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Autosizing Service Hot Water leads to very long run time

Hello folks

I recently worked on a project where I had selected district heating for service hot water. When I ran the annual simulation with the nominal capacity set to 1000000 watts the simulation took about 7 minutes to complete. But when I set that nominal capacity to Autosize the same simulation took more than 1 hour to complete. There was no other difference in the model. Why would autosizing the District Heating object have such a drastic impact? I hope some one can explain. Thanks

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Rdutta's avatar
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Rdutta
asked 2022-03-01 12:59:17 -0500
Aaron Boranian's avatar
14.1k
Aaron Boranian
updated 2022-03-01 14:12:05 -0500
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1 Answer

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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.

rraustad's avatar
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rraustad
answered 2022-03-01 13:28:09 -0500, updated 2022-03-01 13:29:42 -0500
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Thank you for the guidance. I will take a look at the ConvergenceLimits object

Rdutta's avatar Rdutta (2022-03-01 14:01:38 -0500) edit
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