Is it legal to split one room into several zones in EnergyPlus?

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Following the ISO 13790, it is not:

In particular you have not to dived a space in more thermal zone if:

1. Set-point temperatures for heating/cooling of the spaces differ by no more than 4 K;

2. The spaces is serviced by the same heating systeme (if any) and the same cooling system (if any);

3. At least 80% of the floor area are serviced by the same ventilation system;

4. The ammount of ventilation (in cubic metre per square metre of floor area per second) differs by not more than a factor of 4 within 80% of the floor area.

While ISO 13790 typically advises against splitting a room into multiple zones, you can proceed if your setup meets the specific conditions regarding temperature, heating/cooling, and ventilation uniformity. Best of luck with your modeling!