An apparatus includes a pump circuit structured to receive pump data indicative of an operating characteristic of a pump feeding a fluid to a waste heat recovery (WHR) system; a flow circuit structured to receive valve position data indicative of a position of a valve downstream of the pump, estimate a flow rate of the fluid exiting the pump, and estimate the flow rate of the fluid exiting the valve; and a pressure circuit structured to receive pressure data indicative of the pressure of the fluid exiting the valve, estimate a change in pressure of the fluid across the WHR system, and determine a pressure of the fluid in a hot section of the WHR system based on the pressure of the fluid exiting the valve and the change in the pressure of the fluid across the WHR system.