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This paper presents a method to evaluate the off-design performance of a heat exchanger without specifying detailed heat exchanger geometry. Presently, off-design heat exchanger performance evaluation is often done by assuming one of the terms in a lumped volume approach is constant (such as UA, temperature difference, ε etc.) or by producing a draft heat exchanger geometry to evaluate the local heat transfer coefficients in off-design operation. As opposed to these approaches, the method presented in this paper manages to accurately predict off-design heat exchanger performance with very limited information. The method relies on a single parameter beyond the design operating conditions, namely the conductance ratio which is the product of heat transfer coefficient and area on both sides of the heat exchanger. The method is particularly powerful as it allows for the exploration of different off-design scenarios for a given on-design operating point. The paper presents a theoretical introduction of the method along with a validation using data provided by BMPC and Alfa Laval for different types of heat exchangers and working fluids, including supercritical CO2. The method is then used to model the off-design performance of a simple recuperated sCO2 cycle, showing its ability to capture the off-design performance of a heat exchanger without specifying its detailed geometry and the impact of conductance ratio on off-design cycle performance.