The set point temperature and the maximum coverage spacing, which are specified
by listing organizations, currently characterize the performance of heat detectors.
The listed spacing values can be used as a tool, following the procedure given in
NFPA 72, to estimate a fire size at detector activation. However, the uncertainties
associated with this practice are too great for the estimated fire size to be used in any
reliable manner. The confusion created by the leading listing organizations, such as
UL or FM Approvals, by assigning the independent spacing values that may not
agree for the identical type of detectors, does not alleviate the situation.