Present paper investigates the effect of furnace heat flux on the evaporator downcomer circuit of natural circulation boiler. A two-phase flow model based on separated flow assumption is developed for the circuit analysis. The variation of furnace heat flux of real furnace data is implemented in the present algorithm by suitable linear regression method. The axial variation of furnace data is considered here and the effect of position of the burner in investigated by using a burner height factor. It is observed that the mass flux and the steam quality strongly depend on the burner location factor. The tube dry-out or critical heat flux (CHF) also alters with the burner height factor. The results are compared with the average heat flux and it is observed that the average consideration of actual furnace heat flux may over predict or under predicts the tube burnout scenario. The study will help the design and operation engineer of boiler evaporator circuit.