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How do you average peak loads when sizing HVAC Systems?

asked 2015-11-12 14:28:28 -0500

updated 2015-11-12 14:35:45 -0500

When using energy simulation (such as EnergyPlus) for load calculation, which averaging window do you choose to calculate the peak load that will determine the size of your HVAC systems?

I am specially concerned with those cases where you have thermostat schedules with night setup or setback for the design day and you'll get much higher peak values the shortest the time you average the loads. Do you design with different averaging times depending on the project? That is, you choose that one type of project can admit one hour to reach the setpoint while other may need a shorter period?

Which approach do you use? Please, share your thoughts.

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answered 2015-11-13 10:41:50 -0500

Archmage's avatar

updated 2015-11-13 12:01:27 -0500

It depends on what load scenario you want to calculate. I think the usual meaning of a design day "load calculation" is to evaluate the load imposed by the design day's weather conditions. In that case the thermostat schedules for the design day should not include night setup or setback but be flat throughout the day and night. The schedules can be defined separately between the design periods and the regular run period so you can have it both ways.

Calculating the loads imposed by recovery from setback or setup is not a completely unreasonable thing to do, but the problem is under-defined unless also given a timeframe for how long the recovery is expected to take. For example if the HVAC operation schedule starts up a half hour before the occupancy schedule picks up, then a half hour averaging window might be appropriate. The longer the averaging window the more likely the system won't be oversized and it seems that an hour usually works out okay.

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Thanks Archmage, I agree with your second paragraph but I am not so sure about the first. I think that the flat assumption can easily lead to undersized systems and I would consider internal gain and even setpoint schedules at least in cooling design, which also assumes temperature and radiation variations along the day.

ecoeficiente's avatar ecoeficiente  ( 2015-11-13 11:23:27 -0500 )edit

Morning warmup etc. can sometimes be where the peak loads occur so I would agree with ecoeficiente that being able to heat/cool from setback to occupancy temperature is important to include in the system loads calculation.

nickj's avatar nickj  ( 2015-11-20 15:56:20 -0500 )edit

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Asked: 2015-11-12 14:28:28 -0500

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Last updated: Nov 13 '15