The purpose of this study is to determine the regime parameters corresponding to the boundaries of high-frequency thermoacoustic instability of vapor-generating channels. The authors use the nonequilibrium polydisperse model of the bubble structure in underheated boiling flows. The nonsteady relative enthalpy perturbations are assumed to be small with respect to relative pressure perturbations, which allows to eliminate the energy equation; this seems to be valid at least in the high-frequency case. The instability boundaries are determined by using the linearized equations and hodograph method. A comparison with experimental data shows the accuracy and effectiveness of the developed approach.