Calculating steam boiler savings based on a heating hot water supply temperature reset

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I'm currently working with a facility that is circulating 170F water year round in its hot water loop for reheat boxes, and talking with the operators that's what they need to set it at in the winter to fulfill all the heating needs. Logically they won't need such hot water year round, but I'm having difficulty coming up with a good way of calculating the energy savings of lowering that hot water temperature during warmer times without knowing what the new loop dT would be. I was thinking of suggesting a reset between 170F HW-ST below 40F OAT and 140F above 70 OAT.