How to Choose the right cooling capacity?

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Hello, Well, I would say the traditional approach for sizing a cooling plant would be to run design load calculations for all zones connected to the chiller plant and use the coincidental sum of all zones.

I use designbuilder, which has built in heating and cooling design calculations. These calculations use a different weather file that represents the desired design conditions. The cooling design goes through an hourly simulation on each of the specified design days of the summer (usually June-September, one design day per month).This is where I would follow the procedure described above.

In OS or EP, I think you could use ZoneHVAC:IdealLoadsAirSystem, and you will need to look into the weather file options. I suppose you could output the Zone Ideal Loads Zone Total Cooling Rate [W] for each zone, sum the outputs together and find the coincidental peak.