Thermostat and humidistat

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OS does not automatically assign a humidity/RH setpoint. In many cases, and especially throughout California and much of the western/arid US, humidity is often neglected when designing HVAC systems. In these cases, only a dry-bulb temperature setpoint/schedule is necessary to specify in order to do a load calculation. Of course, there will be humidity, depending on the outside air moisture content, the amount of ventilation air supplied by the system, and internal source of moisture, but that does not make it necessary to control for humidity; it is perfectly acceptable to control for only dry-bulb air temperature, except perhaps for critical space types such as operating rooms and other spaces where humidity control is required. For example, all of the DOE/PNNL commercial prototype models do not control for humidity, except for a single zone in Outpatient Healthcare (Floor 1 Operating Room 1).

Openstudio does not apply a humidistat in thermal zones automatically nor does it need to. Thermostats and humidistats are used only to control a specific variable of the air of the thermal zone. Thusly, if you only have thermostats your HVAC system will operate only to keep the temperature of the thermal zone between you setpoints, ignoring the humidity of the thermal zone. So to sum it up, Openstudio does have a specific state of air in each timestep but thermostats and humidistats are just control signals for your HVAC system. However, in order to calculate the airflow rates required for the thermal zone, openstudio uses the parameters you inputed in thermal zones for supply air which among others is the supply air temperature and humidity ratio.