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Thermostat and humidistat

asked 2021-04-19 04:10:26 -0500

wjsjch gravatar image

updated 2021-04-19 04:10:54 -0500

In OS, to finish HVAC sizing, I only need to assign the thermostat schedule to the corresponding thermal zone instead of providing both thermostat schedule and humidistat schedule. But to describe a certain state of air, it needs both dry bulb temperature and relative humidity. And I'm sure that OS can't calculate the cooling/ heating load with just one parameter (dry bulb temperature).

So, I believe OS must have assigned a certain RH value to the thermal zone automatically. And my question is: what principle is OS based on when it automatically assigned an RH value to a thermal zone.

Looking forward to your reply!

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answered 2021-04-19 13:50:41 -0500

updated 2021-04-19 13:53:20 -0500

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).

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It's really interesting to know such a different design principle in the US. Thank you very much!

wjsjch gravatar image wjsjch  ( 2021-04-19 19:45:04 -0500 )edit

answered 2021-04-20 11:05:15 -0500

Petros Dalavouras gravatar image

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.

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Asked: 2021-04-19 04:10:26 -0500

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Last updated: Apr 20