Flow instabilities in a natural circulation boiling loop at a low pressure are reported. The oscillations at boiling incipience are primarily chaotic and bifurcate to quasiperiodic ones depending on inlet subcooling T-sub and heater power Q. They also strongly depend on water volume phi in the loop. We have presented power spectrums, attractor reconstructions, and Hurst exponents for the analysis of the experimental data. The analysis shows that the primary oscillations are very similar to geysering instability. Chaotic oscillations occur at low T-sub or high Q, whereas quasiperiodic oscillations occur at high T-sub or low Q. Our experiments also suggest that wall superheat exceeding a critical value triggers the instability. (c) 2013 American Institute of Chemical Engineers