Numerical simulations of concealed residential sprinkler head activation time in a standard thermal response room test

Charlie Hopkin, Michael Spearpoint

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It is common for sprinkler heads in residential buildings to be of the concealed type. Key parameters for the thermal sensitivity of sprinkler heads are the response time index (m ½s ½) and conductivity factor (C factor, m ½s −½). BS 9252:2011 and BS EN 12259-14:2020 outline test methods to determine whether sprinkler heads achieve adequate thermal sensitivity, stipulating that a room test be undertaken for concealed heads. In carrying out this test, neither the response time index nor C factor is defined. This paper uses computational modelling tools Fire Dynamics Simulator and B-RISK to simulate the room test. In running these simulations, maximum values for the ‘effective’ response time index and C factor needed to pass the room test can be estimated, assuming concealed heads can be represented as equivalent pendent heads. Initial deterministic simulations are used to calibrate results between Fire Dynamics Simulator and B-RISK. Monte Carlo modelling is then utilised within B-RISK, with variable parameters for the response time index and C factor (C), to estimate the relationship between the two parameters necessary to pass the room test. As a result, it is proposed that this relationship can be represented using a simple, empirical power law equation of response time index = 100 (5.4–C) 2/3, where C < 5.4. Practical application: The results indicate that the minimum RTI and C factor values needed to pass the room test are greater than those needed to pass wind tunnel testing methods. In observing that equivalency is not demonstrated by the room test, and by defining the RTI/C factor relationship needed to pass the test, this paper provides fire safety engineers with amended values for concealed heads to be adopted in future assessments. In the absence of any detailed specification for sprinkler heads, it is recommended that an RTI of 290 m ½s ½ and a C factor of 0.5 m ½s −½ may be applied.

Original languageEnglish
Pages (from-to)98-111
Number of pages14
JournalBuilding Services Engineering Research and Technology
Issue number1
Early online date1 Sept 2020
Publication statusPublished - 1 Jan 2021


  • concealed sprinklers
  • conductivity factor
  • fire safety
  • response time index
  • standard test


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