Getting building thermal demand as output

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You can find an overview of output variables that the ZoneHVAC:IdealLoadsAirSystem can generate. I would assume that you are most interested in the Zone Ideal Loads Supply Air Total Cooling Rate and Zone Ideal Loads Supply Air Total Heating Rate. Note that an ideal loads air system object is connected to only a single zone, so if you are trying to determine the thermal demand of the whole building, you will need to parse through multiple columns of data in the output CSV file because one column is generated for each output variable generated by each object. For example, if you wanted to see thermal demand in two zones, you would parse through four columns (heating and cooling for each zone).

Reading your question, it seems you're perhaps looking after the total heating/cooling demand of your building rather than individual zones. If you only use IdealAirLoads, and don't have any special plant loops with District heating or cooling, you can just request the Meter for District Heating and District Cooling.

Possible appropriate meters are Heating:DistrictHeating or DistrictHeating:Facility (works only if you have your IdealAirLoads as Districtheating) or DistrictHeating:HVAC (possibly the most appropriate for your use case). See eplusout.mdd for more info including making sure the meter your end up selecting has what you require and only that.

Indeed, as stated in the doc for ZoneHVAC:IdealLoadsAirSystem

The energy required to condition the supply air is metered and reported as DistrictHeating and DistrictCooling.

See also DistrictHeating > Output Variables, Output:Meter, and the Output Details and Examples > eplusout.mtd.