire models were also carried out and measured and calculated data were compared. An upward gas flow observed along the walls heated by radiation from the fire plume and the hot gas layer was briefly discussed. This wall flow, not considered in existing two layer models, was shown in these experiments to develop a layer between the neutral plane and the hot fire gases. The layer had moderate temperature and contained very little combustion products, which was verified by measuring C02 concentration along the height of the doorway.