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Is there a reason why I have unmet hours when I have ideal air loads and mostly everything is autosized?

asked 2024-04-16 01:15:10 -0500

Mike117's avatar

I am gradually adding HVACs to a building. But, I noticed when I started out and ran the first simulation I was getting unmet hours. Everything in my model is autosized and ideal so I find it strange that the thermal zones would have a large number of unmet hours.

unmet hours in the openstudio report below: image description

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answered 2024-04-16 03:01:26 -0500

updated 2024-04-16 03:02:19 -0500

Common cause would be the schedules you used.

  • Check that the Winter/Summary design day rules are in line with the regular rules
    • Design Day conditions are used for sizing, so if you specified you wanted 28C in cooling, it'll size on that, but if your default day schedule wants 24C you'll be missing capacity to meet the load at runtime obviously
    • (You also want to check other schedules like lighting, equipment, people, typically they should be zero on Winter DD and one for summer DD).
  • The heating unmet hours could be because of a very abrupt recovery from a setback.
    • You should request the Output:Variable for unmet hours at the Detailed Timestep, and check when they occur. You might find they also do at 6 AM for eg, when your heating schedule was 15C until 6:00 AM, and at 6:01 you request 20C or something. You could instead slowly ramp the schedule every 15min.

You should also go check the global Sizing factors (Sizing:Parameters) object (that's on the "Simulation Settings" page in the OpenStudioApplication)

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Asked: 2024-04-16 01:15:10 -0500

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Last updated: Apr 16