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# Unreasonable "Sizing Runs" requirement in ASHRAE 90.1-2019

ASHRAE90.1-2019 Appendix G specifies that internal loads schedules for cooing sizing runs shall be equal to the highest hourly value used in the annual simulation runs and applied to the entire cooling design day.

Before seeing this requirement, I was using the same schedules for annual simulation and sizing runs.

When I followed this requirement and changed the internal loads schedules in my project, the cooling demand (peak cooling load) became almost three times higher than before changing the schedules. Before changing the schedules, the peak was at 15:10, but after changing the schedules it was at 6:10am. It makes sense because when air conditioning is turned on in a midsummer morning, inside of the building is hot, and adding peak internal loads to that situation generates an extremely high cooling load.

Is everyone complying with this requirement?

Honestly, I don't want to follow it because it's unrealistic. Or is my interpretation wrong?

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Your interpretation is right, and to be honest, most design engineers have been doing that. I actually see a scenario where the engineers would make all internal loads 100% of their peak power at all times.

One change that I can suggest is that your HVAC should be ON through the whole day. Otherwise, you will get those high-peaks for the heat gain that has been accumulating in the space.

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Thank you for your answer. I followed your suggestion and the cooling demand became only about 10% higher than the original one, which is acceptable to me.

( 2021-08-07 11:34:14 -0600 )edit

I agree with keeping ALWAYS_ON the HVAC system too

( 2021-08-12 04:51:22 -0600 )edit

This answer suggests modifying the Timesteps in Averaging Window input in the SIzing:Parameters object to smooth out the spike in the load associated with recovering from a night setback/system off period.

I think if you maintain the averaging window to one hour you are within the intent of the generally accepted engineering standards referenced in G2.2.3.

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Thank you for your answer. At first Number of Timesteps per Hour of my model was 6 and I haven't set Timesteps in Averaging Window. I changed Timesteps in Averaging Window to 6, but the cooling demand hardly changed. I increased Timesteps in Averaging Window to 12 and 18, and the cooling demand decreased a little, but it was still1.5 times more than the original. I think 18 is too long as Timesteps in Averaging Window. So, in my model, I chose to change the HVAC schedule to 24 hours instead of changing Timesteps in Averaging Window. But I'm glad to know this function. Thanks!

( 2021-08-07 11:12:32 -0600 )edit