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# OpenStudio/EnergyPlus Thermal Zone Outdoor Air Balance with Exhaust Fan Only and 100% Outside Air System

I am attempting to model an outdoor rest station with two large bathrooms. Based on ASHRAE 62.1 the restrooms have an exhaust airflow requirement of 70 cfm/toilet. Additionally, I am modulating the CFM by the occupancy schedule since there is very low occupancy during the winter.

In OpenStudio, I am using a Fan:ZoneExhaust object in each thermal zone with the Maximum Flow Rate option matching the CFM for the number of toilets. Using a 100% outside air system, I was hoping that the system would supply the correct amount of ventilation air to the space to supplement the exhausted air, without specifying any ventilation requirements.

However, in the error report, I am receiving the message:

  ** Warning ** GetOAControllerInputs: Controller:MechanicalVentilation="CONTROLLER MECHANICAL VENTILATION 1
**   ~~~   ** Cannot locate a matching DesignSpecification:OutdoorAir object for Zone="THERMAL ZONE: 1 FEMALE RESTROOM".
**   ~~~   ** Using default OA of 0.00944 m3/s-person and 0.0 m3/s-m2.


and

   ** Severe  ** Controller:MechanicalVentilation="CONTROLLER MECHANICAL VENTILATION 1", Design Specification Outdoor Air Object Name blank
**   ~~~   ** For Zone="THERMAL ZONE: 2 MALE RESTROOM".
**   ~~~   ** This field either needs to be filled in in this object or Sizing:Zone object.
**   ~~~   ** For this run, default values for these fields will be used.


Which appears to be applying a Design Specified Outdoor Air that doesn't really balance with the Fan:ZoneExhaust object.

My question is, using the Fan:ZoneExhaust object with the Decoupled mode and Unbalanced (as seen in the documentation), is my fan being properly balanced with the 100% OA packaged rooftop system?

Should I specify a 0 DesignSpecification:OutdoorAir or will this incorrectly size my system?

What is the best method to balance my ventilation and exhaust flowrates for these spaces?

What about when I add energy recovery ventilation? How do I force the exhaust air back to my central system while keeping the correct ventilation requirements for the restroom?

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You need to specify ventilation requirements for the space. In your case, it seems like the best option will be to specify your ventilation requirements to match your exhaust flow. If you only want your exhaust fan to run when the DOAS is supplying ventilation air to the space, you could use the Coupled option, but if they are scheduled the same, it shouldn't be necessary. If the exhaust fan is unbalanced, the exhaust fan flow rate will be subtracted from the return flow for the DOAS. For the case in which DOAS flow matches exhaust flow, return flow will be zero. If you want to add energy recovery to account for heat recovery from the exhaust flow you will likely not want to set the exhaust fan as Unbalanced. If you set the Balanced fraction to 1, no flow will be subtracted from the return air stream and you can recover heat from the full "exhaust" stream. You can technically also connect the outlet node of the exhaust fan to the return path of a heat exchanger, but you most likely won't want to take that approach if you are trying to recover heat centrally from multiple exhaust streams.

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Thank you for your advice. I thought that might be the best solution, it is good to get confirmation. After specifying the same ventilation, I changed the Fan:ZoneExhaust to be Balanced so that the air was drawn from the zone. I was hoping that the the Exhaust Fan would override the need for ventilation air directly to the space but EnergyPlus must still need it for system sizing. I will need to look into air transfer between zones later in the project.

( 2018-05-30 09:00:35 -0600 )edit

I also tried this approach and I can see that return or supply fan are not affected by the exhaust fan flow. However, it seems the infiltration or the zone air heat balance transfer rate are not affect as well. This made me wonder where the air coming out of the exhaust fan is coming from? Do we need to manually change the infiltration for the spaces in the zone or to add a zone transfer item?

( 2019-08-01 11:57:21 -0600 )edit