A spatial-averaged model of a boiling hydraulic channel is presented. Linearized analytical results are compared, with reasonably good agreement, with several transfer functions measured by Zivi, Wright et al. in a boiling loop at atmospheric pressure using both natural and forced circulation. The necessity of applying a heat source correction to the experimental data is discussed, and the role that the dynamic pressure plays is presented. The physical mechanism causing the experimentally observed hydraulic instability is shown to be an interaction between the transient flow and friction pressure drop in the two-phase region. The experimentally observed increase in unstable oscillation frequency with inverse boiling length is also shown analytically. The position of the boiling boundary in the channel is shown to be important in stability considerations. By comparing analytical results with experimental data of Wissler et al. and Becker et al. it is concluded that the least-stable situation results when the boiling boundary is partway up the channel. Since the position of the boiling boundary is directly related to the degree of subcooling, the existence of this crucial position is used to explain the influence of subcooling on stability.