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Can DOAS AirLoop flow rates be scheduled?

asked 2021-01-13 10:54:49 -0600

Abarker gravatar image

updated 2021-01-14 08:14:21 -0600

We are trying to model a dedicated outdoor air system that modifies its flow based on a time of day schedule.

By adding a fractional schedule to the Maximum Fraction of Outdoor Air Schedule Name in the Controller:OutdoorAir object, this achieves the desired reduction in outdoor air. However, the flow in the AirLoop itself does not change and the system becomes a recirc systems at hours where the max OA fraction is scheduled to be less than 1.

As a result we don't see the fan power savings we expect, and also now have to contend with recirc air which isn't real. Is there a way to get the whole air systems flow rate to follow our fractional schedule?

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@Abarker it sounds like you are using EnergyPlus, but are you using one of its interfaces (DesignBuilder, OpenStudio, etc.) to create and edit the EnergyPlus model?

Aaron Boranian gravatar image Aaron Boranian  ( 2021-01-13 15:17:15 -0600 )edit

Attaching your sample file (idf or whatever format you have) would also be helpful to describing the system you're modeling.

Yueyue_Zhou gravatar image Yueyue_Zhou  ( 2021-01-13 16:34:43 -0600 )edit

Hi @Aaron Boranian, the model geometry was built using the OS SketchUp plugin and then edited further in IDF editor. All HVAC objects start as HVACTemplate and are then modified / refined further as needed.

Abarker gravatar image Abarker  ( 2021-01-14 08:18:50 -0600 )edit

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answered 2021-01-13 17:03:23 -0600

Maximum Fraction of Outdoor Air Schedule doesn't change airflow in the loop, it just applies desired fraction of OA. As per docs indicates, "The current schedule value is multiplied by the current system mixed air flow rate to set the maximum outdoor air flow rate. This schedule is applied after all other limits (except EMS and demand-limiting overrides)." If you want a 100% OA system it should be 1 all the time. But where the mixed air flow is specified should be modified. I cannot tell without more details of how you model it. The least case is that you can modify the airflow by actuate fan air flow rate via EMS, where you can read in your schedule (via sensors) and multiply it by maximum/full air flow rate then assign the desired air flow rate to fan actuator. I haven't tried it but it looks reasonable in theory.

Good luck.

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Hi @Yueyue_Zhou, I believe I have now edited the original message to attach the model file but I don't see it anywhere (apologies, I'm not familiar with how to do it). Of course, to maintain 100% OA the fraction should always be 1.0, that makes a lot of sense. I just wonder if there is any other way to schedule the flow rate of an AirLoop without using EMS. Though if that is the only or best way to do it, so be it, and thank you for the suggestion.

Abarker gravatar image Abarker  ( 2021-01-14 08:23:03 -0600 )edit

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Asked: 2021-01-13 10:54:49 -0600

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Last updated: Jan 15