Installation of exposed fire sprinklers may cause inconvenience in areas where architectural and interior presentation is significant. In order to overcome this inconvenience, recessed and concealed sprinklers were created and are applied widely. Response Time Index (RTI) and C-factor are the thermal sensitivity (intrinsic parameters) used to characterise a sprinkler. They are also used as input parameters in computer fire models to simulate sprinkler response time. However, the RTI and C-factor are not published by the manufactures. Therefore the RTI and C-factor of the recessed and concealed sprinklers have been analysed and determined in this research. In order to obtain the RTI of the recessed and concealed sprinklers, four of the most commonly used sprinkler models (two recessed and two concealed) in New Zealand have been investigated in plunge test experiment by using a wind tunnel in this research. The UC3 wind tunnel used to conduct the plunge test has been fabricated in this research. This work has demonstrated that the UC3 wind tunnel could provide a very stable and uniform temperature profile in the test section. However, the velocity uniformity of the tunnel needs to be improved in the future. The "apparent" RTI for different recessed and concealed sprinkler models (two recessed and two concealed) have been determined in the plunge test experiment. It should be noted that the "final calculated RTI" for each tested recessed and concealed sprinklers has been denoted as "apparent RTI" in this study. BRANZFIRE computer model has been used to model the fire scenarios in the full scale fire tests conducted by Bill and Heskestad (1995). The best input fire object location, the best input sprinkler distance below the ceiling and the input "apparent C-factor" in BRANZFIRE for the flush, recessed, concealed and the recessed sidewall sprinklers have been determined in this research. This work has generally improved the guidance available to fire safety engineers for the RTI and C-factor of the recessed and concealed sprinklers.