Autosizing Service Hot Water leads to very long run time

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I would guess that the plant capacity did not size large enough for the load and the simulation ran at shorter timesteps and more iterations since the plant loop temperature was not stable. Try using the ConvergenceLimits object to see if the simulation times are reduced. See the Input Output Reference section for this object. You don't want to always use this to speed up simulations but this object can help you diagnose issues with your inputs that are causing problems. Once those issues are fixed you can remove this object or increase the iterations to get more accurate results.

ConvergenceLimits,
  0,                       !- Minimum System Timestep {minutes}
  5,                       !- Maximum HVAC Iterations
  2,                       !- Minimum Plant Iterations
  3;                       !- Maximum Plant Iterations

1.5.3 ConvergenceLimits

This item is an “advanced” feature that should be used only with caution. It is specifically included to assist some users “speed up” calculations while not overly compromising accuracy. The user must judge for him/herself whether the reduced run time is useful.