Recently, low-frequency measurements (0.1-Hz region) of the elastic susceptibility in KSCN near the order-disorder phase transition at $T_c=410$ K revealed a relaxation that could be explained by entropy fluctuations. We present a phenomenological theory describing the heat-diffusion central peak in the macroscopic ($q=0$) dielectric susceptibility. It results from the Landau free energy and the heat-diffusion equation. The model predicts a relaxation in the ferroelectric phase at low frequencies that depends on sample size. It does not follow a simple Debye behavior, but reveals a long high-frequency tail.