Combustion-driven oscillations are a common problem with boilers, water heaters, and furnaces. Sound is produced by the flame and is reflected downstream from the combustion chamber. The reflected sound waves disturb the mixture flow or equivalence ratio, increasing the heat-release pulsations and the accompanying sound produced by the flame. This phenomenon has been described using a positive feedback-loop in prior work. The current paper applies the feedback-loop stability model to two boilers, which exhibited combustion oscillations. Additionally, a feedback loop model was developed for equivalence ratio fluctuations and validated. For the first boiler, the combustion oscillation problem was likely related to the geometry of the burner and the intake system, though results are inconclusive. For the second boiler, the model indicated that the combustion oscillations were due to equivalence ratio fluctuations. Accordingly, the combustion oscillations could be alleviated by changing the geometry of the intake system.