The LWF Blog
Fire Safety Engineering for Design – Sprinkler Heads – Part 235
May 19, 2025 10:33 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 234, LWF discussed other devices/sprinkler systems for particular uses. In part 235, we give an overview of the thermal sensitivity of sprinkler heads.
Thermal sensitivity of sprinkler heads can best be described as the speed at which the heat-sensitive element of a sprinkler head reacts to local thermal conditions, i.e. how fast it reacts to a fire situation in the area.
The speed of response is dependent on various factors, such as the size and structure of the bulb or fusible link; the size and shape of the sprinkler head; the type of fitting the sprinkler fits into, etc.
During the approval procedure for a sprinkler system, the speed at which sprinkler heads react can be measured and compared using standard apparatus. The RTI is a measure of sprinkler thermal sensitivity and sprinklers are graded according to sensitivity range. There are three potential response classes, as follows:
Standard response A: corresponding to RTI values between 80 and 200 (m1/2 · s1/2)
Special response: corresponding to RTI values between 50 and 80 (m1/2 · s1/2)
Quick response: corresponding to RTI values of 50 (m1/2 · s1/2) or less.
Specific equations may be used to determine sprinkler reaction times for ceiling jet velocity and temperature; and temperature rise and may be found in CIBSE Guide E.
Horizontal sidewall, recessed or concealed sprinklers are not classified and are referred to as unrated.
When sprinklers are to be used as part of a fire engineered solution for a building, their speed of operation must be predicted and the RTI of the head should be used as part of the calculation. It is important, therefore, that further information is gained from the sprinkler manufacturer, before their use can be confirmed as appropriate. Ad-hoc testing of concealed pattern sprinklers fitted with a sprinkler with a ‘fast response’ element have indicated the reaction times are within the ranges of a sprinkler classified as a standard response unit.
Quick response sprinklers are designed to operate earlier in the fire’s development and therefore when the fire size is smaller. Smaller fire size means less water may be required and the hydraulic demand on the system should be reduced. Equally, a smoke ventilation system operating in the same building may be subject to reduced demand when fire size is restricted.
The impact on reduced costs and reduced risk of damage make the benefits of quick response sprinklers easy to justify and employ when appropriate for the fire safety design.
In part 236 of LWF’s series on fire engineering we will look at the types of sprinkler system. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.