Difference Turn on ideal air loads and ZoneHVAC:IdealLoadsAirSystem

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Hello to all,

I would like to simulate the heat demand of a building (independent from the heating system). Therefore, I want to use Ideal Air Loads. Now I am confused about the settings.

Option 1: I use the tick box "Turn on Ideal Air Loads" and a thermostat schedule (min. 21°C) --> Then I get some results for "District heating". Unfortunately District heating is only for the whole building and not for single thermal zones which I would like to evaluate.

Option 2: I use the ZoneHVAC:IdealLoadsAirSystem. Applying that, the tick for "Turn on Ideal Air Loads" gets out automatically. - Why?

My questions: Whats the difference between these two options? Which option should I use? In both cases: What is the correct output I should look for?

Thank you in advance for any help!