How to model a R744 (CO2) transcritical HP for DHW in EnergyPlus?

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Since CO2 needs 2 pairs of curves (1 for subcritical and 1 for transcritical operation), he probably has to do 2 simulations and combine the results according to the outside temperature

I don't think so. The curves are Heating Capacity Function of Temperature, Heating Capacity Function of Air Flow Fraction, Heating Capacity Function of Water Flow Fraction, Heating COP Function of Temperature, Heating COP Function of Air Flow Fraction, Heating COP Function of Water Flow Fraction and Part Load Fraction Correlation Curves. CO2 is transcritical at the compressor outlet even if the outside temp is low. Even if CO2 becomes subcritical at the compressor outlet with low outside temp, I think such changes as a function of temp can be approximated by one regression curve.

You are correct, it's for DHW so the discharge will always be above the critical point (31.1oC) thus the operation will be transcritical. I got carried away from refrigeration