Compact and thermally efficient, Printed Circuit Heat Exchangers (PCHEs) are favored for use in next generation nuclear power plants. Containing thousands of small working fluid channels distributed in a solid 316H or 800H block, PCHEs can handle high pressures and operating temperatures required by generation IV nuclear plants. Advanced nuclear reactors will require the certification of a nuclear service PCHE design by construction codes, such as BPVC Sec-3. Compliance with this standard requires Creep fatigue and ratcheting analyses be performed for expected loading service transients. Realizing this analysis in PCHEs requires a simplified and flexible modeling approach that can be run over dozens of transients for multiple heat exchanger geometries. The Rich Environment Heatex-changer Transient (REHT) model is being developed to provide a full PCHE model needed to properly resolve Sec-3 loading conditions without the complexity inherent in resolving all facets of the PCHE geometry. This work introduces the thermohydraulic model that is the core of the REHT model. An example problem modeling an experimental scaled PCHE is presented. The ability of the REHT model to simulate fluid flow through a directional varying microchannel core of two heat exchanging streams is demonstrated. The REHT model resolves PCHE thermohydraulics using simple model definitions and minimum computational overhead, making it an ideal design tool.